planetary formation - Why haven't asteroid belts turned into new large bodies?

From Wikipedia:
http://en.wikipedia.org/wiki/Asteroid_belt#Formation

Planetesimals within the region which would become the asteroid belt were too strongly perturbed by Jupiter's gravity to form a planet. Instead they continued to orbit the Sun as before, and occasionally colliding.[27] In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,[28] preventing the formation of planet-sized bodies. Orbital resonances occurred where the orbital period of an object in the belt formed an integer fraction of the orbital period of Jupiter, perturbing the object into a different orbit; the region lying between the orbits of Mars and Jupiter contains many such orbital resonances. As Jupiter migrated inward following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other.[29]

Magnitude, satellite flare and the Heavens Above app

The astronomy "magnitude" scale works backwards: smaller numbers indicate brighter objects. Back in the days before precision measurements of brightness, stars were categorized by eye, with the brightest being "stars of the first magnitude". When more precise measurement became possible, this scale was retained, and extended into the negative numbers for very bright objects like Venus, the Sun, and a few of the brightest stars.

The Iridium satellites have enormous, mirror-like antenna arrays. When one of them is angled correctly, it will reflect sunlight straight at you, producing an incredibly bright flare visible even in broad daylight. Very few other satellites have large reflective surfaces other than their solar panels, and solar panels are kept pointed straight at the Sun, so they never generate flares.

To give some points of comparison, the Iridum flare listed in your screenshot, at magnitude -5.2, is comparable to Venus at its brightest, visible during daylight if you know where to look. The other satellites listed, with magnitudes in the 2-3.5 range, have brightnesses between that of Polaris, and that of the dimmer stars making up Ursa Minor.

human biology - How does laser surgery correct accommodation problems?

Laser eye surgery works by altering the shape of the cornea. The cornea works together with the lens to focus rays of light onto the retina. The cornea accounts for two-thirds of the optical power of the eye (1) (i.e. the eyes capability to focus light), however unlike the lens is of fixed power. It is the lens that changes is shape by the action of suspensory ligaments and ciliary muscles in order to focus light by the correct amount depending on how far away the light originated from (and consequently the angle of incidence with the cornea). I'm sure that you are aware of all this, however for the benefit of future readers this is fully explained diagrammatically on this webpage.

Three forms of accommodation problems can be treated by laser surgery:

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Hypermetropia (long-sight) is where light focusses behind the retina:

Laser treatment is used to make the cornea thicker, resulting in a greater degree of refraction of light and correction of the focus.

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Myopia (short-sight) is where light focusses before the retina:

Laser treatment is used to make the cornea thinner, resulting in a reduced degree of refraction of light and correction of the focus.

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Astigmatism is where the cornea is not the correct shape - it is closer to a rugby ball rather than the sweeping curve demonstrated in the first diagram. This results in multiple focal points therefore a blurred image.

Laser eye surgery is used to alter the shape of the cornea until it is more normal.

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Presbyopia is an age related condition where the lens becomes more rigid and is less able to change its shape to accommodate the light.

As this is a problem with the lens rather than the cornea, it can not be treated with laser eye surgery. It must be corrected with glasses, as shown in the above diagram.

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(1) Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.

gravity - What is spacetime 'made' of?

General relativity is often explained as saying spacetime is curved by gravity, what does this mean?

It means that general relativity can be formulated in a way in which its mathematics have a very direct analogue to differential geometry on a curved four-dimensional manifold. In other words, the way test particles would behave under the influence of only gravitational forces is exactly how they would behave if moving freely on a curved four-dimensional manifold. The mathematics have a direct correspondence: nothing more, nothing less.

Electromagnetism has a description in which the electromagnetic field strength is the curvature of a connection on a line bundle. I realize that this statement is very cryptic to someone who hasn't studied gauge theory, but it's important to realize that an essentially geometric description is not special to gravity. What's special to gravity is that it couples to all stress-energy-momentum equally, and gravitational freefall of a test particle is completely independently of composition.

Because of this universality, it is possible to interpret the properties of the gravitational field as properties of spacetime, i.e. as property of the arena on which everything else happens. We don't have to do so, and indeed there are some presentations of general relativity (e.g., Weinberg's) in which the geometric interpretation is relegated to an unimportant side note, but we can--and geometry is how general relativity was originally developed.

How could we perceive a curve in spacetime when there is no external "straight" reference frame for instance?

We could measure it.

As a conceptually (but not practically) simple way to do so, we could set up a small ball consisting of initially comoving test particles. With no curvature of the gravitational field, every such ball would keep the same shape and volume because they're all the test particles are moving in the same direction with the same speed. But if the gravitational field has Ricci curvature, the volume of the ball would either start shrinking or expanding. Similarly, changes in the shape of the ball would give information about Weyl curvature.

This is the same kind of answer as in the case of electromagnetism: the field strength is also a kind of curvature (though not of spacetime), but how do we perceive it? Well, we could measure it by seeing how test charges behave.

cosmology - How to disentangle a very distant star's relative velocity vs. redshift distance

Conrad is almost right. It is true generally that if a Galaxy is close enough to take spectra of individual stars (e.g. luminous supergiants) then it is not far enough away to be regarded as part of the "Hubble flow" and so applying Hubble's law to this star, or its host galaxy, would not yield a reliable distance in any case, but would reflect the "peculiar motion" of that galaxy.

To put some numbers on this. Galaxy peculiar motions tend to be a few 100 km/s, as do the individual velocities of stars with respect to their galaxies. Taking a Hubble constant of 70 km/s per Mpc, we see that we need to be at distances of 15 Mpc before Hubble recession velocities ($v = H_0 d$) become large compared with peculiar motions. At these distances we cannot observe individual stars - they are too faint and unresolved from the bulk of the Galactic light.

The exceptions are supernovae. The redshifts of individual supernovae, that briefly outshine their galaxies, can be measured right across the universe. Here you are correct that the measured redshift is a combination of cosmological redshift due to the expansion of the universe and a velocity of the star relative to the Hubble flow at that distance. There is no way to distinguish between these two unless velocity measurements could be obtained for other objects in the same galaxy. Given the rarity of supernovae, we might wait a long time for this.

But does it matter? Even if we look at a "low redshift" supernova at $z=0.1$, its Hubble recession velocity is 30,000 km/s and far in excess of any peculiar velocity contribution at the level of $sim 1$%.

supermassive black hole - Time according to the gravity of Sagittarius A?

Not at all a dumb question. As you have heard, it is true that time is affected by gravity. The stronger the gravitational field, the slower time passes. If you're far from any gravitating matter, time passes "normally".

But to answer your question, we must specify what is meant by "the black holes's time" (let's call the black hole $mathrm{BH}_mathrm{Sgr,A^*}$; see note below on the nomenclature), since it depends on how far from Sgr A* we are talking. The time pace at a distance $r$ from the center of a BH is given by
$$t = t_infty sqrt{1 - frac{r_mathrm{S}}{r}},$$
where $t_infty$ is the time "at infinity", i.e. far from the BH, and
$$r_mathrm{S} equiv frac{2GM}{c^2} simeq 3,mathrm{km},times left( frac{M}{M_odot}right)$$
is the so-called Schwarzschild radius (the "surface" of the BH), which is where not even light can escape. Here, $G$ is the gravitational constant, $M$ is the mass of the BH, $c$ is the speed of light, and $M_odot$ is the mass of the Sun.

The last equality shows that a BH with the mass of the Sun would have a radius of 3 km. The mass of $mathrm{BH}_mathrm{Sgr,A^*}$ is some 4.1 million Solar masses, so its radius is $r_mathrm{S} = 12.4$ million km.

Plugging in the other numbers, we can see that at a distance from $mathrm{BH}_mathrm{Sgr,A^*}$ of

1. 1 lightyear, time runs slower by a factor of 1.0000006557, i.e. unnoticeably.


2. 1 astronomical unit (the distance from Earth to the Sun), time runs 17% slower.


3. 1 million km from the surface, time runs slower by a factor of 3.7.


4. 1000 km from the surface, time runs slower by a factor of 111.


5. 1 km from the surface, time runs slower by a factor of ~3500.


6. 1 m from the surface, time runs more than a million times slower.


7. At the surface, time stops.

Note that this time dilation is what a distant observer (i.e. the guy with the $t_infty$ time) would measure for an observer at the distance $r$. The person at $r$ would just measure his/her own time as usual. For instance, according to point 5 above, if you were hovering 1 km from the surface, waving your hand every second, then I, choosing to stay at a safe distance of 1 lightyear but with a magically powerful telescope, would see you wave approximately once every hour. And when you run out of fuel and plummet into the BH, then when you cross the surface you wouldn't notice anything particular, but I would see you frozen in time. This is the concept of relativity.

Finally, let me use this chance to clarify something that people, including myself, often have gotten wrong: Sagittarius A (without an asterisk) is a radio source in the center of the Milky Way. It consists of three parts: Sagittarius A East (a supernova remnant), Sagittarius A West (dust and gas clouds), and Sagittarius A*, or Sgr A*, which is a very bright and compact radio source believed to be formed by a supermassive BH. Sgr A* isn't actually the BH itself. I think the BH doesn't really have a name, so I'll call it $mathrm{BH}_mathrm{Sgr,A^*}$. Maybe that's a bad name…

botany - How long will a vegetable live for after being harvested?

The short answer is that as long as the vegetable/fruit is fresh looking - i.e. the cells have not disintegrated - they will be respiring, many cells will be functioning quite normally, and the plant is still technically alive. In cases where the part of the plant we treat as a vegetable is a part intended for reproduction (e.g. a seed, or a tuber like a potato) the plant will keep growing.

The point at which the plant dies is not clearly defined like it is in animals, but generally if you can still eat it, it's still alive.

Death in plants is quite different from that in animals - we refer to it as senescence. The key difference is that it happens to tissues and organs which can die and separate from the organism. Individual leaves can die without the plant's health being affected. Once this has happened to all the parts, the organism is considered dead, but if there is any respiring tissue left, it's still alive.

angular resolution - Would Adaptive Optics be Useful in Radio Astronomy?

In fact, the techniques of adaptive optics are already being used in radio astronomy. They are implicit in the basic imaging algorithms (e.g., CLEAN) used to produce maps from radio interferometers. In those cases, they are usually being used to correct for the artificial structure introduced by the way the interferometer samples the sky, rather than for structure imposed by the intervening material. But at low frequencies (1 GHz and below, certainly) they are also used to correct for the artificial structure imposed on the incoming radio wavefronts as they pass through the ionosphere. Current large low-frequency instruments (such as the LWA and LOFAR) rely heavily on these methods.

general relativity - Extra dimensions

String theory, Kaluza Klein theory etc. need extra-dimensions.
For string theory these are compactified.
My feeling is that these extra dimensions are not of our 4-D space, but are only of the space in which our universe is EMBEDDED, and what we feel as electro-magentism and the other interactions are related to the extrinsic curvature of our 4-D space in these extra dimensions.
I.e. gravity is the only one which derives from intrinsic curvature, all other forces are "tidal forces" connected with extrinsic curvature.

I.e. a spiral curve in 3-D space is intrinsic flat, but it should be somehow different for a 1-D creature to go on a straight line or on a spiral.
From the point of view of a 3-D creature looking to the 1-D creature, the latter has to experience a centrifugal force.

What planet is better than earth to infer solar system configuration?

The mankind had to work some centuries to infer the real configuration of solar system, starting from greeks, Ptolemeus, until Copernicus, Galilei, Kepler, Newton etc. Is there any planet where we could better/faster determine the configuration?

(For example, having a moon is an advantage. Maybe having two moons(or none) or having a thinner atmosphere or being closer/farther to the sun may helped.)

The area of the question starts from inferring the form of the planet(round) until the discovery and orbits of all eight planets.

PS: The question ignores the fact that life is not possible on another planet. It is from pure astronomical viewpoint. On Neptune you have a different sky, so other questions/answers.

How can I keep HEK cells alive while expressing NMDA receptors?

I am trying to express functional NMDA receptors in HEK293 line cells for single channel recording experiments.

The HEK cells are maintained in the standard way (Thomas & Smart 2005) and transfected with NR1 and NR2x subunit cDNAs and also GFP, using either lipofectamine or calcium phosphate precipitation. GFP expression suggests successful transfection, but cells exhibiting green fluorescence are uniformly swollen and dead and it's more or less impossible to obtain a successful patch. Untransfected cells seem to remain perfectly viable.

On the basis that the toxicity might be a consequence of Ca²⁺ influx through open NMDARs, I've tried including a cocktail of blockers in the growth medium, including AP5, kynurenic acid and Mg²⁺, but the transfected cells continue to die.

Can anyone suggest anything else I should be doing to keep the cells alive? Or am I just on a hiding to nothing? Other researchers seem to have managed this (eg Medina et al 1995, Vicini et al 1998) and do not seem to be doing anything substantially different, so I'm a bit at a loss.

milky way - What happens when two black holes collide?

There are many hypothesis saying that there will a collision between
the milky way and Andromeda galaxy...so what happens when two black
holes will collide??

There's a few answers in the links in my comment above copied, here and here, but it's such a fun question I thought I'd answer it anyway.

With Andromeda and the Milky way's super-massive black holes in their respective centers, those objects are so massive, they are likely to be largely unaffected by any stars in their path and they will just fly towards each other at whatever rate and direction gravity dictates and by the time they get fairly close to each other, their respective gravity should have them moving quite fast towards one another. I don't know if anyone knows if they will orbit around each other for an extended period of time or spiral in fairly quickly. That will depend on the angle of approach. It's possible they could pass by each other, miss and each could get sent far out away from the other, part of an enormous orbit around each other, taking perhaps billions of years to lead to an eventual collision.

There's a few simulated videos out there on what happens when 2 super massive black holes spiral into each other. Here's one.

As Super-Massive black holes approach, each will pass through roughly half of the other galaxy, disrupting the orbits of any stars they pass hear-by, though they'd likely need to pass within less than a light year to have significant effects on the star's direction, which wouldn't happen that often, but it would happen.

Sagittarius A is huge for a black hole, but quite small compared to the space between stars. It's estimated to be about 44 million KM in diameter, which means it would fit (Just barely) inside the orbit of Mercury and our Sun. Mostly it will fly past stars. Andromeda's super massive black hole may be several times larger, but still fairly small compared to the distance between stars it's likely to pass on the way towards their mutual collision.

It's possible that one or both stars will pass through each other's galaxy relatively collision free but it's possible that one or both of them will get close enough to a star to generate a large accretion disk. While black holes don't have charge, their accretion disks do, and if that happens, that could be an interesting and not very well understood interaction between the 2 super-massive objects and their accretion disks. Also, as they spiral in towards each other, several of their near-by stars will be cast any-which way, some of them, inevitably inside towards one of the 2 black holes. It could be a very impressive show.

Finally, as they merge, which, might take quite a bit of time if they end up orbiting each other, perhaps millions, even billions of years. If/when they do merge, there could be some curious and not very well understood gravitational wave effects. bending and stretching of space like ripples. (Gravity already bends space, but not in measurable ripples. Our observation's of gravity warped space is a smooth curve.

I'll re-post this article here from comments that says it's possible for 2 black holes to repel each other if they bend space in opposite spin-directions and approach each other on a level plane. Less close and they could still easily disrupt outer planet orbits, casting some planets every which way.

And how will it effect the other objects revolving around them??

Imagine jellybeans in a salad spinner going as fast as you can spin it, and you remove the top. That's basically what will happen to any near-by stars. Both galaxy centers are quite crowded with stars (and perhaps several black holes) orbiting their centers. The gravity assists from 2 super-massive objects moving towards each other will be significant and, basically stars will be flying all over the place. Stars have tiny masses compared to those objects and they could be sent in any direction.

asteroid belt - Planetary orbital resonances

This is actually a very subtle question, much more so than the answers to the similar questions provided in the comments give it credit for. When I was in graduate school at Ohio State I routinely asked this question to visiting dynamicists and invariably got different answers.

The very basic answer is that if you have two sufficiently strong resonances sufficiently close together, then the resonance will be unstable. Otherwise, the resonance will be stable. But what determines "sufficiently strong" and "sufficiently close" is where things get very complicated quickly. A basic criterion is the Chirikov criterion. (The Scholarpedia article is somewhat more detailed.) However, the Chirikov criterion is not universally valid.

If you have overlapping resonances, then an object gets bounced back and forth between these two resonances chaotically. These different resonances perturb the orbit in different ways, and eventually they will perturb the orbit into an unstable orbit, thus leading to depletion of the resonance. If a resonance is "distant" from other resonances, then the resonance tends to keep objects locked in place, leading to an excess of objects in the resonance.

Most of the resonances in the asteroid belt are fairly close together, which leads to them being unstable. The Kirkwood Gaps are the most prominent manifestation of these instabilities. For example, the Alinda family of asteroids are in a 1:3 resonance with Jupiter, and are very close to a 4:1 resonance with the Earth. This leads to instability, and hence very few asteroids in this family. However, in the outer Solar System, the resonances are generally far apart, and so are mostly stable. The plutinos are one example of such a stable resonance, being in a 3:2 resonance with Neptune.

Origin of the Universe - Astronomy

Not entirely sure as to what question you're asking exactly but many theories of the origin of the universe have risen for many decades now. But as you would know, the Big Bang theory is the most accepted one. Since I'm typing this from my phone, I won't go into detail but I did once write an essay on this.
There are obviously parties that are against the theory and parties that are for it. But some of the other theories of origination sometimes fail to cover certain aspects or even suggest of multiple dimensions beyond what we already believe exists.
But none are as well researched or factual as the Big Bang theory.
Hope that helps :)

mrna - Do gene expression levels necessarily correspond to levels of protein activation?

I have seen a lot of research into molecular mechanisms of diseases/phenotypes use measures of RNA as a 'proxy' for the level of protein available in the cell. Is this actually valid?

My problem with the assumption that RNA levels correlate with that of the active product (i.e. the protein) is that a lot of post translational regulation occurs, including co-factor binding and phosphorylation, to name but 2. Does anyone know of any studies that have looked into the correlation between RNA levels and protein levels, and separately into the correlations between RNA levels and active protein?

It makes sense to me that RNA would correlate with protein certainly, but whether this relates to the proteins active function is what I wonder - i.e. there could be a pool that is replenished as and when the protein levels drop, but the proteins are only actually active for short periods in response to specific stimuli. So, does anyone know of any studies that have looked into the correlation between RNA levels and protein levels, and separately into the correlations between RNA levels and active protein?

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Update (04.07.12)

I have not accepted any answers as yet because none address my question about levels of protein activation, but I concede to Daniel's excellent point that proteins are not all activated in the same way; some are constantly active, some require phosphorylation (multiple sites?), some binding partners... etc! So a study looking at 'global' activation is not yet possible. Yet I was hoping that someone may have read some specific examples.

I today found an unpublished review by Nancy Kendrick of 10 studies that have looked at the correlation between mRNA and protein abundance - still not relating to activation. However she finishes the paper as follows;

The conclusion from the ten examples listed above seems inescapable: mRNA levels cannot be
used as surrogates for corresponding protein levels without verification.

If this is her conclusion about protein levels, then any correlation between protein activation and mRNA abundance seems unlikely (as a rule. Some protein levels do correlate with the RNA - see the paper).

I am still interested in any answers that give any information about specific examples of protein activation and mRNA levels - it seems highly unlikely there are no such studies, but I have been as yet unable to find any!

gravity - How does dark matter interact with black holes?

It interacts gravitationally.That's all there is to it.

There is a big difference though to the way that normal and dark matter interact with black holes - dark matter is far less likely to be captured.

Given two lumps of matter, one normal one dark, with the same mass and angular momentum with respect to the black hole; only the normal matter is capable of shedding its angular momentum (normally an accretion disc is involved), which allows its orbit to shrink enough to be captured (within three Schwarzschild radii). Dark matter is dissipationless; if it has too much angular momentum it won't get captured.

pluto - Has New Horizon's data updated Charon's orbital elements?

For years I've been fascinated with the mutually tide-locked bodies Pluto and Charon. In July 2012, The Astronomical Journal published an article The Orbit of Charon Is Circular by Buie, Tholen and Grundy. The authors thought that the eccentricity of Charon's orbit is very close to zero.

It seems to me their opinion is somewhat speculative given the low quality of images from Hubble. Has New Horizons data verified Charon's circular orbit? Is there an online source giving more precise orbital elements from New Horizons data?

I'm also interested in the obliquity of Pluto and Charon.

evolution - Is it the case that all changes in phenotype during life are not inheritable?

In general, Darwin's theory has been supported over and over again by experiments - our modern understanding of evolution is fundamentally what Darwin suggested. However, apart from appreciating many more details than Darwin ever could have, we also now know that Lamarck may not have been so crazy as he was later portrayed.

Inheritance in the Darwinian sense involves the digital information of DNA, i.e the sequence of bases. But we also know that DNA can be altered structurally - i.e. in the way it folds, or whether bases are methylated - and that these structural alterations can affect the expression of genes. In some cases, these epigenetic modifications can be trans-generational; they can be passed on to offspring.

Here are the mechanisms that I know of (perhaps others can expand on this):

• X-chromosome inactivation (XCI): this is when one of two copies of the X-chromosome in females is completely inactivated by being packed into heterochromatin, preventing the DNA from being transcribed. Which chromosome (the maternal or paternal) is deactivated initially is random, but the decision can be inherited by all daughter cells. Skewed x-inactivation is when a cell very early in the cell line passes on its XCI decision, and can result in a particular phenotype being activated in a whole organ or tissue (such as patches in tortoiseshell cats). It has been shown that in mice and in humans, the somatic cells can sometimes have their XCI decision influenced by the mother, and that this can lead to early skewing of the XCI in the offspring, thereby passing on a decision about which alleles are present without affecting the DNA sequence.


• Parental imprinting: in this case, individual alleles derived from one parent are preferentially activated or deactivated by methylation or histone modification. This change is passed on to the zygote, and alters expression in the offspring. Several human heritable diseases are associated with this kind of modification, such as Prader-Willi Syndrome.


• Paramutation: first discovered in maize, this is when the presence of one allele in a genome can affect another allele in a heritable way. I.e. if allele A is present in the same genome as allele B for a single generation, allele A is permanently inactivated so that if you breed out allele B, allele A will not be active in the offspring.

Finally, there is also a phenomenon called structural inheritance, whereby a structural feature of an organism is inherited in a non-genetic way. There is less written about this, so the mechanism is not entirely clear as far as I know, but an example is that the 'handedness' of the spiral pattern on the shell of a protozoan Tetrahymena is inherited without any genetic change (Nelsen et al., 1989).

References:

Nelsen, E.M., Frankel, J. & Jenkins, L.M. (1989) Non-genic inheritance of cellular handedness. Development (Cambridge, England). 105 (3), 447–456.

the sun - Does the sun itself present the problem of global warming is it the main cause?

The energy input of the Sun stays constant (mostly, there are some minor variations), so no, the Sun is not responsible for climate changes.

The temperature of the Earth has to do with the balance between the energy input, and the energy radiated back into space. If the temperature is not changing, they are the same.
Global warming is caused by gasses in the atmosphere limiting the energy radiated into space, therefore, the temperature rises, until the energy radiated is again equal to the solar energy input.

human biology - Where do the bacteria within the vagina originate from?

Most of the initial colonisation is said to be coincidental ('happenstance' as the textbook puts it!) exposure.

It's then fairly predictable depending on:

• type of delivery (as Larry commented);


• feeding; and


• receipt of antibiotics.

In terms of feeding, there are differences in flora between babies fed human milk and those that are given cow's milk.

There's a section called 'Establishment and Composition of Normal Flora' in chapter 187 of Principles and Practice of Pediatric Infectious Diseases (3rd ed) by Long which discusses the above.

It's also said that hormones may influence indigenous flora. For example, premenarcheal and postmenopausal vaginal flora are very different to those present during the childbearing period.[1].

1. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 7th ed. 2009. Churchill Livingstone.

How to form Copper from Calcium in a supernova explosion?

Copper is not thought to be primarily made in a supernova. It is thought to be mainly produced by the s-process of slow neutron capture onto iron-peak nuclei that already exist inside a star. These reactions are endothermic.

The source of the neutrons is still somewhat debated, it could either be from the decay of $^{13}$C in relatively low-mass asymptotic giant branch stars or is more likely from the decay of $^{22}$Ne in more massive evolved stars (e.g. Pignatari et al. 2010).

Below you can see a typical route for producing copper from $^{56}$Fe. The axes of the plot are neutron number on the x-axis and proton (atomic) number on the y-axis. Three neutron captures are followed by a beta decay, a neutron capture, a beta decay, then 3 more neutron captures followed by another beta decay to form $^{63}$Cu.

The net process is
$$ ^{56}{rm Fe} + 7n rightarrow ^{63}{rm Cu} + 3e + 3bar{nu}_e$$

The copper is then distributed into the interstellar medium by a later supernova explosion in the same star.

If there are no "seed" iron-peak nuclei (e.g. in the first/second generation of stars), then copper can be inefficiently produced by explosive r-process neutron capture during supernovae explosions. However, this would not contribute very much to the copper we see on the Earth.

orbit - Why aren't all planets in the same plane?

Your reasoning is correct: if Mercury orbited in the same plane as Earth, we'd see it transit the Sun every 4 months or so. In fact these orbital planes are inclined 7 degrees to each other, and the other major planets' orbits are inclined 1 to 3 degrees relative to Earth's.

The planets perturb each other's orbits slightly, so no planet's orbit is perfectly planar.
However, the Solar system average plane of all orbits and rotations is
invariable,
and most individual planets' orbits will remain near it for millions of years.

homework - Is secondary follicle or Graafian follicle arrested in the second metaphase of oogenesis?

Three follicular stages are recognised,
1) Primary/Preantral Follicles (with primary oocyte inside)
2) Secondary/Antral Follicles (with primary oocyte inside)
3) Pre Ovulatory Follicles (with secondary oocyte inside)

Pre ovulatory Follicles are formed ~36 hrs before ovulation at time of LH surge. This coincides with completion of Meiosis I and formation of the secondary oocyte. The secondary oocyte immediately enters Meosis II.

Appx. 3 hrs before ovulation, the Pre ovulatory Follicle (with the secondary oocyte inside) is arrested in the metaphase of Meiosis II and is extruded out of the ovary - a process called as Ovulation.

It comes to rest in the ampulla of the fallopian tube and here it is fertilised. As the sperm pronucleus is released into the cytoplasm, the Secondary Oocyte (now called the ovum) completes Meiosis II and releases the second polar body.

Hope this helps.

human biology - What is the eye muscle status when you stare at distant view through a glass wall?

In the situation you describe, the eye would be focused on the distant mountain. This would mean that the lens would be stretched and thin in order to minimize the focussing power of the eye. Therefore the ciliary muscles would be relaxed.

When you are looking out of the window, it is possible to make a conscious decision to focus on the window pane itself (thus adjusting the focus to be more powerful as the ciliary muscles contract), however then the distant object will be out of focus and uncomfortable to look at.

This is because the light rays reflecting from the mountain are barely affected by the pane of glass, hence its transparency.

If quasars are powered by black holes, why are they so bright?

A black hole, in deep space is basically black, and very hard to detect. But if a black hole is surrounded by material, that material will fall towards the black hole and enter into orbit about it. (Black holes don't suck, they gravitate)

The material may come from, for example another star, or in the case of the giant black holes at the centre of many galaxies, from the gas, dust and stars that are found in the cores of galaxies.

As objects orbit the black hole they will tend to collide with each other, releasing energy in the form of heat, and causing them to fall to lower orbits. This process tends to cause the gas orbiting a black hole to form into a disk, called an accretion disc. Pretty soon any larger objects will be broken apart, and the accretion disk will be composed of gas, and as it heats up, plasma.

Now as objects fall to lower orbits they speed up. And for a black hole, this speed up is extreme. The gas will be orbiting at speeds that approach the speed of light. This makes friction and collisions between the particles that are orbiting the black extreme as well. The accretion disk heats up, to immense temperatures.

Now something weird happens, and the physics of it is not really sorted out. Magnetic fields get tangled up in the hot matter and cause a portion of it to be ejected away from the black hole in a jet, perpendicular to the accretion disk. The speeds of the particles in the jet is close to the speed of light. Massive amounts of electromagnetic energy is also released along this jet.

Quasars are active galaxies that happen have their jets pointing towards us. The large amounts of gas need to supply a massive black hole with the energy to make a quasar were more common in the early universe, so many quasars are very distant and very old, but the youngest is only about 700 million light years distant, and there is every reason to suppose that quasars still exist today.

You don't need a binary black hole to make a quasar, but the merging of two black holes could also release massive energy, and may be a type of gamma ray burst, and should also release gravitation waves.

Very bright star in the east at northern hemisphere. What is it?

As other people have pointed out, it is hard to work out which star it is, without knowing your general location. However, after checking on Stellarium, there seem to be a couple of likely suspects:

• Sirius - the brightest star in the sky. I've seen it myself - and on a good, dark night, it can really stand out.


• Jupiter - the king of the planets is also rising at about the same time. It is brighter than any star in the sky, by a wide margin (though fainter than Venus), and it can really stand out.

Other than that, there aren't really that many objects rising in the East at the time you specify that could really stand out.

There are a couple of useful ways to tell the two apart:

• Sirius is a bright white object - perhaps with a subtle bluish tinge to it, whereas Jupiter has a slight yellow tint to it.


• Jupiter is currently rising in the North-East, and can get very high in the sky at the moment from the northern hemisphere, whereas Sirius rises in the South-East, and doesn't get that high (though that does depend on location).


• Sirius tends to twinkle, and 'flicker', as its light is disturbed by air currents, whereas Jupiter remains very steady - perhaps not twinkling at all.

As mentioned earlier, the best method is usually to use software like Stellarium, which will tell you exactly where everything is, and hopefully give you a definitive answer to which object it is.

fundamental astronomy - Calculation of hour angle

I need to determine Right Ascension and Declination from Azimuth and Altitude, working in C#. The problem is that the formula for calculating hour angle, for some reason, doesn't work. Here's the code:

        az = az * DEG_TO_RAD;
        alt = alt * DEG_TO_RAD;

        lati = latitude * DEG_TO_RAD;

        // Julian day
        JD = CalculateJDN(year, month, day, h, m, s);

        // Greenwich mean sidereal time
        GMST = CalculateGMST(JD);

        LST = GMST + longitude / 15;

        dec = Math.Asin((Math.Sin(lati) * Math.Sin(alt)) + (Math.Cos(lati) * Math.Cos(alt) * Math.Cos(az)));

        ha = Math.Atan2(Math.Sin(az), (Math.Cos(az) * Math.Sin(lati) + Math.Tan(alt) * Math.Cos(lati))); 

        ha = ha * RAD_TO_DEGREE / 15;
        dec = dec * RAD_TO_DEGREE;

        ra = LST - ha;

        // Input data for Mintaka (delta Ori):
        // az = 47.5, alt = -35.3 on 13:57 UTC, 1 Dec 2015
        // latitude = 43.897, longitude = 20.344
        // Required output: dec = -0.19, ra = 5.5
        // Given output: 
        // dec = -0.19, ra = 17.5, ha = 2.5

Az and alt are given in degrees, so they are first converted into radians. Functions for calculating Julian day number and GMST are correct, since I've already tested them. Formula for declination is good, but for some reasons formula for hour angle (ha) doesn't work. I don't know where's the error.

galactic dynamics - What happens to galaxies when they die?

Well, it would be useful to define what a 'dead' galaxy is. Probably the most simple method would be a galaxy that is no longer producing new stars. We might also consider a galaxy that no longer produces significant light in the visual spectrum, or perhaps EMR across the entire spectrum.

Generally, there's unlikely to be a firm line between living and dead, and not nearly as dramatic as larger stars. More akin to watching a camp fire burn itself out. Star formation is largely dependent available gases, but as more and more stars fuse those gases into heavier elements, there is less gas available for star formation. For your average sized galaxy, this will eventually result in running out of gas. Eventually the galaxy will dim and go dark, a process purported to begin at the center of the galaxy, where star formation is heaviest according to research based on Hubble images of giant galaxies. (Tacchella, et al.) The matter ought to (mostly) all still be there and still orbiting the (presumed) SMBH, but with no energy coming from fusion, it's going to be a dark, cold, and barren place. Sounds dead to me.

There are some complicating factors. It's believed that encounters with nearby galaxies can affect available gases. The gravity from a larger galaxy could potentially strip the gases from a smaller one, a fatal blow for the smaller galaxy. Fortunately, it won't suffer much as the death will come (relatively) quickly. This process has been deemed 'strangulation' by a study published in Nature several years months days ago. (Ping, et al.) Note that as the study indicates, the methods of death are proposed solutions - not conclusive understanding of the exact processes that result in a galaxy's death.

---

S. Tacchella, C. M. Carollo, A. Renzini, N. M. Förster Schreiber, P. Lang, S. Wuyts, G. Cresci, A. Dekel, R. Genzel, S. J. Lilly, C. Mancini, S. Newman, M. Onodera, A. Shapley, L. Tacconi, J. Woo, and G. Zamorani. Evidence for Mature Bulges and an Inside-out Quenching Phase 3 Billion Years After the Big Bang
Science 17 April 2015: 348 (6232), 314-317. [DOI:10.1126/science.1261094]

Y. Peng, R. Maiolino & R. Cochrane. Strangulation as the primary mechanism for shutting down star formation in galaxies Nature 521, 192–195 14 May 2015 [DOI:10.1038/nature14439]

Andrea Cattaneo. Astrophysics: The slow death of red galaxies Nature 521, 164–165 14 May 2015 [DOI:10.1038/521164a]

genetics - Is sexual reproduction outside the same biological family possible? Has it ever occured successfully?

Are there any examples of two species taxonomically classified in different biological families that have successfully hybridized and produced viable offspring? If not, is there an example of where reproduction occured with non-viable offspring?

To be clear, I mean regular sexual reproduction that could occur in the natural world outside a lab. Even so, I'd be curious to know if it could even occur in a lab without direct genetic manipulation.

For example, grolar bears which are ursid hybrids between different species in the Ursus genus are known to exist. Also ursid hybrids between bear species in different genuses have been produced in captivity (sloth bear Melursus ursinus x Malayan sun bear Ursus malayanus and sloth bear x Asiatic black bear Ursus thibetanus). Would an extra-familial hybridisation be possible? Would this be more likely in the plant kingdom?

This question is inspired by a separate question on the Gardening SE which hints at a general lack of understanding of the genetic similarity required for cross-pollination in plants. It made me wonder whether there are any exceptions to the general assumption that extra-familial hybridisation is impossible.

the sun - What happens to oxygen produced on the Sun (or other stars)?

The Sun is a small main sequence star. It does not produce oxygen via fusion. It can't. The temperature and pressure in the Sun's core are too low. Fusion in the Sun is currently limited to production of helium. This will remain the case for several billion years.

That said, there is oxygen in the Sun, about 1% by mass. This oxygen was produced long ago by other stars at the end stages of their lives. Our Sun is a third generation (or more) star. Most of the Sun is far too hot for those oxygen atoms to combine chemically. One exception is sunspots, relatively cool areas on the Sun's photosphere. (Relatively cool means less than 4500 kelvins, so still quite hot.) Molecules can form at these lowish temperatures, and scientists do see signatures of many different molecules in the light coming from the Sun.

Update, in response to edits to the question

Molecules cannot form inside of a star. The temperatures are just too high. Molecules decompose (split apart) into their constituent parts at high temperatures. The Sun's photosphere is about 5800 kelvins, which is already too hot to sustain very many molecules. Temperature rises rapidly with increasing depth below the photosphere. The Sun's core temperature is about 15 million kelvins (27 million Fahrenheit), and the Sun is a small star. Larger stars have even higher core temperatures. At 15 million kelvins, there aren't even atoms, let alone molecules. There are instead atomic nuclei and electrons. Atoms are stripped of their electrons at those extreme temperatures.

In five to seven billion years, our Sun will have fused all of the hydrogen in the core into helium. That's when our Sun will become a red giant. Even then, it still will not produce oxygen. The first stage a one solar mass star experiences after leaving the main sequence is the red giant phase, where the core is an inert mass of helium surrounded by a shell of fusing hydrogen.

Eventually (after another billion years or so), the temperature of that helium core will rise to the point where the helium starts fusing into carbon, plus a little bit of oxygen via the first step on the alpha ladder. At this point, the Sun will leave the red giant phase and join the horizontal branch of the Hertzsprung–Russell diagram. This is a rather short-lived phase of a star's life. The carbon and oxygen produced by helium fusion quickly (in stellar timeframes) form an inert core. At that point, our sun will become an asymptotic red giant.

The red giant and asymptotic red giant phases are rather messy affairs, wracked by convulsions where the star expels lots of gas. Our Sun will lose about half its mass to such convulsions. Molecules do form when this expelled gas cools. This results in some of the prettiest pictures in astronomy, shown below.

light - How long can you be trapped orbiting around a black hole for?

I have a story I wanna write but I want to be sure it's not completely scientifically irrelevant.

I know there is black hole modelisation called the Kerr black holes, in which there is a limit around black holes called the event horizon. It's a spatial limit ; no light ray crossing this limit will ever be able to escape the gravity of the black hole. That's the definition of event horizon.

By extension, I guess any physical object (such as a spaceship) crossing the event horizon will be doomed to orbiting around the black hole until it gets destroyed. My question is about the time you can spend between the moment you cross the event horizon (= you are doomed) and the moment you actually die (for example by the tide effect, which basically destroys bodies because of the difference of attraction force between the feet and the head of a human body)? Could this moment last... 100 years ? 1000 years ?
(for my story the more the better)

I know black holes studies are very theoretical, I'd just like to avoid any huge scientific plot holes.

I am quite unskilled in that domain of astronomy, about time dilatation, etc. So if anyone has any idea of the order of magnitude of the time a object can spend beyond a black hole event horizon before it gets destroyed ?

Thanks for your answers :)

How does a cell sense its size?

This is a question that has been the focus of study for the last century (e.g., Amodel of cell size regulation - Ycas et al, J. Theoret. Biol. (1965) 9, 444-470). Cell size regulation may be in part determined by ribosomal activity (through mTor regulation) and is a critical checkpoint in cell division.

How the cell senses its size, however, is not understood. In 2009, two reports suggested that protein gradients could be responsible for the sensing of cell size. You can read a commentary about those articles in Cell size control: governed by a spatial gradient. - Almeyda and Tyers, Dev. Cell. (2009) 17(1), 3-4:

The phenomenon of cell size homeostasis, whereby cells coordinate
growth and division to maintain a uniform cell size, has been an
outstanding issue in cell biology for many decades. Two recent studies
in Nature in fission yeast demonstrate that a gradient of the polarity
factor Pom1 is a sensor of cell length that determines the onset of
Cdc2 activation and mitosis.

These articles demonstrate one way in which cells may sense their size, but most probably several other mechanisms are also in place.

botany - Most suitable biodiversity index

I am conducting an investigation into the effect of two different grass management techniques (grazing vs. machine-mowing) upon floral biodiversity.

I have collected my data and now need to process it in a way that will yield meaningful and valid results. My data is in the form of 25 samples per area with %-abundance measured for each species in a 0.25m² quadrat.

I am currently using a non-standard diversity quantification technique, called Disney's index (which I have been led to believe is named eponymously after R.H.L Disney, however I am unable to find any references describing this), in which we assign each species a weight based on percentage abundance, as follows:

                                           

We then use these weights to calculate the index as follows (i.e. by computing the sum of the weights of the species, over the number of species):

                                                                    

I want to know if this is the best possible diversity index I could use for this type of analysis, or whether there are others which I should be considering.

Thanks in advance!

lab techniques - Can I image Coomassie and GFP in gels at the same time with a fluorescence scanner?

I'm working with a GFP-tagged protein and am routinely using a fluorescence imager (GE Typhoon) and a standard optical scanner to capture fluorescent and absorption images, respectively, of my SDS-PAGE gels. The Typhoon supports multiple channels, so is there some way that I can scan for the two on the same device (GFP + total protein)? I assume I can't do absorption on a fluorescence scanner..

I could manually register the two images, but it would be laborious as they are at different resolutions, slightly different rotations, and the gel can stretch ever-so-slightly when placing it on the platens.

zoology - Is the appendix a vestigial structure in all vertebrates?

Aside from humans, it is largely rodents and most notably rabbits that have an appendix. Therefore, using rabbits as my example:

In rabbits, the appendix is thought to have a key role in the development of the immune system. Specifically it has been shown experimentally that when neonatal rabbits are given an appendectomy levels of Immunoglobulin A and G (IgA/IgG) fall dramatically. Both these polypeptides are prominent antibodies - IgA plays a key role in mucosal immunity whilst IgGs in humoral immunity. These effects were localised to the small intestine of the rabbits, however were statistically significant. ¹

In infants, the rabbit appendix resembles the chicken bursa and sheep ileal Peyer's patch (both performing similar functions as described above).²

This has led to some ongoing research as to whether the human appendix has a similar properties in having function in infants ³:

If the human appendix functions as a primary lymphoid organ, it may occur during the first few months of age when the GC T-cell density is low.

---

¹Neonatal appendectomy impairs mucosal immunity in rabbits. Cell Immunol. 1997 Nov 25;182(1):29-37

²The appendix functions as a mammalian bursal equivalent in the developing rabbit. Adv Exp Med Biol. 1994;355:249-53.

³A morphological and immunohistological study of the human and rabbit appendix for comparison with the avian bursa. Dev Comp Immunol. 2000 Dec;24(8):797-814.

light - Could someone see anything while being inside black hole?

The answer is most definitely yes, or at least yes, as far as our current understanding of how gravity works goes. It is observationally untestable (let's be more specific - nobody could report the results of an observational test!) since no signal can emerge from inside the event horizon.

The scenario is treated in some detail by Taylor & Wheeler ("Exploring Black Holes", Addison, Wesley, Longman - highly recommended) in terms of what an observer would see on a direct radial trajectory into a non-rotating "Schwarzschild" black hole. I won't bore you with the maths - it is fairly complex.

A star situated at exactly 180 degrees from the observer's radial path will always appear in that position as the observer looks back - right down to the singularity. The light will be gravitationally blueshifted by an ever-increasing amount - essentially tending towards an infinite blueshift at the singularity.

For stars at an angle to the radial path, their positions will be distorted such that they appear to move away from the point at which the observer has come from (and are also blue-shifted). In the final moments (it takes less than $1.5times 10^{-4}$ seconds of proper time to fall from the event horizon to the singularity of a 10 solar mass black hole, but a huge $sim 60$ seconds for the black hole at the centre of our Galaxy) the light from the external universe will flatten into an intense ring at 90 degrees to the radial direction of motion. So you would end up seeing blackness in front of you, blackness behind and the sky split in two by a dazzling ring of light (almost seems worth it!).

Can there be life in black hole?

If we lived inside a black hole... Well, we wouldn't actually live. We would rather be ripped apart by gravity. Or we wouldn't even be anything actually, because the gravity inside a black hole is huge. So huge that not even light can escape.

I suggest you edit your question to make it more understandable if you want more details. Also, why the caps ?

Is it possible that some stars are already black holes yet we see light emitted from before becoming a black hole?

https://xkcd.com/1342/ and http://www.explainxkcd.com/wiki/index.php/1342:_Ancient_Stars

https://xkcd.com/1440/ and http://www.explainxkcd.com/wiki/index.php/1440:_Geese

could it be that they are already black holes (the sufficiently large
ones) and we are just seeing the light emitted during it's main phase?

For some stars, yes it could be. There are certain stars that we can individually observe, and that we believe could go supernova more-or-less at any moment. Betelgeuse is probably the most famous example. It's 640 light years away, it could go type II supernova any time (expected within the next million years, but could conceivably happen within the next 640 years as we observe it, such that it's already happened). It might become a black hole when it does (although a mere neutron star is also on the cards, I believe, and probably more likely).

Furthermore, when we observe a distant galaxy it is certain (as far as our theories are concerned) that some of the stars contributing to the light from that galaxy have since become black holes. However, since we aren't resolving individual stars in those galaxies even with our best telescopes, you could have an argument whether we're "seeing" the stars or not.

is it safe to say that most of the stars are already dead that we see?

Stars in our galaxy, the Milky Way, no. It's only around 100k light years across, it contains 100 billion - 1 trillion stars, of which we can individually "see" only a small proportion. Most of those are very close, the furthest around 1500 light years, and most of those are not supernova candidates at all, let alone black hole candidates. Even allowing for our best telescopes, we still only resolve a small proportion of stars individually.

The galaxy probably experiences roughly one supernova every 50-100 years. So even if all of those were visible stars (which they aren't: even of the largest stars in the galaxy only a small proportion can be seen with the naked eye), and all the supernovae resulted in black holes (which they don't), then at most 15-30 naked-eye-visible stars could already be black holes, and only up to 1000-2000 total stars in the whole galaxy regardless of visibility are in the duration between going supernova, and earth entering the future light cone of that supernova. This extremely loose upper bound is by no means "most of the stars that we see", and in fact the chances are that none of the stars you see when you look up at the night sky are already black holes. It might well be that none of them ever will be.

It's also unlikely that any star you observe with the naked eye is dead, since their lifetimes are of the order of 10 million years (for the largest) to billions of years, and their distances from earth are 1500 light years or less. But there are a few stars, like Betelgeuse, that we know are near the ends of their life and that therefore might be "already dead" in this sense.

Using telescopes we can see some individual stars in nearby galaxies, or at least indirectly observe them since they're lighting up nebulae that we see, up to a few tens of millions of light years away (these tend to be blue supergiants, I think). Of these, those on course to turn into a black hole may well have lifetimes less than or equal to their distance from us. It's not necessarily the case that all or even most of them become black holes, I'm not sure what's typical of the types of star in question, but for those stars you can say that many or most are already black holes. The most distant individually-resolved stars are all dead one way or another.

If you allow for us "seeing" each star in a very distant galaxy, even though we can't resolve any of them individually before they supernova, then the numbers change again, but my instinct is that it's not the case that "most" of the observable universe is already collapsed into black holes. But the stars may well be dead even though they aren't black holes. We can see galaxies 10+ billion light years away, which is about the entire lifetime of a star like the Sun. To come up with "most", though, you have to firstly agree what counts as a star and secondly survey what kinds of star those galaxies contain. If you end up concluding that most stars are (by comparison with the Sun) tiny little red-brown things with lifetimes in the tens of billions of years, then most stars aren't already dead no matter how far away the galaxy they're in :-) On the other hand, the brighter objects that contribute most of what we observe via telescope in the most distant galaxies, sure, certainly die in far less than a few billion years.

Note that in special relativity the concept of "simultaneous" is a bit difficult to pin down. But I've followed the convention that if something is X light years away, then anything we observe from it between now and X years from now, has "already happened". We're not necessarily entitled from where we're standing to say that it's "already happened", since it's not in our past light cone, but good enough :-)

the sun - Will Neptune be visible with the naked eye if I am standing on its satellite

I'm probably misunderstanding the question, but
http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html
notes that Neptune's magnitude from Earth is 7.8 at opposition, when it's
4347.31 million km away.

If you get 3 times closer, 1449.1 million km away, Neptune
will appear 9 times brighter, bringing its magnitude to 5.4,
well within our visible range.

As Stellarium notes below, Neptune is almost visible
(magnitude 5.66) from Uranus, an entire planet away.

My version of Stellarium can't simulate the view from Naiad
(Neptune's closest moon), but at a distance of only 48,227 km
(Naiad's semi major axis), Neptune's magnitude would be almost
-2, much fainter than our own moon, but brighter than Sirius
appears to us.

This brightness would be spread out across Neptune's disk, but
the brightness at any point would be about magnitude 3, still
quite visible.

Moreover, Naiad probably has a much thinner atmosphere (and no
light pollution), making Neptune even easier to see.

The other outer planets are visible from Earth (Uranus just
barely), and so would also be visible from their own moons.

I haven't done the calculations for Pluto, which is
technically no longer a planet.

What was that Asteroid/Comet in the 90s?

Was it Comet Hale-Bopp? It was discovered in 1995, but made a very close approach in 1997, earning it the nickname "The Great Comet of 1997".

Wikipedia states

As it passed perihelion on April 1, 1997 the comet developed into a spectacular sight. It shone brighter than any star in the sky except Sirius, and its dust tail stretched 40–45 degrees across the sky. The comet was visible well before the sky got fully dark each night, and while many great comets are very close to the Sun as they pass perihelion, comet Hale–Bopp was visible all night to northern hemisphere observers.

^{Image courtesy of Wikipedia user LoopZilla under the Creative Commons Attribution-Share Alike 2.0 Austria license.}

My second guess is Comet Hyakutake, which made an extremely close approach to Earth in 1996 - becoming "The Great Comet of 1996".

Are we sure there are no planets inside Mercury's orbit?

Yes, Vulcanoids larger than 5.7 km diameter are considered to be ruled out:
A Search for Vulcanoids with the STEREO Heliospheric Imager.

An object sufficiently large to be called "planet Vulcan" would change Mercury's orbit in a detectable way by gravity.

So, there are two lines of evidence, that such a planet doesn't exist.

More to read in Wikipedia:
https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)

telescope - What would be the maximum theoretically possible angular resolution?

I have studied some basic astronomy, but I have difficulty conceptualizing the physics of luminosity and optics. We use electromagnetic spectra to detect existence and properties of distant planets, but I am curious about the extent of what could theoretically be optically captured.

(I am unsure if optical is the correct term, but I'm referring to what could be directly captured vs. computed.)

Assuming the use of technology far beyond what we currently are capable of, what is the maximum possible optical angular resolution we could theoretically obtain of a distant sky object? Or, for example, of the surface topography of an outer planet/moon/dwarf planet?

What would be the limiting factor at the extreme end of possible angular resolution?

the sun - Using the Sun as a Gravitational Lens

Yes, it is possible to use the sun as a gravitational lens and to achieve better telescopic viewing. As you know space is curved by mass and so light is deflected by mass, it is possible to focus light using gravitational lenses and thus achieving greater telescopic viewing.

However, the sun does have corona fluctuations around it. So, to better exploit the gravitational lensing effect of the Sun, one should try to focus a little bit further away from the sun.

Actually, it was already being planned for a space mission to exploit the sun's gravitational lensing effect to communicate interstellarly. The mission is called FOCAL (For "Fast Outgoing Cyclopean Astronomical Lens").

For more further information, search for either "Dr.Claudio Maccone" or "FOCAL Space Mission"

When Phobos collides with mars (or breaks up), will it have any noticeable effects on Earth?

No. Phobos is small - just 11 Km across - the size of a small city. Mars (and Phobos) is so far away that a Phobos impact will not affect Earth much. (Mars ranges from 100 billion meters away to nearly 400 billion meters depending on its and Earth's position in their orbits.)

When Phobos hits the Roche limit as it will break up and become a thin light-grey planetary ring around Mars for a few million years.

When it breaks up, (almost) all the debris should stay near Mars because the escape velocity from its orbit around Mars is greater than the velocity imparted to boulders of Phobos as it self-destructs.

orbital elements - Why don't the stars in a binary star system of equal masses always orbit their center of mass in a circular orbit?

There's no reason why 2 bodies of equal mass couldn't have elliptical orbits around each other. There's an example of that here

.

The simple way to think about this is, if two bodies of similar mass approach each other, one of two things can happen, they either have sufficient velocity to pass each other with some hyperbolic curving of both objects or there's enough gravitational attraction that they capture each other in orbit, but the shape or eccentricity of the orbit depends on the ratio between the tangential velocity and the orbital speed at the closest pass. For a circular capture the ratio needs to be exactly 1, and that kind of exactness is rare, so we nearly always get an elliptical orbit with an orbital capture.

Solar-system planet or moon formation (not moon capture) tends to be much more circular, because elliptical orbits of objects tends to cancel out as large bodies coalesce, but you're never going to get 2 similar mass objects orbiting each other in a condensing gas cloud. Most of the mass inevitably collects in the gravitational center. There can be, however, a tidal influence that circularizes orbits over time, never reaching a full circle, cause that's impossible, but slowly becoming more circular.

orbit - Is this Universe scenario correct?

Like it was already pointed out in the comments, your assertions and assumptions are way off today's well-accepted theories. Nonetheless, I'll try to answer you questions.

Will our solar system die of old age in 5.4 billion years

Our sun is a G-type main-sequence star with an estimated lifespan of roughly 10 billion years. Like you mentioned, it is about 4.6 billion years old and will stay in the main sequence for another 5.6ish billion years. The sun's mass is not enough to end with it's life with a supernova. Instead it will become a red giant with a radius of about 1 AU (= astronomical unit), meaning that it will most likely devour planet earth, but won't expand further.

or will we be consumed by the Universal black hole?

I've never heard of something like an universal black hole, but it appear's to me that you might have a big misconception about black holes in general. If (for some completly unknown and unphysical reasons) the sun would all of the sudden turn into a black hole with the same mass, nothing much would change (on a cosmological scale). Planet earth's main energy input would seep away and we all would freeze to death, but gravitational, nothing would change. The planets orbits would be exactly the same and nothing would become 'consumed'. Black holes don't accrete mass (that's what I suppose you mean by consuming) due to some magical properties, but due to gravitational attraction, which is only dependent on the masses and the distance of the objects.

How long should it take for the 93 percent of universe to be consumed by the black hole?

Speaking of 93% of the universe might not make any sense. Today's measurements suggest that the curvature of space is flat ($Omega_{tot} = 1.00 pm 0.02$) and that yields in a possible infinite universe. Please see $lambda CDM-Model$
and Planck 2015 results. XIII

dna - Are there viruses that affect cells across different species?

Firstly, it's important to recognize that "plant viruses" do not exist. There are only "viruses that affect particular plant cells", or "viruses that affect a particular cell type". You'll see why in a moment.

One of the structural components of many virus is its protein coat. Different types of biological molecules protrude from the surface of this protein coat, deemed signalling molecules. The structure of these molecules are specific to a certain type of virus.

Analogously, specific cell types have specific biological molecules that protrude from their plasma membrane, deemed receptor molecules. For a virus to affect a particular type of cell, their signalling and receptor molecules must "fit within each other", like lock and key. Once they do so, a virus may interact with this molecule, in whichever way it does.

Although your hypothesis wasn't entirely correct, as viral contact with a cell must first be established, viruses do interact with DNA replication in different ways, as in the lytic or lysogenic cycle.

radio astronomy - The contents of potential "message" in the Wow! signal lost?

I'm not an expert in this, but it's a fun little blip in the history of SETI. Pretty much the only blip I think.

I understand the incredibly high signal strength it entails.

I wouldn't call it "incredibly high". It peaked at 30 times normal. source, and that's inside the "waterhole" a frequency range where the background radiation is the lowest in the cosmos. 30 times a normally very low signal is still pretty small, I think. It was unusual, no question, but looking at the sheet of paper most often reprinted in articles about this, each space is 12 seconds, there are a handful of visible spikes.

Just on that one printout there's a 3,3,2,space,7 and a 3,6,1,3 and a 4,3,4,4
The Wow, 6,E,Q,U,J,5 or 6,15,26,30,20,5 compared to those numbers is maybe 6 or 7 times larger than some fairly common spikes. Statistically speaking, that's high, but not enormous. It's still an unexplained oddity, but it wasn't an "incredibly loud" signal, just unusually loud.

In 1977 they couldn't turn the dishes, so they had to wait 24 hours to scan that part of the sky again and 24 hours later, and every day since, and no similar signals have ever been found, so yeah, it's a mystery.

The way they had it set up, they couldn't tell which dish either, so there's 2 tiny slices of the sky that it might have come from, or it could have (theoretically) come from a ship of some sort transmitting it's location back to it's home planet, and the earth just passed through (or it could have been a mouse chewing on a wire, or a Russian prank) . . . it could have been lots of things.

In 1980 they set up the ability to have the dishes track a specific part of space, which would have come in handy in 1977, but new new signals of that strength were ever heard.

And, you're right, the big ear wasn't designed to record. As I understand it, the system was designed to eliminate background signals, so if both dish's caught similar wavelengths while pointing at slightly different parts of the sky the duplication could be eliminated. It was what one received that the other didn't that was printed on paper, but no way to tell which dish did the receiving.

Some cool pictures and write-ups about the Big ear here and here. It was a monster of engineering for the 1950s when it was designed but it's primitive by today's standards. The Wow signal has no triangulation to verify if it was from outer-space vs a closer event, and they couldn't turn their equipment to track it. It was initially designed to look deep into space, by the way, not to record alien signals. Seti began using it in 1973 after it was somewhat obsolete for more traditional astronomical pursuits, but throughout the 60s it was one of the better observatories. Another problem, it didn't detect a wide range of radio waves, just a tight band, which is good for looking for a signal, but less good for eliminating rare stellar events that might have spanned a broader range of radio-frequencies. Some possibilities here for stellar explanations and without triangulation, the possibility of rare atmospheric reflection of some kind seems possible too.

The 6 data points looks like a bell curve and that's exactly what we should expect if we pass through a tight signal. It's not what I'd expect if a signal was directed at us, but passing through a tight beam would explain the bell curve of the spike. A black hole in that part of space might explain it too.

Is there a possibility that the burst could have carried information,
however unlikely? It could have carried a one-time "message"?

The "one time message" idea strikes me as very unlikely because I don't see any point to it. I also don't think a lot of information could be contained in 72 seconds, but some could, but there's no point in sending information for just 72 seconds and just once. The point of sending information is so the person on the other end receives it.

But if it was Technology based and not natural in origin, Radio-waves can carry a lot of information, otherwise radios and antenna televisions wouldn't work, but the information is contained in signal variation usually in either amplitude or frequency, like these images below

or this:

Source

The big ear measured total signal variation between the disks every 12 seconds, so it wouldn't detect Amplitude or Frequency variations that would happen in much shorter intervals than 12 seconds. We simply have no way of knowing. Another possible method to transmit a information could perhaps be polarization variation in a consistent signal, but big ear couldn't pick that up either.

The problem is, we simply don't have enough information to know. It's like somebody snapped a terrible picture of something that might have happened. You can look at the picture forever, but it will never give you any new information.

More on it here, and I liked this article here. Hope that wasn't too wordy or too much already known information.

space - Why can't gravity repel things?

As far as our current understanding of gravity, no. The common analogy is a rubber sheet with marbles. The sheet can only be pulled downwards, so you cannot have a repelling force.

To pull the sheet upwards would require something like inverse gravity, which is not yet known to current physics.

Of course dark energy seems to look like this, but as the origin of dark energy is currently completely unknown, it is not useful to speculate on this at this point in time.

How are whole Haplotypes for Sequencing isolated?

Is it possible to reliably isolate and amplify DNA from individual sperm and/or eggs (from a fish)? I'd imagine that the small amount of DNA would make the PCR a bit wacky. We've considered irradiating sperm and attempting to create haploid females via gynogenesis, but I was wondering if there was a faster and easier method that could utilize gametes directly.

How much do the mascons on the moon affect surface gravity?

Upon some google-ing and wiki-ing I found this image of a gravity map of the moon:

That scale up the top is measured in milli-Gal which is thousandths of a cm/s^2. For scale gravity is ~9.81m/s^2 which equals ~981000mGal. The difference between gravity at sea level and the top of mount everest is 2Gal, or 2000mGal, which is 0.2% of average gravity. On the moon, the variation seems to be about 1000mGal, with the average being 162000mGal, which is about 0.6% of average gravity.

In conclusion, on the surface, there is no noticeable difference. Not to a human anyway.

star - Can we know the orbital planes of extraterrestrial planetary bodies?

I presume what you mean is how does the plane of the orbit compare to the equatorial rotation plane of the star?

The answer is, you can sort of estimate this, by using something called the Rossiter-McLaughlin effect (see also Rossiter 1924; McLaughlin 1924).

You can find plenty of information on the web - I'll add a couple of links when I have a moment - but to summarise:

The rotation of a star broadens its spectral absorption lines. The hemisphere coming towards us emits light that is is blue shifted, the hemisphere receding is redshifted. If we now take a transiting planet, during the eclipse it crosses the disc of the star and obscures regions that are blue or red shifted by various amounts.

Now what you do is measure the line-of-sight velocity of the star. During the transit you would not expect this to vary due to the "Doppler wobble" caused by the exoplanet, except that if the planet obscures a blue shifted portion of the stellar disc, the net spectral absorption line that remains shifts to the red, and vice-versa. The pattern of red, then blue shift (or vice versa) as the transit progresses is known as the Rossiter-Mclaughlin effect.

A schematic showing how the Rossiter-McLaughlin effect works and how a different transit geometry with respect to the rotation axis of the star leads to a different line-of-sight-velocity signature in the spectral lines of the star. (Image credit: Subaru Telescope, National Observatories of Japan.)

If the planet orbits in the same plane and in the same direction as the stellar rotation (as the planets in our solar system nearly do), then the blue shifted limb of the parent star is obscured first, followed by an equal amount of redshift as the planets moves to obscure the receding stellar limb (see image above, left). Thus the stellar absorption lines show a redshift followed by a symmetric blueshift. If the planet was retrograde it would be symmetric but occur in the opposite order. A Polar orbit would show no RM effect. Inclined orbits would have an asymmetric RM effect - ie perhaps more blueshift than redshift (see image above, right).

The RM effect cannot give an exact orientation, it gives the projected angle between orbital and rotation axes on the plane of the sky. Nevertheless, that is sufficient for us to know that a lot of the transiting exoplanets have highly misaligned orbital and stellar rotation axes.

gravity - Would time go by infinitely fast when crossing the event horizon of a black hole?

(I will assume a Schwarzschild black hole for simplicity, but much of the following is morally the same for other black holes.)

If you were to fall into a black hole, my understanding is that from your reference point, time would speed up (looking out to the rest of the universe), approaching infinity when approaching the event horizon.

In Schwarzschild coordinates,
$$mathrm{d}tau^2 = left(1-frac{2m}{r}right)mathrm{d}t^2 - left(1-frac{2m}{r}right)^{-1}mathrm{d}r^2 - r^2,mathrm{d}Omega^2text{,}$$
the gravitational redshift $sqrt{1-frac{2m}{r}}$ describes the time dilation of a stationary observer at a given Schwarzschild radial coordinate $r$, compared to a stationary observer at infinity. You can check this easily: plug in $mathrm{d}r = mathrm{d}Omega = 0$, the condition that neither the radial nor the angular coordinates are changing (i.e. stationary observer), and solve for $mathrm{d}tau/mathrm{d}t$.

The conclusion is that if you have the rocket power to hover arbitrarily close to the horizon, you will be able to see arbitrarily far into the history of the universe over your lifetime. However, that doesn't actually cover what happens to an observer that crosses the horizon. In that case, $mathrm{d}rnot=0$, and the coefficient of $mathrm{d}r^2$ above becomes undefined at the horizon: as in the other question, the Schwarzschild coordinate chart simply fails to cover the horizon and so is ill-suited for talking about situations cross the horizon.

But that's a fault of the coordinate chart, not of spacetime. There are other coordinate charts that are better adapted to questions like that. For example, the two Eddington-Finkelstein charts are best suited for incoming and outgoing light rays, respectively, and the Gullstrand-Painlevé chart is adapted to a freely falling observer starting from rest at infinity.

If this is correct, would you see the whole universe's future "life" flash before your eyes as you fall in, assuming you could somehow withstand the tremendous forces, and assuming black holes don't evaporate?

No. I think this is best seen from the Penrose diagram of Schwarzschild spacetime:

Light rays run diagonally. In blue is an example infalling trajectory, not necessarily freely falling. Note the two events where it crosses the horizon and where it reaches the singularity. Shown in red are inward light rays that intersect those events. Thus, the events that the infalling observer can see of the external universe consist of the region between those light rays and the horizon. The events occurring after that won't be seen because the the observer will have already reached the singularity by then.

Now suppose the observer tries a different trajectory after crossing the horizon, accelerating outward as much as possible in order to see more of the future history of the external universe. This will only work up to a point: the best the observer can do is hug the outgoing light ray (diagonally from lower-left to upper-right) as much as possible... but since the observer is not actually allowed to go at the speed of light, seeing all of the future of history will be impossible. The best the observer can do is to meet the singularity a bit more on the right of the diagram.

By the way, since the light ray worldlines have zero proper time, trying to do that will actually shorten the the observer's lifespan. If you're in a Schwarzschild black hole, you would live longer if you don't struggle to get out.

The above is for an eternal, non-evaporating black hole, as that's what you're asking about here. (The 'antihorizon' is there because the full Schwarzschild spacetime is actually an eternal black hole and its mirror image, a white hole in a mirror 'anti-verse', which not shown on this diagram. That's unphysical, but not relevant to the situation we're considering here.)

If it is correct that black holes evaporate due to Hawking radiation, would you be "transported" forward in time to where the black hole fully evaporates?

An evaporating black hole is morally the same as above: only an ideal light ray can reach the point when the black hole fully evaporates; everyone else gets the singularity. (Since this ideal light ray right along the horizon would be infinitely redshifted, arguably not even that.) You can repeat the above reasoning on its Penrose diagram yourself:

---

Addendum:

I have thought a bit about this, and does this solution take into account the relativistic time effects near the horizon of the black hole (e.g. is my understanding correct that the observer would observe time in the universe passing approaching infinitely fast when approaching the event horizon)?

How much time dilation happens depends entirely on what coordinates we're talking about (more generally, which frame field). What a given observer will actually see, however, is completely independent of choice of coordinates. In particular, Penrose diagrams illustrate the light cone structure of the given spacetime, and what an observer can in principle see depends entirely on what light rays intersect the observer's wordline. So yes, it's taken into account by default.

If you're actually falling in it, no, your understanding is mistaken, for reasons explained above. For additional motivation, flip the question around: what does the very distant stationary observer see of the infalling object? On the above Penrose diagram, outwardly directed light rays are diagonal, from lower-left to upper-right. Draw some some outward light rays from the blue infalling worldline. You will see that no matter how far into the far future (up on the diagram) you pick an event outside the black hole to be, you can connect that event with an outward light ray originating from the blue infalling worldline before it crosses the horizon. The conclusion would be that an observer that stays outside the black hole would be able to see the infalling object arbitrarily far into the future. No matter how much time passes for someone who stays out of the black hole, the image of the infalling object would still be visible as it was before it crossed the horizon. (In principle at least; in practice it will get too faint to see after a while.)

Thus, the usual result of "infinite gravitational time dilation makes the image of the infalling object hover forever near the horizon" is also straightforwardly deducible from the diagram, and so is completely consistent with the infalling object being able to see a finite part into the future of the external universe. Perhaps it is best to emphasize that the situation is not actually symmetric: what the external observer sees of the infalling object is not some straightforward flip-around of what the infalling object sees of the external universe. The black hole itself breaks that symmetry.

the sun - Would dark energy save the earth for a while as the sun heats up?

I don't think this is a bad question, and I think Pela is essentially correct, the effect is very small.

Dark energy doesn't expand objects, for example, cause those are bound together, but dark energy should expand space between the sun and the earth though the effect is tiny.

If we take this number, 68 KM per second per 1 million light years (link below), that's 245,000 KM per hour per million light years or about 1 KM every 4 hours per lightyear. With 63,000 Astronomical units per light year, between the sun and the earth, we can expect the earth (assuming it's in a perfectly stable orbit around the sun - which it's not, but lets say it is), a velocity of 1KM/30 years. And that's over the age of the universe. The earth/sun is only 1/3rd the age of the universe, so dark energy has accelerated the earth maybe 1 KM/90 years velocity since the formation of the solar system. That's a pretty small push. A velocity change of a bit a meter per year over the age of the solar-system. The effect of the solar wind and the Jupiter and Venus orbital tugging are probably greater than that.

Source: http://www.space.com/26279-universe-expansion-measurement-quasars-boss.html

Now, over a few billion years, a small velocity change of 1 meter per year would add up - so I think dark energy is part of the solar system's long term orbital calculation. I might be wrong on that, but I think it has a measurable effect over billions of years even in distances as small as planetary orbits.

But would it save the earth - I doubt it. The sun's heating up accelerates over time. It will at some point in the future, move too fast and get too hot for the earth, so, we'll have to save ourselves, either by moving the earth or terraforming mars, or building space colonies or some combination of all 3. The good news, we have tens of millions, maybe hundreds of millions of years to figure that stuff out.

Dark energy has been useful though. It's probably kept us from crashing into Andromeda and/or the great attractor - however far away that is. Certainly delayed those collisions (We're never going to crash into the great attractor, I gather, thanks to dark energy), and crashing into Andromeda, when it happens, might harmless anyway, but I'm just kind of thinking out-loud.

I do think it's a good question as there probably is some small expansion due to dark energy between the earth and the sun. If my numbers are wrong, I welcome correction.

human biology - Why do mammalian red blood cells lack a nucleus?

just to add on to the previous reply...

Shown to be in mice & rats (and sick humans), the cell-cell interaction between a macrophage (this is a big engulfing cell required for immunity) and young red blood cells (RBC), is known as the erythroblastic island (commonly known as EBI). If u googled it, there is a scientific review in 2008 that describes this structure.

At the embryonic stage (in humans), we still retain our RBC nuclei. But as we developed into fetus and adult, we no longer have RBC nuclei. This is thought to be related to the EBI present (in the fetal liver and adult bone marrow respectively). Currently, there is a lack of information of the EBI in other mammals. The only proven ones are mice, rats and sick humans. It is widely assumed (not proven) that mammals have EBIs. Besides mammals, some other animals (e.g. birds) have enucleated RBC, and some dont. It is unclear why is it so. Our lab thinks that it might be related to the formation of the EBI.

In addition to engulfing the RBC nuclei, it is believed that the macrophage acts as a "nurse" cell as proposed in the 50s. In other words, possibly providing iron, and possibly providing some proteins required for young RBC to mature. In early 2013, for the first time, it was showed that these macrophages are important in animal models(published by 2 research groups in nature medicine journal).

As for enucleation (the removal of erythroid nuclei), the exact mechanisms are unknown. But cytoskeleton proteins are important players in enucleation. However, there isnt enough information, as these proteins are essential for other important cellular activities as well. For example, bringing in nutrients, development and cellular migration. Most animal models that lack these proteins are unavailable for studies, and these animals usually die at the embryonic stage.

The research mentioned by EdoDodo is a proposed model on how enucleation takes place, and is a widely accepted model. Currently, our lab are working on another model that could partially explain how enucleation is being triggered.

Advantages of enucleation:
In addition to better oxygen diffusion across the membranes, some older scientific papers mentioned that it lightened the cardiac workload. Each extruded RBC nuclei is approximately 40 picograms. A normal healthy adult individual would produce about 2 million RBC per second. That would be 0.08 milligrams of weight per second are required to be removed. However, I couldnt trace the scientific evidence for this claim, but this have been cited by some scientific papers.

The other advantage would be to reduce risk of hemolysis when transversing through the microvasculature. In other words, mature RBC can move along tiny blood capillaries by changing their biconcave shape (to bell-shaped I think), so that they will not rupture (and die).

I hope this helps.. :)

P.S. I forgot to add that, not all RBCs have similar shapes and sizes. You might want to google it for more information. I think camels have slightly different RBC morphology.