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.

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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.