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:

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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.

Can impact craters on the moon act like giant radio telescopes?

Could large craters on the moon be used as reflective lenses for radio
signals?

You'd have to line the surface with something reflective to microwaves, like a metallic mesh, or similar materials.

Secondly, the shape of the crater is probably not quite ideal, so it would have to be adjusted a little, carved up a bit in various places. But it's a good start, and definitely better than starting with a flat ground.

There is also the question of stability - you need to make sure that whatever changes you make (carving a different shape, lining it with mesh) do not affect the stability of the crater, or else various parts may slide or collapse. This is an engineering problem.

Acting like a large radio telescope reflecting radio waves to a
satellite positioned over the crater.

Not possible unless the crater is exactly on the equator, and even then it would be tricky.

But a crater like the one in your picture is so strongly curved, the focal length is about the same as the diameter. In other words, if the diameter of the hole is X, the altitude of the receiver is pretty close to X - give or take something like 50% or so, depending on the exact curvature. It might be easier to just build a giant arch over the crater. Again, this is a matter of engineering.