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.

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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!).