Why nuclear fusion is a controlled process in stars?

Because in most stars, the pressure where nuclear burning takes place is proportional to temperature. In order for the nuclear reaction rate to increase, the temperature and hence pressure must increase. This would cause the gas to push out the layers above it, then to expand and cool.

Conversely, if the nuclear reaction rate fell, so would the temperature and pressure. The smaller pressure would allow the outer layers to move inwards, compressing the gas, heating it up and speeding up the nuclear reactions.

In this way the nuclear reaction rate (for hydrogen burning) can be kept almost constant. The small changes (increase) that occur during a star's main sequence life are driven by the increasing mean atomic mass as hydrogen turns to helium. This means you need a gradually higher temperature to maintain the same pressure.

Where the temperature and pressure are decoupled - for instance in the degenerate helium core of a low(ish) mass star near the red giant tip, where electron degeneracy pressure is independent of temperature, then runaway nuclear reactions can be initiated - the "helium flash".

immunology - What's the advantage of autocrine signalling?

In the antibody-mediated immune response, when the helper T cell gets activated by the costimulus (IL-2 and TNF-α secreted by the APC) which in turn produces IL-2, IL-2 acts in an autocrine manner. I'm just wondering why does IL-2 have to be secreted? Why doesn't it just exert an affect while it's already inside the helper T cell? What's the point of autocrine signalling?

I hope the answer isn't going to be, "Well, that's just the way it is..." because paracrine and endocrine make sense and have advantages, but autocrine just seems a bit extra.

How do astronomers detect gases that are in the atmosphere of exoplanets?

Exoplanets are planets that are located outside our solar system - whether that be orbiting a star or drifting past one. Now, the closest star to us is Alpha Centauri which is just over four light-years away. So, how can astronomers detect gases in the atmospheres of planets over this distance with any degree of accuracy (or at all)?

black hole - Explanation for the first spinning neutron star detected in the Andromeda galaxy?

A new class of Neutron stars has been found in the Andromeda galaxy (M31), This is the first time a Spinning Neutron star has ever been detected by astronomers in M31. The article states-

“pulsars” can be found in stellar couples, with the neutron star cannibalizing its neighbor. This can lead to the neutron star spinning faster and to pulses of high-energy X-rays from hot gas being funneled down magnetic fields onto the neutron star.

“It could be what we call a ‘peculiar low-mass X-ray binary pulsar’ — in which the companion star is less massive than our Sun — or alternatively an intermediate-mass binary system with a companion of about two solar masses,” said Paolo Esposito of INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan, Italy.

While the precise nature of the system remains unclear, the data imply that it is unusual and exotic.
http://www.astronomy.com/news/2016/04/andromedas-first-spinning-neutron-star-has-been-found

Question-
Could the companion of this Spinning Neutron Star not be a star at all but a Black Hole? This might explain the unusual spin and that all neutron stars do not consume other binary companions stars but rather a Black Hole is in the process of consuming the neutron stars.

The Black Hole can devour all of the outer energy of any active star leaving only the dense core of heavy metals that will eventually cool off entirely creating the rouge brown dwarf.
Supernovae never derived from dying neutron stars but rather from Binary star Collisions.

universe - If I were to point into the sky, how many galaxies would be in this line?

On average, you will point at one galaxy.

The argument goes as follows:

Looking at the spectrum of quasars, which are effectively point sources, and which lie at distances of the order of the size of the observable Universe, it turns out that typically, roughly one damped Lyman $alpha$ system (DLA) is detected. DLAs are huge reservoirs of neutral gas, which are early stages of galaxy formation.

At least in a handful of cases, the galaxy counterpart of the DLA has been confirmed. These observation are very challenging, since they are a faint sources at very small projected distances from the very bright quasar. Here's the setup:

So, accepting a DLA as a galaxy, I'd say that you will typically point at one galaxy.

How does gravity affect spacetime

In fact time always "moves" at the same speed to any given observer. It appears to move at different speeds to an external observer, but you cannot experience slowed time as such - except in the sense that you might observe external clocks ran faster than the watch you had your wrist.

An attempt to explain: The supposition - very strongly supported by the experimental evidence (the latest discovery of gravity waves being the jewel in the crown) is that we live in a four dimensional spacetime. An individual always experiences time as "moving" at a constant speed, but as has been remarked by others, externally gravity appears to add curvature to the "straight line" we are moving along on the time dimension and so we "gain distance" in this direction more slowly when seen externally.

Has Science observed and recorded a stars birth?

The star formation process from giant molecular cloud to unobscured protostar is thought to take about a million years.

So the answer is no.

Similarly, there are very few large scale physical processes that occur in the universe on a human timescale. Nevertheless we are sophisticated enough to understand that you do not necessarily have to see something happening to know that it has occurred and work out how it happened.

Or are you asking whether all the separate phases of the star formation process have been observed in different places? The answer to that is broadly yes. The rarest (shortest) phases is the initial collapse to a "core" that is embedded within an obscuring molecular cloud. Nevertheless, such objects can be seen at sub-mm and radio wavelengths.

The moon: how to determine its size, speed and distance from earth using a simple telescope?

Finding the distance is tricky. To find the distance you need to carefully note the position of the moon (relative to the stars ) from two different places, *at the same time *. The moon will appear to be in a slightly different position from the two viewpoints (an effect called paralax). That done, finding the distance, and size is a simple exercise in trigonometry, and Khan academy has a page which shows hoebto find the distance to the moon.

Greater accuracy could be obtained by measuring the time that a star is hidden by the moon, from two locations, and using that to determine the position of the moon to greater accuracy than possible by direct measurements. Hipparchus apparently used a solar eclipse to obtain two positions atbthe same time needed for calculation of paralax.

Knowing the distance makes finding the diameter and speed simple to do, by observing the angular size, and using a little trig. Working with photographs takenby the telescope is convenient but not strictly necessary.

redshift - Why are there so many seemingly blue-shifted galaxies in deep space

You cannot gauge the redshift of a galaxy by looking at a false colour image. The images taken through different filters are stacked and colourised to suit. You can say that the blue galaxies are indeed bluer than the red galaxies, but there is no absolute scale with which to judge redshift by eye.

Secondly, there is no detail in the NASA web page, but the ACS and WFC3 cameras have near infrared capabilities. So I would think that this image is a visual false-colour image of information that extends well redward of what the eye can perceive. So even the things that look blue might have a spectrum that peaks at redder wavelengths, whilst anything that looks red might actually be infrared!

However, beyond this, in order to judge what the redshifted appearance of a galaxy would be, you would need to know what the galaxy looked like with zero redshift. That is, light that is emitted in the ultraviolet could be redshifted into the visible part of the spectrum. It is entirely possible that visual colour of a redshifted galaxy would not change very much at all if the galaxy emitted lots of UV light in its frame of reference or equally, if it emitted lots of very red light that was then redshifted out of the telescope's sensitivity range.

Finally, there could be some genuine astrophysics going on. Many distant galaxies are bluer than nearby galaxies because they are undergoing intense star formation. Massive star forming regions emit copious UV light that is redshifted into the optical.

Finally, finally! Many of the galaxies in the picture will be quite close and will not be very redshifted.

human biology - Why are some menstrual cycles irregular?

There's two phases of a menstrual cycle, before ovulation known as the follicular phase and after ovulation known as the luteal phase. The second phase is not very variable, it's the same length almost always as it is governed by how long the corpus luteum (remnants of the follicles after the ovum bursts out) survives. The first part is variable and is what causes periods to be irregular. In this phase the eggs mature. The body picks several eggs to mature but only one will "win". However, how fast it wins changes. It all depends on how fast it grows and much it manages to suppress growth of the other eggs and that in turn depends on the quality of the egg. But then this also branches out, as it is controlled by levels of hormones this is heavily affected by stress and diet among other things.

It's unfortunate that the first phase is variable otherwise knowing which day you were going to ovulate would be easy.

orbit - Is it possible that the Sun has a binary partner (the Nemesis Theory) that has eluded detection?

The first part of your question has been asked before: Is Sun a part of a binary system? and the current (lack of) evidence for such a companion is discussed on the relevant wikipedia page about "Nemesis".

To summarise: if it were a small companion star, or even a brown dwarf that had been cooling for 4.5 billion years and it had a 26 million year orbit, then Kepler's third law tells us how far away that object would be. It turns out that this is not far enough away that the object would have eluded detection in all-sky surveys. The recent WISE survey in the near-infrared should have been capable of detecting even a very cool brown dwarf (and indeed it has detected some very cool brown dwarfs, just not that close to the Sun - e.g. a 250K brown dwarf only 6 light years away Luhman et al. 2014) that was close enough to the Sun to be a Nemesis candidate.

Since the details do not appear to be on the wikipedia page, I'll fill some in. If we take a very low mass brown dwarf - say 20 Jupiter masses - in a 26 million year orbit, then Kepler's 3rd law tells us this will be about 90,000 au (1.4 light years) from the Sun (assuming a circular orbit). According to the evolutionary models of Saumon & Marley (2008), such an object has an intrinsic luminosity of $10^{-7}$ times (1 ten millionth) that of the Sun and a temperature of 400 Kelvin and would appear to have a spectral type of late T or early Y.

From the calibration of absolute magnitudes versus spectral type for cool brown dwarfs in Marsh et al. (2013) we know that at 90,000 au, such a brown dwarf would have magnitudes of $H=14$ and $W2=8$. The former is bright enough to have been seen in the 2MASS all-sky survey and the latter is easily detected by WISE. The combination of data would also easily have revealed the large parallax of such an object. We can conclude that an object would have to have much lower mass than this to remain undetected.

Now the middle part of your question: The star will "wobble" in response to its companion with exactly the same period as the orbit. So, if you are prepared to wait for an appreciable fraction of 26 million years, then yes, the presence of the binary companion might be revealed! Otherwise not. Measuring the "wobble" by doppler methods is currently capable of detecting Jupiter-mass objects in orbital periods of 10-20 years. Measuring the wobble "astrometrically" - that is measuring the position displacement of the star due to an unseen companion is more sensitive to distant companions, but still, one is limited by the fact that the wobble period will be the same as the orbital period of the companion. An assessment of the performance of the Gaia astrometry satellite by Perryman et al. (2014) suggests detection of planets with orbital periods out to 10 years might be possible.

For the final part of the question - yes, the motion/wobble of a star can be decomposed (using Fourier techniques) into components that are due to each planet. There are numerous examples of multiple planetary systems that have been discovered by the radial velocity technique in this manner. Similar techniques can and will be employed to analyse any astrometric wobble. An interesting plot is to show the motion of the Sun in the plane of our solar system, in the rest frame of the solar system centre of mass. The Sun executes a complicated trajectory in this plot due primarily to the influence of Jupiter (on an 11 year orbit), but with superimposed "epicycles" due to the influence of the smaller planets on different orbital periods. See below.

Motion of the Sun around the solar system centre of mass (from http://commons.wikimedia.org/wiki/File:Solar_system_barycenter.svg )

neuroscience - Are CN3, CN7, CN9 and CN10 the only Parasympathetic Cranial Nerves?

It has been my thought for a long time that this is the case, but I am unsure currently, since the parasympathetic tract of colon sigmoideum does not seem to have connection with CN 10. It connects with pelvic splanchnic nerves.
Discussion about it here.

If there is no connection from pelvic splanchnic nerves to CN 10, like CN 10 running within the nerves, then my though must be wrong.

Are CN3, CN7, CN9 and CN10 the only Parasympathetic Cranial Nerves?

immunology - Do antigens protrude through the capsule/slime layer in prokaryotic organisms where these features are present?

They don't have to -- there are times in life of a bacteria (cell division, hunger, mutations, attacks of lysozyme and other enzymes, cell lysis) when any possible antigen gets less or more exposed.

About escaping phagocitosis, there are numerous strategies to achieve it -- from forming a large slime-covered colony, through killing or disabling phagocytes up to stopping digestion, escaping the phagocyte back to the environment or even living inside it (Listeria monocytogenes is a prime example -- it can even directly move from one cell to another).
EDIT: Here is a nice overview.

white dwarf - How can a low-mass star increase its mass to 1.4 Msun?

You were taught wrong. Stars of up to about 8 solar masses will end up as white dwarfs. But it is only their cores that become degenerate and end up as the white dwarf. The rest of the envelope is lost during the giant phase due to a dense wind.

There is a non-linear, but probably monotonic, relation between the initial progenitor and final white dwarf masses (see below, from Kalirai 2013) - the Sun will likely end as a 0.5 solar mass white dwarf, but in normal stellar evolution, degenerate white dwarf stars can only be produced up to about 1.25 solar masses by the most massive progenitors. Any more massive than this and it is likely that the core does not become degenerate before igniting and burning through the heavier nuclear fuels. The most massive, probably single, white dwarf known is "WD 33" in the cluster NGC 2099 and has a mass of $1.28^{+0.05}_{-0.08} M_{odot}$, is likely made of an O/Ne mixture, and had an estimated progenitor mass on the main sequence of $>3.5 M_{odot}$ (Cummings et al. (2016).

In order to get a more massive white dwarf, up to the Chandrasekhar mass (about 1.38 solar masses for a C/O or O/Ne white dwarf in general relativity), it almost certainly needs to accrete mass from a close binary companion or be the result of some sort of merger.

This is the leading candidate to explain type Ia supernova.

jupiter - Do great spot like features favour appear in south hemisphere?

Given that we have very few examples, we could not, statistically, draw any significance from any apparent bias. It's also hard to see any theoretical basis for expecting a bias, so my gut feeling would be there probably is no bias.

It's also worth remembering that human observations of these things are essentially a very brief snapshot in a huge period during which we have no idea about what may have been happening. So again the statistical significance of what happens at the moment needs to be carefully considered.

Chances of man-made satellite colliding with space debris

Firstly, has a man-made satellite, or indeed any spacecraft, ever collided with space debris and been destroyed? What are the chances of this happening? (I imagine very slim, but how slim?) And do we check that there is no space debris where we are going to send our spacecraft before we launch it?

telescope - What visual artifacts are expected from the JWST?

Are you asking about the PSF (point-spread function)?

There are some simulations at http://www.stsci.edu/jwst/software/webbpsf ; there are some basic images available at that site as well as a downloadable package you can use to compute the PSF for a particular instrument and wavelength. Since the telescope hasn't been fully assembled yet these are based on simulations.

zoology - Why is blood pressure higher the more distal an artery is?

The cardiovascular system is affected by three types of pressure. These are:

• Heamodynamic - caused by the contraction of the heart, which would give the view of pressure being higher closer to the aorta as in Kevin's answer and my first thought too.


• Kinetic - caused by the action of skeletal muscles in movement in squeezing primarily veins to return blood to the heart.


• Hydrostatic - the combination of fluid density and gravity that leads to pressure on the vessel endothelium. The pressure at a given point is proportional to the volume of fluid above it. This means that pressure is highest at the bottom of the vessel.

Arteries can of course be treated as one long tube due to their lack of valves, therefore the hydrostatic pressure could be significant in contributing to total blood pressure. If hydrostatic pressure is more significant than heamodynamic pressure then this would explain why pressure would be higher the more distal the artery is - when standing there is simply more blood pushing down on it.

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Critical Care Nurse. 2002;22: 60-79

interstellar medium - Population of excited H levels in a Strömgren Sphere

In chapter 2.2 of Astrophysics of Gaseous Nebulae and AGN Ostriker and Ferland claim that, as far as ionization is concerned, one can assume all atoms to be in the ground state in a Strömgren Sphere setting (static, homogeneous $napprox 10/mathrm{cm}^3$, isothermal $Tapprox 10^4,mathrm{K}$ pure hydrogen cloud around a single star). While I do not doubt the validity of this assumption, I have some trouble with how they justify it.

They estimate the inverse ionization rate for ground state hydrogen at a typical distance from the star, which is of course the lifetime against ionization from the ground state $tau_mathrm{ion}^{1^2S}$. Then they say that the estimate also holds for ionizations from excited levels and they find

$$tau_mathrm{ion}ggtau$$

where $tau_mathrm{ion}$ and $tau$ are the typical lifetime against excitation and the typical lifetime of excited states respectively.
So far so good. In the next step, however, they end their chain of argumentation by saying that consequently any atom in an excited state will have enough time to decay to the ground state before it can be ionized, so all ionizations are from ground state. That is my understanding at least.

My problem with this is the following: suppose I have some mechanism that keeps a good fraction of my atoms in an excited state. In that case, even if $tau_mathrm{ion}ggtau$, I will have ionizations from this excited level simply because it is always populated. So what one should really look at in my opinion is the lifetime of an atom against excitation.

My first question is: Am I missing something or did I get something wrong?

And secondly: If my argumentation makes sense, one has to compare the lifetime against excitation with $tau$, right? Where can I find the corresponding cross sections for radiative and collisional excitation? I was only able to find ionization cross sections.

Which is the most accurate stars catalogue for J2000 epoch?

I'm developing a planetarium software. I have no idea about how to obtain stars right ascension and declination.

Searching on Internet I have found that

• Wikipedia has AR and DEC for a lot of stars. But RA and DEC on Wikipedia that the data shown in Stellarium program.


• Tycho 2 catalogue also has a lot stars, but it doesn't have stars
  like Rigel. There are two files, suppl_1.dat and suppl_2.dat with another stars like Rigel not found on main file catalog.dat.


• Stellarium stars catalogue.


• Hipparcos catalogue?

My problem here is that I don't know which one has the most accurate values for RA and DEC.

Which one do you recommend me?

I have found this page, Recommend stars catalog, with a lot of catalogues.

observation - Stellarium 0.10.4: planet orbits change over time?

Edit (I looked, unfortunately most of the sources are astrology, not astronomy), but Mars last went in Retrograde March 2014 and it's not due to go again till May 2016 (every 2 years, 2 months). My guess below was incorrect (though it's kind of cool, I'll leave the picture up).

Mars does have a tilted orbit compared to earth and that could be the reason. That and the angle from the earth changes too.
Source

Mars, from our point of view, slows down, temporarily reverses direction then returns to it's original direction and continues on. The entire "S" motion takes maybe 3-4 months and your 2 photos are 24 days apart. (but this won't happen till May 2016)

Source of photo and explanation here:

gene regulation - Modularity of transcription factors

The NF-κB family of transcription factors is very modular, with different combinations having different effects. The active (nuclear, DNA-bound) TF is a dimer, composed variously of RelA/p65, RelB, c-rel, NFKB1/p50, and/or NFKB2/p52 subunits. For example, the "canonical" p65/p50 dimer is activated in response to stimulants like TNF-α (tumor necrosis factor alpha, released in response to inflammatory signals like the presence of pathogens) and LPS (lipopolysaccharide from the cell walls of Gram-negative bacteria), while the RelB/p52 dimer plays an important role in the development of B cells (the immune system component that produces antibodies). The AP-1 transcription factor is also heterodimeric, containing proteins from the Jun, Fos, JDP, and ATF families, and there are numerous other examples of multimeric transcription factor complexes. This recent article in Nature Immunology (disclaimer: I was not involved in that research, but my old lab did similar work) shows DNA sequence-specific binding by different NF-κB dimers.

Polymerism in general, and dimerism in particular, are quite common modes of transcriptional activation and regulation. The large number of ways in which a relatively small number of transcription factors can be combined allows for the exquisite control of genes, responding to a huge variety of cellular situations. Unfortunately, it also means that mutations in key, common components can result in transformation, unrestricted growth, and the generation of tumors.

the sun - Does the Sun rotate?

Yes, the Sun rotates. This can be observed by tracking a variety of features on the Sun, such as sunspots, X-ray brightpoints, coronal holes, filaments, and small magnetic flux elements. Another way to determine the rotational speed of the Sun is to measure spectral lines at the edge of the Sun's disk and determine their redshift.

It is thought that the rotation of the Sun is due to the way the primordial gas cloud collapsed in on itself to create the Sun. Also, it is likely that the Sun originally rotated much faster when it initially formed, than it does to day. This slow-down was probably caused by 'magnetic breaking' in which strong magnetic fields threading our primordial Sun out into the solar wind resisted the rotation.

Today, the reason that the Sun rotates at a different angular speed at different solar latitudes is due to hydromagnetic effects. One cause is thought to be the nature of convection within the outer third of the Sun. Interestingly, below a the convective envelope, beneath a boundary called the tachocline, the Sun rotates as a rigid body. You can find more about this in Schou et al., 1998

Globular cluster star density as a function of distance from the center

The usual thing is a King model.

There are indeed free parameters. These are the central density, the "core radius" and a tidal truncation radius.

The background and rationale for these models is given in the link. They provide a pretty good representation of the surface density of globular clusters (or indeed open clusters). They require a numerical scheme to "deproject" from the plane of the sky to 3D.

If you find the nitty-gritty of Abel integrals too tricky in the deprojection, then you could always approximate with a Plummer model. This is analytically and computationally easier to deal with, but lacks a bit of physical realism. Central density and a characteristic radius are free parameters here.

biochemistry - How can I produce milligram quantities of an isotope-labeled DNA oligomer?

What about PCR using labeled nucleotides? Might have to run several reactions to get miligram quantities you need, but 35 nucleotides seems really really small for growth in bacteria, and purification would be extremely difficult. But 35 bases might even be too small for PCR, hardly bigger than your primers.

If you do need to grow in e coli, it might make sense to create a plasmid with multiple repeats of the 35 base sequence separated by the same restriction site. You can grow e coli in c13 n15 media, but it's expensive and you have "train" the cells over several generations because the heavier isotopes create differences in chemical kinetics that make the labeled nutrients hard for enzymes to use, but I've seen people do this for making proteins for NMR.

I bet you need this labeled DNA for NMR, don't you?

Have you considered phosphate NMR? No need to label it, P-31 is already NMR active.
If it must be C13 N15, I highly recommend having it synthesized, it will be expensive, but that 35 base length and the requirement for single strandedness are ideal for chemical synthesis.

observation - How many stars are there in a Globular Cluster of 10^5 solar masses?

I was wondering whether there is an easy way to approximate the number of stars in a Globular Cluster (GC) with 10^5 solar masses.

Can one, for instance just assume the GC is made of sun-like stars and therefore has 10^5 stars?
Or is this too simple?

Thank you for your answers!

microbiology - Why is there now only one Salmonella species?

Once upon a time, I chanced upon an old microbiology book that detailed the rather colorful world of enterobacteria. Salmonella in particular stood out, as it seemed there were a lot of species: typhi / typhosa, paratyphi, gallinarum, typhimurium, choleraesuis, and quite a bunch of others that I have now forgotten.

Flipping through a newer book, it now seems that all of these "species" were collated under choleraesuis (and now more recently enterica), with all those species being demoted to "strains" (or maybe I should use the current term of art, "serovar"). Unfortunately, the book didn't give much in the way of explaining about this merger.

So, why is there now only S. enterica? If "S. typhi" is a mere serovar, how come the species name is still used in the literature?

x ray - Stratospheric Balloon with an Xray camera

Long ago I was part of a group that flew high altitude balloons to study x-rays from pulsars etc. How much you detect and at what energy depends upon how much atmosphere is above your detector. Lower energy x-rays are absorbed more than higher energy x-rays. Realistically you won't detect anything except maybe the Sun at energies below 20 keV and at altitude below 25km. We aimed for altitudes near 40km. To detect high energy x-rays the cheapest method is to use a proportional counter. Not something you can buy off-the-shelf.

Also the Sun only emits a lot of high energy x-rays during solar flares.

An interesting experiment would be to fly a geiger counter see http://earthtosky.net/

solar sytem - Can a drastic change in a stars cycle cause harm or affect a nearby star system?

I'm not sure if this is a misunderstanding or just a matter of scientific terminology, but the "star cycle" is not something you can "drastically change".

The life cycle of a star (as we currently understand it) is a fairly predictable set of stages of evolution from the formation of a star from a cloud of gas, along the main sequence (the stable stage out own star is currently in) and then to "death" of the star as it's internal fuel runs out, and material can be released back to the interstellar medium.

For more details see the Wikipedia page, this video from NASA, or this very simplified diagram from the BBC.

But to answer the underlying question, stars at different stages of their life cycle can definitely affect other stars, planetary systems, or interstellar matter that is nearby in many ways. Here are a few examples:

• When young stars are born they may be quite close together and their mutual gravitational forces will disrupt circumstellar disks (where planets form), which may inhibit the formation of planets, but the specifics of this are debated in the scientific community.(see this paper)


• At the end of their life cycle, more massive stars may explode in a supernova, which releases lots of energy and throws matter out. This can change the structure of clouds of gas and can compress it, possibly triggering new stars to form in the more dense material. This is also a topic of current study so the exact details and mechanisms are not very well understood, but the theory is logical and observations have been found that seem to confirm this.(see this paper)


• A white dwarf star and a red giant star in a binary system can have transfer of mass from the red giant to the white dwarf. A white dwarf has a maximum stable mass (Chandrasekhar limit) so when the mass exceeds this, the white dwarf will go supernova - this is , as you stated, a drastic change in one star due to interaction with another star.

What can the timing of human urination tell about the human's physical condition and circadian rhythms?

I've noticed a peculiar phenomenon. A subject drinks 400 ml of water, then observes time until the urge to urinate is felt. The time is 15 minutes. The subject releases water. 14 minutes later another urge to urinate is felt. The subject releases water again.

I'm particularly interested in what kinds of biological systems are involved in timing of such events.
Does the time depend on how full the subject's stomach is? Does caffeine and other diuretics play a part?
Is it time of day (circadian rhythm) sensitive?
Does that predict anything about the suppression/release of diuretic hormones?

What I'm trying to understand is if the timing between human urges to urinate after water consumption can be used to make predictions about the human biological clock and the state of various systems within the body (for example the digestive system).

I will be conducting this experiment at different times of the day. My hypothesis is that at night, when diuretic hormones are suppressed, the timing would be longer for the same amount of water consumed. This is based on my limited research in the area.
Update: I did perform the same experiment at night, the time was 75 minutes for the same amount of water. The experiment was performed at the end of one of sleep cycles, which makes me think that 75 minutes was the duration of the subsequent sleep cycle.

I appreciate your input on the subject, along with any keywords that can help me advance my research in this area.
Thank you!

astrophysics - Mass limit of planetary ring

A couple of points on this. While faint rings can exist outside the Roche Limit (like Saturn's G and E rings) - photo is too big to link but click here. rings of any significant mass are only going to reside inside the Roche Limit, so the practical limit of the mass of a planet's ring is about the mass of the largest moon a planet is likely to have, roughly speaking.

A ring around a planet can form in a couple of ways, by giant impact, by a moon passing inside the Roche limit and breaking apart or by accretion, like how Enceladus feeds Saturn's faint e-ring.

Also, rings don't have to be particularly massive to be impressive. The mass of Saturn's rings estimated here (and there's some uncertainty to that estimate, but close enough for our purposes)

Based on Voyager observations, the total mass of the rings was
estimated to be about 3 x 10^19 kg. This is a small fraction of the
total mass of Saturn (about 50 ppb) and is just a little less than the
moon Mimas

So the mass of Saturn's rings is the mass of a pretty small moon, a little under 200 km in radius, but that wasn't your question, so, moving on.

A planet with rings 200 times the size of Saturn (40,000 times the surface area) was observed and if we assume the same density as Saturn's rings (which is a very bad assumption to make, but this is just an approximation), 1.2 x 10^24, or about twice the mass of Mars. Now we're getting somewhere, but there's a few points to make on our friend J1407B

First, it may not be a planet at all. It's mass estimate is 10-40 jupiters which suggests it's in the brown dwarf category. 2nd, it's a young solar system, just 16 million years old, so the ring system may still be in the process of forming a moon system and not be a permanent ring at all. Source. Also, same article, it mentions that parts of the ring system block 95% of the sun's light, so it's a far denser system than Saturn and there's a moon in there thought to be 0.8 times the mass of earth, but I still don't think that should counts since it may be an accretion disk remaining from the formation of that solar-system.

If we set the planetary mass limit to be about 13 Jupiter masses, which is roughly the cut-off between heavy Jupiter and brown dwarf, and we take some estimates of the size of moons to planets, during formation, it's unlikely that moons can be more than maybe a ratio of one to several hundred, due to limitations in angular momentum and likely size of formation. In our solar system, the largest moon to planet ratio, not counting giant impacts or captures is about 1 to 4,200 (Saturn to Titan). Jupiter to Ganymede is 1 to about 12,800 and Tritan to Neptune (and Tritan may be a captured moon), about 1 to 4,800. If we assume a heavy Jupiter of about 13 Jupiter masses and a moon in a decaying orbit that breaks up with a mass ratio of 1 to 400 - and needless to say, that's very ballpark, then this theoretical estimate works out to a practical limit around a heavy jupiter to have a moon system of roughly 10 earth masses. (1/400th the mass of a 13 jupiter mass planet).

Now, there are creative ways to increase it, like, lets say there are two moons, perhaps in Trojan points to each other, and they both spiral in gradually. Or, lets say a passing object of greater mass flies past, but the problem with a passing object is that, due to escape velocity speed, such an object, if it passes within the Roche Limit, it would have a highly elliptical orbit, which wouldn't lend itself to a ring system, since the momentum of the elliptical orbit would only pass through and not stay inside the Roche sphere. What you need for a good sized ring system to form is a slowly decaying circular orbit, not a recent capture of a large passing object.

See Shoemaker Levy 9 which passed through but quickly went back outside Jupiter's Roche limit.

Now if we imagine a scenario of a double planet, which is probably a rare scenario but not impossible, and over time due to proximity and tidal forces, the two planets form a decaying circular orbit around each other, in this theoretical, lets say we have two heavy Jupiter orbiting each other, one of 12 Jupiter masses and the other of 6 Jupiter masses (purely theoretical) and the 6 Jupiter mass planet begins to break up as it enters the Roche limit of the 12 Jupiter mass one. There's 2 problems with assuming you'd get a ring with half the mass of the planet. One is, hydrogen and helium gas are likely at least 80% of the mass of the planet that's breaking apart, perhaps 90% and hydrogen and helium don't easily bind into ice particles, so over time, much of that gas is likely to get blown away by the solar wind. You're left with a rocky and icy mass of quite a bit less than 6 Jupiter masses, probably less than 1. The other problem, with an orbiting object that large, it doesn't break apart all at once, but it's denser core stays together and spirals closer to the planet while the lighter ices around the surface which do begin to form the ring, can get spun off and flung either into the planet or flung outside the Roche limit, as the denser core of the smaller of the two planet moved slowly closer, there would be a vacuum cleaner effect wrecking havoc on the very same ring that planet was forming. For smaller moons, this effect can be seen around saturn as tiny moons create tiny breaks in Saturns' rings. With a much more massive core of the planet that was forming a moon, you'd get orbital chaos and it wouldn't be a good scenario to form a ring, even with all that available ring material. You'd still probably end up with a ring, but I think only a small percentage of the original smaller planet's mass would survive the process.

A similar problem arises with giant impacts. If the impact is too large, the planet breaks apart. If it's not quite that large, like the giant impact on earth was thought to be, 4.4 billion years ago, then debris is blown outside the Roche Limit and forms a moon, in our case 1/81st the mass of the earth, though at the time it might have been a bit more, maybe 1/72-74 or so, and there were probably two moons initially not one, but I digress. The point is, to have debris blown off the planet but staying inside the Roche limit so it forms a ring, not a moon, you need a smaller impact than the one that formed our moon, and a smaller impact implies less debris, so with Rocky worlds, what does that mean, 1/100th? 1/200th? In there somewhere. With gas giants the impact method is even worse due to the outer layer of gas giants being so much hydrogen that you don't get good material for a dense permanent ring structure from giant impact.

So, there are practical limits to the size of a ring system you're likely to see (if we ignore young solar-systems still information like J1407b. It's a very ballpark just barely educated guess, but I think the practical limit for a ring system is probably pretty tiny compared to the mass of the planet, like a ratio of 1 to 200 or so, which if you have a planet the mass of 13 Jupiters, that's still quite a bit of mass. If we use the 1 to 200 ratio and 13 jupiters, that's the mass of 20 earths. I have a very hard time seeing how a ring system around a planet could get much more massive than that.

Now, in theory, like, lets say we want to build a ring system, and we design this enormous snow blower and we blow ice crystals around a planet, just for fun. A ring system would probably remain stable well beyond that mass. I don't have the means to calculate where instability might come into play, but if you build your own ring system around a planet, you might be able to get the ring system as massive as 1/10th the mass of the planet, maybe a bit more before some kind of gravitational instability took over. I don't think you're ever likely to see that in a natural situation, but theoretically I think it could be done.

Now, to answer your questions:

does it depend on the mass of the planet?

absolutely. The mass of the planet (and it's density) determines the size of it's Roche limit, which defines how far the ring system can extend. As pointed out above, planets can have rings beyond the Roche limit but only faint ones. Dense/Thick rings only form inside the Roche Limit and the more massive the planet, the larger the Roche Limit.

presence/absence of moons?

Small moons don't matter much. Saturn has small moons inside it's rings and they create small breaks in it's ring system. Large moons, particularly if they are close to the planet, can create gravitational disruptions and wouldn't be good for a permanent ring system.

distance to the star or other parameters?

Beyond the frost line is best, cause that's where ices don't melt. Ices make up more mass of our solar system and probably most solar systems than rocky material and they break apart more easily, so statistically speaking, a ring system should do better far away from the sun, in our case, at least Jupiter Distance. Jupiter's enormous magnetic field might also not be long term ring friendly, so in addition to far from the sun you'd want a planet without too strong a magnetic field that's shooting high velocity charged particles through the ring. Here's a fun article about how Saturn's magnetic field erodes it's ring. Jupiter would melt any ice ring it had much faster than Saturn does.

Hope that wasn't too long and it's more my trying to work it out than a definitive answer, but until we get much better telescopes, there might not be a definitive answer to this one.

expansion - Redshift for gravitational waves?

According to general relativity, would gravitational waves experience the same sort of redshift that electromagnetic waves experience due to the expansion of the universe?

And are there astrophysical processes that would be expected to have a characteristic 'frequency signature' analogous to the lyman or balmer series of hydrogen which would enable a measurement gravitational wave redshift.

I'm thinking of processes like baryon accoustic oscillations, certain types of supernovae, neutron star mergers, etc.