Introductory books about evolution - Biology

You won't find a good book that presents the 'weaknesses' of the theory of evolution, and for good reason. There has to date been no convincing challenge to the theory, whilst it has been supported by experimental and circumstantial evidence as well as clear logic over and over and over again, as well as actually being observed taking place numerous times. The closest to what looking for is a book which presents our best understanding of how evolution works, or which discusses competing ideas about how evolution works - there's no question about whether it works.

The Selfish Gene by Richard Dawkins, whilst it contains some errors, has some very clear thinking about evolution in it, and in particular explains why group selection is a misguided idea.

The Logic of Chance: The Nature and Origin of Biological Evolution by Eugene Koonin is a more up-to-date coverage of our current understanding of evolution, in particular with insights from modern genomics.

There are plenty of terrible books about the weaknesses of evolution, but they are deeply flawed. Pick one and I'll be happy to debunk it for you.

galaxy - Trouble understanding speed-dispersion in (elliptical) galaxies

I'm learning about LOSVDs (Line Of Sight Velocity Distributions) and I'm having a bit of trouble understanding the used terms.

As I understand, the LOSVD of a given (elliptical) galaxy is the density distribution of the LOS-velocities. The full LOSVD is difficult to find and it's easier to find 2 parameters of the distribution: $bar v_{_{LOS}}$ and $sigma_{_{LOS}}$ by fitting a (Gaussian) model to the spectrum of the galaxy.

I have a limited understanding of statistics, so I'm having trouble intuitively understanding what these 2 parameters mean later on.

I think that $bar v_{_{LOS}}$ is simply the average value of the LOS-velocity for the whole galaxy while $sigma_{_{LOS}}$ is the equivalent of a standard deviation.

Now later on in my coursebook, there's an explanation of how to get a LOSVDs for every point (/pixel) in the (projection onto the celestial sphere of the) galaxy by using 3D spectrography.
From this we can get a 3D dynamic model from our 2D kinematic model. In this newly found 3D distribution there's also 3 Sigmas, one for every dimension.

But here comes the part I don't understand:
The book is looking at the movement of stars in a spiral galaxy, the Milky way in particular.

We're trying to find a correlation between the age of MS-stars and their dispersion and so there's a dataset of a few stars in the neighbourhood of the sun with their dispersion in every dimension.

What is the meaning of dispersion in this context? How can a single object have a dispersion if it's a parameter of a density distribution?

homework - What is the Giemsa staining of chromosomes?

I got the question in my exam and wrote the following and I do not understand what is wrong in it:

Giemsa staining is a staining method to stain particularly malaria and other parasital diseases. G-bands occur because Giemsa stain
consists of A,T rich material i.e. poor gene such that dark and white
bands occur. Each chromosome has an unique reaction to Giemsa staining
so G-bands occur.

0 points. I do not understand what's wrong with it, since in their comments about the same question in my first exam they wrote also the extra questions: What are G-bands? How are they formed and why? This time I answered the given things and got zero mark.

Probably, the mistake was that I did not answer to the question in the scope of medical Biology in some way. However, I am not exactly sure what it is exactly.

How would you answer to the question when you know that the course was about medical biology?

Please, add the tag Giemsa.

neuroscience - Why are melodies/harmonies perceived as pleasurable by humans?

There are strong connections between the auditory cortex and the limbic system, which includes such structures as the hippocampus and the amygdala.

A recent paper [1] builds on earlier notions of emotional "significance" of music without any lyrics. It adds in lyrics, so giving a perspective of which portions of the brain are reacting to which component of the music.

Additionally, contrasts between sad music with versus without lyrics recruited the parahippocampal gyrus, the amygdala, the claustrum, the putamen, the precentral gyrus, the medial and inferior frontal gyri (including Broca’s area), and the auditory cortex, while the reverse contrast produced no activations. Happy music without lyrics activated structures of the limbic system and the right pars opercularis of the inferior frontal gyrus, whereas auditory regions alone responded to happy music with lyrics.

One of the limitations of this particular study is that the subjects self-selected their own pieces, which may limit the reliability of the results. Of course, defining "happy" or "sad" for every individual is slightly subjective and difficult. They cited an earlier "pioneering" study which standardized the musical selection between subjects. Without consideration of the lyrics:

The first pioneer study using functional magnetic resonance imaging (fMRI) by Khalfa et al. (2005) chose a controlled manipulation of two musical features (tempo and mode) to vary the happy or sad emotional connotations of 34 instrumental pieces of classical music, lasting 10s each. Sad pieces in minor-mode contrasted with happy pieces in major mode produced activations in the left medial frontal gyrus (BA 10) and the adjacent superior frontal gyrus (BA 9). These regions have been associated with emotional experiences, introspection, and self-referential evaluation (Jacobsen et al., 2006; Kornysheva et al., 2010).

As an aside to answer your final thought, in cases like this I think trying to jam everything under an umbrella of one "neurotransmitter system" or another can make things overly simplistic to the point where you lose focus of the diversity of receptors expressed. You can say a system is driven by Dopamine, but D1 and D2 receptors have exactly the opposite effects on the neuron.

[1] Brattico, E., Alluri, V., et al (2011) A functional MRI study of happy and sad emotions in music with and without lyrics. Frontiers in Psychology, 2: 308. doi: 10.3389/fpsyg.2011.00308 (free pdf)

(see also, http://www.sciencedirect.com/science/article/pii/S0028393206003083 and related)

fundamental astronomy - What would the night sky look like if the Milky Way were the only galaxy in the universe?

It wouldn't be too apparent, but there are a few objects that you can see in good viewing conditions with the naked eye that would disappear.

Here they are in order of brightness. I marked the objects in a reddish color.

1. The Large Magellanic Cloud, apparent magnitude 0.9, located in the constellation Dorado. Only visible from the southern hemisphere.
   




2. The Small Magellanic Cloud, apparent magnitude 2.7, located in the constellation Tucana. Only visible from the southern hemisphere.
   




3. The Andromeda Galaxy, apparent magnitude 3.4, located in the constellation Andromeda.
   

And that's it. Seriously. Sure, there are a few others, but they're exceedingly difficult to see.

lagrange point - Can we observe what objects exist at the L3 positiion of planets of our solar system?

You asked

L3 is obscured by the Sun, so is that true for our viewpoint of the other planets?

No. Each planet orbits with it's own period because each planet is at a different distance from the sun. So most of the time we can see the L3 point of each planet's orbit from Earth. Occasionally it goes behind the sun from our point of view.

For the Hilda's of Jupiter have we observed them in that position or is it that we have calculated their coordinates to be in the L3 position?

The Hilda family do not occupy the Jupiter L3 point. They generally orbit closer to the sun than Jupiter, and orbit three times for every two orbits of Jupiter. Their orbits are interesting; they have eccentric orbits (up to 0.3) that in some cases cross the orbit of some of the Trojans. You may have noticed that some of the Hildas also approach the Jupiter L3 point. At that point, and when other members of the family approach the Tojans at L4 and l5, the Hildas are moving more slowly. So the Sun's gravity gradually accelerates them, pulling them back to the lower, faster part of their orbits.

I have linked to the Wikipedia Hilda family page. The Lagrangian point page is also helpful.

Can a star orbit around multiple planets or a planet with massive moons?

There's theoretical ways to do it but it's so unlikely as to probably not exist. Assuming you want a system where the planets are in stable orbits around each other. The basic difficulty is the 3 body problem or n body problem. More on it here and here.

For example, a massive planet could (in theory) have a single super-massive moon of similar mass to the planet, though that would probably be considered a 2 planet system. It's not possible for one planet to have two super-massive moons because that wouldn't be stable. In general, planets are many times the mass of all their moons combined, similarly stars are many times the mass of all their planets combined and when that stops being true the system is no longer stable. It's very difficult to generate sufficient mass by adding moons to a system, or by adding enough planets to get more massive than the sun, unless the planets crash into each other but past a certain mass, they'd stop being planets and become a kind of star when enough planets combined.

If we look at gravitational systems, like our solar System, something like 99.7% of the mass is in the sun, so the sun dominates and everything orbits the sun. A few of the larger objects have moons, and curiously, but only cause they're in relatively calm orbits far away from other planets, a few asteroids orbit each other, but the system is very structured around the sun with much smaller stable orbital zones around the planets.

Pluto also has a rather curious orbital system of it's own, likely caused by an impact, where Pluto and Charon are comparatively large and 4 tiny moons orbit around them.

Source:

But these kind of structured systems are only possible if you have a significant mass differential. When you have 3 or more bodies of similar mass and similar distance you get a high degree of mathematical chaos and instability. There are creative mathematical tricks to make it work, but none of them are stable or likely.

This is what 3 similar mass bodies look like, and in such a system, with constant changes, the most likely scenario is that one of the bodies eventually gets ejected. (source, N-body problem above)

There are star systems with several stars, but they are either unstable or contain significant differences in mass. The gravitational structure applies for large planets the same as it does for stars, and there's an article about that here.

You can create stability by having 2 objects orbiting each other and a 3rd massive object quite distant. (Alpha Centauri is that kind of set-up though Proxima Centauri is quite a bit smaller, but it's the same system).

You can even create the heirarchy where you have 2 objects orbiting each other and then 2 more, orbiting each other, but distant so the 2 co-orbitals orbit around each other, and if you do this enough times, you can kind of create enough planetary mass where a more massive star could distantly orbit the entire thing, but it gets very structured and very distant. It's not what I'd consider a normal orbit.

You could also cheat and have several planets in a wildly unstable general proximity orbit and have a star some distance away orbiting the chaotic mess in the middle, but it wouldn't be stable for long.

Does the Moon's magnetic field affect Earth's magnetic field?

So would the Moon's magnetic field affect the Earth's magnetic field, just as its gravitational pull affects Earth's gravitational pull for oceans?

Yes, but only slightly. Firstly, magnetic fields can superimpose, so the field at any point is the sum of the field due to the Earth and the field due to the moon.

However, the moon is rather far away (and has a weak magnetic pole strength), so the magnetic field due to the moon on Earth's surface is nearly negligible (magnetic field also decreases as an inverse-square law)

In addition, the magnetic field of the Moon may bolster or erode the Earth's field as magnets moving relative to each other tend to either lose magnetization or become stronger. But this process has a negligible effect when we take the Moon and Earth.

astrophysics - Relationship between absolute magnitude of a star and its luminosity?

• Why does this relationship involves the absolute magnitude of the sun and its luminosity?


• How to derive an expression relating the absolute magnitude of a star to its luminosity?

So according to the magnitude equation, $$m - M = 2.5logleft(frac{d^2}{d_0^2}right)$$
$$Rightarrow M = m - 2.5logleft(frac{d^2}{d_0^2}right) (eq1)$$

and Luminosity is $$L = 4pi(d^2) times f$$
$$Rightarrow d^2 = frac{L}{4pi times f} (eq2)$$

Plugging eq2 to eq1 would have seem reasonable, but how is absolute magnitude of the sun and its luminosity used?

evolution - Did researchers evolve multicellular yeast or did they just turn on multicellularity?

You're right:

Within the Fungi, simple linear multicellularity of hyphae occurs in all major clades (see below), but only Ascomycota and Basidomycota display more complex two- and three- dimensional multicellularity in the form of sexual spore- producing fruiting bodies. In both of these groups, reversals to unicellular lifeforms have occurred, for example, Saccharomyces and many other related yeasts in the Saccharomycotina (Ascomycota) or Cryptococcus albidus and related species in the hymenomycete clade of Basidiomycota (de Hoog et al. 2000, p. 130).

Medina, M., A. G. Collins, J. W. Taylor, J. W. Valentine, J. H. Lipps, L. A. Amaral Zettler and M. L. Sogin (2003). "Phylogeny of Opisthokonta and the evolution of multicellularity and complexity in Fungi and Metazoa." International Journal of Astrobiology 2(3): 203-211. doi:10.1017/S1473550403001551
(PDF)

Update: The authors responded to criticism like this on The Loom, here's an excerpt:

Our yeast are not utilizing ‘latent’ multicellular genes and reverting back to their wild state. The initial evolution of snowflake yeast is the result of mutations that break the normal mitotic reproductive process, preventing daughter cells from being released as they normally would when division is complete. Again, we know from knockout libraries that this phenotype can be a consequence of many different mutations. This is a loss of function, not a gain of function. You could probably evolve a similar phenotype in nearly any microbe (other than bacteria, binary fission is a fundamentally different process). We find that it is actually much harder to go back to unicellularity once snowflake yeast have evolved, because there are many more ways to break something via mutation than fix it.

Can we (theoretically) spin the black hole so strong that it will be broken apart by centrifugal force?

Can we (theoretically) spin the black hole so strong so it will be broken apart by centrifugal force?

For a Kerr-Newman (rotating, charged, isolated) black hole of mass $M$, angular momentum $J$, and charge $Q$, the surface area of the event horizon is given by
$$A = 8Mleft[M^2 + (M^2-a^2-Q^2)^{1/2} - Q^2/2right]text{,}$$
where $a = J/M$. An extremal black hole occurs when $M^2 = a^2 + Q^2$. Beyond that, if the black hole is even more overspun or overcharged, is an "overextremal" Kerr-Newman spacetime, which wouldn't really be a black hole at all, but rather a naked singularity.

Thus, I interpret your question as asking whether or not a black hole can be be spun up to the extremal limit and beyond, so as to destroy the event horizon. It's very probable that it can't be done.

Wald proved in 1974 that as one flings matter into a black hole to try to increase its angular momentum, the nearer to an extremal black hole it is, the harder it is to continue this process: a fast-spinning black hole will repel matter that would take it beyond the extremal limit. There are other schemes, and though I'm not aware of any completely general proof within classical general relativity, the continual failure of schemes like this is well-motivated by the connection between black hole dynamics and thermodynamics.

For example, the Hawking temperature of the black hole is $T_text{H} = kappa/2pi$, where
$$kappa = frac{sqrt{M^2-a^2-Q^2}}{2Mleft(M+sqrt{M^2-a^2-Q^2}right)-Q^2}$$
is the black hole's surface gravity. Thus, even reaching the extremal limit is thermodynamically equivalent to cooling a system to absolute zero.

space time - What caused gravitational waves detected by LIGO?

It couldn't be Jupiter or the moon because

1. The frequency of the detected waves is too fast. The detector found something that rotated several hundred times a second, and then stopped.




2. The amplitude is too big. Even though Jupiter is close, the gravitational waves it produces in its orbit are extraordinarily weak. Far too weak for us to detect them (good thing too, because if it was producing powerful gravitational radiation, it would start to spiral into the sun)

We know one thing that can produce a chirp of this frequency and amplitude, and that is a black hole merger. In the last few moments before merging, these two black holes converted about three solar masses into gravitational radiation: That is a colossal amount of energy. About $$500,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000$$Joules. That is why for one moment it was the "brightest" point in the gravitational sky

(Note. standard form is for wimps)

solar system - What good evidence exists for the 9th planet as spoken of by Caltech?

I'm sure Caltech has answered this in some way, but I think it's a good question have on this site.

What good evidence exists for the 9th planet as spoken of by Caltech?

As I see it, Caltech has apparently looked at the orbital parameters of many Trans-Neptunian-Objects (TNOs) and noticed that the lines between their perihilion and aphelion (i don't know what the technical term might be) all run more or less in the same direction. This is evidence that some large body has gravitationally "synced" these planets over millions of years.

Except that alone isn't good evidence. Here are two stats first:

1. Known TNOs: about 1,750 according to the Minor Planet Center's list.




2. Estimated existing Kuiper-Belt Objects: 120,000+ according to Britannica. (That number is only estimating objects of 100-km diameter or more, but that's fine for our purposes.) Note: 120,000 is only for KBOs. There are other TNO sets such as the oort cloud (and the scattered disc if you define it separately), so I expect the number to be much much bigger than 120,000.

Given that we've only measured about 1% or less of TNO orbits, how can we be sure we have a good random sample to say that a massive body must have aligned these orbits? In other words, all the other orbits might be randomly distributed, which would be compelling evidence that some giant body is not "syncing" orbits or else way more than 1% of them would be "synced".

If I recall my statistics class correctly, 1% is not good enough for a random sample. The border-line good enough area starts at 3%, but there's still a catch: It has to be a random sample. There are still observation-bias questions here because obviously we've only mostly discovered the "nearby" TNOs, because they are easier to discover when they're "nearby". That certainly would not be a random sample even if we had 3% of them.

TLDR: There are at least 100,000 distant objects, of which less than 1% of them have some kind of aligned orbit as far as we know today. Is this actually good evidence that something has aligned them?

EDIT: Another analogy that might help. You have a bag of 100 marbles. You reach in and pull out 5 from the top of the bag (notice I didn't say any random 5!). They turn out to be 4 red ones and 1 black one. Is this actually good evidence that about 80% of the whole bag is red?

light - Can't we see all of the Milky Way's supernovae with the naked eye?

So from this I can only come to the conclusion that supernovae did in
fact happen in the Milky Way in the last two centuries, but that we
didn't see any of them.

But now I'm confused. I read about supernovae that have happened in
galaxies billions of light-years from here that lit up as the
brightest source of light in the sky for days. Surely we would be able
to see all of the supernovae that happen in our Milky Way

One problem with estimating how many supernovae have happened in the Milky Way in the last two centuries, is that the remnant from a supernova is far dimmer than the nova itself and far harder to find. Several supernovae could have happened in the Milky Way in the last 2 centuries and remain undiscovered — and just for clarity, we define the date of a supernova by the date the light from the explosion reaches the Earth, so saying saying a nova "happened" in the last 200 years refers to the date the light from the event reached Earth, not the actual date of the event, which you probably already know, but just to clarify.

So for argument's sake, let's say that a supernova's light reached the Earth about 50 years ago, but it took place on the far side of the galaxy. To find that, we'd have to look for a nebula on the far side of the galaxy and that's a hard thing to see. Similar to looking for the theoretical Planet Nine, finding old supernova remnants on the other side of the Milky Way takes a lot of looking. Even with modern telescopes, it's still a needle in a haystack, and especially if the view is blocked by dust like much of the Milky Way is.

A Milky Way supernova remnant was discovered in 1985, supernova remnant G1.9+0.3. It was thought to have "happened" around 1868, though it went unobserved and probably wasn't visible to the naked eye at the time. There's probably been several others more recent than that one. G1.9+0.3 would have been visible if not for interstellar dust. From article above:

It was a type Ia supernova believed to have exploded about 25,000
years ago, and the signal began reaching Earth around 1868. The light
from the supernova would have been visible to 19th century
astronomers, had it not been obscured by the dense gas and dust of the
Galactic Center.

I'm not sure the 4.6 supernovae per century from your article is accurate. It might be, but the number I'm used to hearing is about one per century. But regardless of which number is actually correct, it still doesn't imply high mathematical improbability because many Milky Way novae would have gone unnoticed if they were far enough away. In short, I agree with what you said here:

I can only come to the conclusion that supernovae did in fact happen in the Milky Way in the last two centuries, but that we didn't see any of them.

Here's a related article on Milky Way visibility.

Observation of a distant galaxy nova is possible if we have telescopes looking in that direction. A nova is much more detectable at the time it goes nova. Much less so, years later.

Today, however, with neutrino detection in 7 locations around the globe, I think it's virtually impossible that we'd miss a supernova in the Milky Way, so we have a good chance of seeing one in our lifetime and it's likely that none have occurred since somewhere around 1980. A supernova was detected in the Andromeda galaxy in 1987 by that method, and our neutrino detection has improved since then to give us early warning and pinpoint location.

As far as visibility, size matters, but what matters more is how close and how much dust is in the way. Most of the recorded supernovae were quite bright and fairly noticeable to someone who was familiar with the stars in the sky (list of known supernovae).

Eight Milky Way novae have been recorded by history and observed by the naked eye in the last 2,000 years, well below the number that should have happened in that time. Five of those eight had brightness greater than $-3$, which is brighter than Jupiter and would have been immediately noticed by anyone familiar with star charting. Two others had magnitudes around zero, which is still brighter than most stars. SN386 was less bright, but is still easily visible and recorded by Chinese astronomers. And finally, Cassiopeia A was quite dim, but it was still observable. Only about 10% (ballpark estimate) of the Milky Way is close enough and unobstructed enough to provide supernovae that would visibly get noticed. Most would have gone unnoticed until recently.

Hope that's not too wordy, I can try to clean up if needed.

the sun - Term for altitude of the sun?

The term for when the sun is at its highest is "solar noon".

https://en.wikipedia.org/wiki/Noon#Solar_noon It is the moment the sun crosses the meridian. The meridian is an imaginary half-circle that goes from the northernmost point on the horizon to the highest point directly above you and then to the southernmost point on the horizon. The sun is always at its highest point in its daily path when it crosses the merdian.

You're right that in earlier centuries, each town would set its clocks at noon for when the sun was at its highest point. Those of us who are familiar with astronomy and history would call that type of timekeeping "apparent solar time". You can't guarantee an exact 24 hour day with "apparent solar time", so some people used "mean solar time" instead. (Historical note: Having each town choose its local noon became a nightmare for railway companies that wanted to set train schedules. So the railway companies urged the creation of 4 time zones in America and encouraged each small town to use "standard time" instead of "apparent solar time".

And as rgettman already mentioned, the term for the sun's height above the horizon is "altitude". Astronomers use the term "elevation" for a person's distance above sea level.

general relativity - What's the relation between Einstein's Gravitational Theory and Dark Matter?

Dark matter was originally hypothesised because there is a greater degree of rotational energy in galaxies than visible matter would allow for - crudely they rotate so fast that they ought to spin apart and therefore it was hypothesised that there is an additional source of gravitational force in some form of invisible matter.

This is not as a result of a discrepancy in Einstein's General Relativity. Newtonian gravitation is a very good approximation of Einstein's GR at most ordinary scales and energies and this is true in this case also.

I am not sure, exactly, what your "what about dark energy" question means, but assuming you mean, does this imply an incompleteness in GR, then again the answer is no, or at least not necessarily, but it's a more subtle point.

Dark energy is the hypothesised source of the energy that causes the universe to expand at an accelerating rate. It can be inserted as a term into the field equations of GR (other explanations of DE exist though) - but that is just a mathematical term, rather than an explanation of the physicality of it.

Einstein originally inserted such a term into his solutions - to predict an essentially static universe. When it was shown that the universe was expanding he described this as his "greatest mistake".

The evidence for the accelerating rate of expansion is relatively recent and came long after Einstein's death so he never lived to see the essential idea - of a "cosmological constant" in his equations - revived.

This explains more - https://en.wikipedia.org/wiki/Cosmological_constant

exoplanet - Are there any common characteristics of habitable planets?

I think I will answer your question from two vantage points. Firstly, what do we have to measure about a new exoplanet to consider it potentially habitable? Keep in mind that in detecting these 1000 or so new exoplanets, we've only gone so far as to show that they (statistically) exist and to measure some very very basic properties of them. It is on these very basic properties that we make our judgement of habitability and further research and observation is necessary to confirm these suppositions. When a new exoplanet is announced, we generally know the following information, which we use to determine habitability:

1. Distance from the star. This is a crucial piece of the puzzle because it tells us generally how hot or cold the planet is. We presume that life cannot exist on planets which are boiling hot or freezing cold. This information can be ascertained from either the radial velocity method or the transit method.


2. Size of the planet. Note here, size generally refers to diameter or radius. This is important because we want to generally know what type of planet we've detected. Gas giants are, well ... giant and inhospitable to life as we know it. We want to make sure the planet we've found is small enough that it is likely terrestrial. Although size is not a guarantee that it is terrestrial, its a piece of the puzzle. We can measure size by the transit method, however there is some degree of uncertainty to this process because we need to know the size of the host star to a good degree of accuracy, which is not always possible.


3. Mass of the planet. This is a crucial component in determining habitability. Size alone may tell us that a planet is what we call a "Super Earth" or "Sub Neptune" in that it is slightly larger than our own planet. The problem with that is, if the planet sits on the size border between Earth and Neptune, how are we to know whether it is rocky or gaseous? This can be resolved by looking at the mass, which is ascertained by the radial velocity method. The mass combined with size, gives us density and density is a surefire way to know if the planet is made of dense material or light gases.

That's really about it for basic information gleaned from simple planet detection methods. If you have distance from star, size, and mass (all of which you can get from the RV and transit methods) then you can presume that the planet is Earth-like and at a good temperature. That's enough to claim potential habitability, but not enough to actually show it can support human life. For that, you need to investigate the planet further and show it meets a variety of other conditions. As of know, some of those conditions are measurable with enough work, some aren't. These conditions may include, but are not limited to:

1. An atmosphere with water in it. We have the technology to measure this fact, but I'll say that it is very very difficult to do so and I don't know if anyone has really successfully shown water exists in an exoplanetary atmosphere (some have convincing evidence though). Generally, the process to do this is to measure the radius of the planet as a function of wavelength. Different wavelengths of light will refract through the atmosphere differently (just like in a prism) and so you measure a different radius of the planet (radius being the physical size of the planet plus the extent of the atmosphere) at different wavelengths. From this information, you can model what the atmosphere must be made of to cause the radius to change depending on wavelength. The issue with this process is that is requires removing all possible false detections and having a great signal to noise which is very hard.


2. A magnetosphere. We believe this is a necessity for life. Every second of every day our planet is being bombarded with high energy particles from our Sun. Most of these particles are harmlessly deflected by our magnetosphere and so we're protected. Planets without this protection would have a hard time hosting life as that life would be destroyed by radiation (or at least life like us). There is currently no way to detect whether a planet has a magnetosphere, although I believe there are potential ideas on the chalkboard.


3. The presence of a moon. This one is a bit iffy. Some people will argue our Moon has been instrumental to our development and for life in generally. The argument generally goes that the Moon has helped stabilize our tilt and thus kept our seasons and global weather well regulated. While we might be able to exist on a planet without such conditions, some believe life cannot begin on such a harsh planet without that regulating moon. There is also the benefit that the Moon has shielded us from a large number of impacts by taking the blow itself. Again, we could live on the planet, but the lack of a moon might not foster growth of life naturally. Moons, at this time, are out of our reach to detect.

There are numerous other factors for what helps define whether a planet is habitable, be it by us or in hosting new life. The list could go on and on, but these are the major points I think.

homework - Does chicken embryo form the disc called "discoblast" in the cleavage and blastulation of chicken?

This thread is related to my previous thread which is still unsolved mainly. I need to be able to compare chick and human cleavage and blastulation of zygote.
My friend says that

Chicken have discoblast, human not. That is the main thing to get the idea of the difference.

I know that chicken have yolk sac with yolk, while human have yolk sac without yolk.

My notes say that

Cleavage in birds is partial discoidal (embryo forms disc and cells called blastodisc are on top of the yolk).

So embryo forms some disc, lets call it #1. Some cells called blastodisc are on top of the yolk. So are the cells called blastodisc on top of the yolk and the disc #1?

What is the name of the given disc that chick embryo forms?
It seems that human is not forming the given disc. Apparently, the name of the given disc is discoblast, the thing he says is very important.

human biology - What actually happens when my leg 'falls asleep'?

The explanation in the link Polynomial posted is essentially correct.

Whenever there is a reduced or blocked blood supply (ischaemia) to your extremities, the 'five P's' can occur: pulselessness, pain, pallor (colour), paresthesia (numbness) and paralysis (or weakness).(1).

The numbness and weakness happen after the blood flow have been reduced for a particularly prolonged period.

Cells in our body require a blood supply to stay alive (think about a stroke or heart attack for example). So a reduced supply can cause them to function abnormally or after a time (depending on the cell or tissue type) die.

So with a 'sleeping leg', staying in an awkward or particular position where arterial blood supply is blocked or reduced to the leg, the muscle, nerve tissue etc all lack supply hence causing sensory disturbance and weakness.

The possible buildup of metabolites could also contribute to the symptoms (pain).

Hope that helps!

1. Miller's Anaesthesia - Miller.

human biology - Why does scar tissue change color?

The main mechanism behind the scar changing its color is the dilation of blood vessels within the scar tissue. The dilated vessels lead to the congestion of blood flow. The red blood cells moving more slowly leave more oxygen due to its diffusion to the tissue and the hemoglobin -- the oxygen holding protein -- changes its color from scarlet (bound to oxygen) to cyan (released/bound to CO₂). Since scars are not covered by skin, the blood vessels are clearly visible here and you see this characteristic color.

The triggering mechanism here is cold that increases the energy barrier for many biochemical reactions, first of all those required for smooth muscle contraction.

Cold-induced vessel dilation (that can be seen, for example, on cheeks and extremities) with mild circulatory disturbances ultimately undergoes into so-called cold vessel paralysis when smooth muscles of blood vessel walls completely loose their ability to contract and are maximally dilated. The cyan color of the scar is usually seen at the very last stage, that can occur unexpectedly quickly because scars do not have any warmth insulation layer compared to the regions covered by skin.

fundamental astronomy - How to plot orbit of binary star and calculate its orbital elements?

If it's a binary, it's fairly simple (in comparison with a ternary system), because the stars of the binary orbit about their common barycenter in Kepler ellipses.

An orbit simulator, see here; replace the central star by the barycenter of the binaries.

Calculation of an Orbit from Three Observations. Kepler problem on Wikipedia.

If you don't find a ready-to-use software, try to solve it numerically by simulating the orbit of the binaries according to Kepler's law, and vary mass, distance, excentricity assumptions until they match to the observations. Use optimization methods, e.g. hill climbing algorithms, or gradient methods.