biochemistry - How do antioxidants affect human metabolism?

Anti-oxidants affect human metabolism by altering the redox states of the cell and redox-regulated functions and signaling mechanisms.

The following quotes are from The Redox Stress Hypothesis of Aging (Free Radic Biol Med. Feb 2012)

More recently, in a major conceptual shift, ROS have been found to be
physiologically vital for signal transduction, gene regulation, and
redox regulation, among others, implying that their complete
elimination would be harmful. An alternative notion, advocated here,
termed the "redox stress hypothesis," proposes that aging-associated
functional losses are primarily caused by a progressive pro-oxidizing
shift in the redox state of the cells, which leads to the
overoxidation of redox-sensitive protein thiols and the consequent
disruption of the redox-regulated signaling mechanisms.

Many proteins contain cysteine residues which can undergo reversible modifications such as S-nitrosylation and S-glutationylation.

The redox state affects the oxidation levels of these redox-sensitive protein thiols and the consequent disruption in protein activity and the redox-regulated signaling mechanisms.

Under normal conditions the usual types of oxidation modifications of these protein thiols are reversible, and is a way to protect the thiol in transient periods of oxidative stress.

When the redox state shifts to a more oxidised point then more irreversiable protein thiol modifications occurs, i.e. damages the protein.

As shown in Figure 6, prolonged exposure of sulfenic acid, formed at
the active or regulatory sites, to H2O2, sequentially leads to the
formation of sulfinic- (−SO2H) and sulfonic-acids (−SO3H), which are
deemed to be largely irreversible reactions. In addition, protein
disulfides may undergo over-oxidation, leading to the formation of
thiosulfenate and thiosulfonate, which are also believed to be
relatively irreversible reactions. The level of oxidation of protein
cysteinyl thiolates depends upon the strength (concentration) and the
duration of exposure to H2O2.

Protein activity and enzyme catalytic efficiency which affects metabolism, such as sirtuins, are affected by the these thiol redox modifications.
(eg SIRT1 is a redox-sensitive deacetylase that is post-translationally modified by oxidants and carbonyl stress and A redox-resistant sirtuin-1 mutant protects against hepatic metabolic and oxidant stress.)

The primary arbiter of redox state is glutathione, i.e. the GSH/GSSG couple. The effect of anti-oxidants appears to vary depending on whether it affects the redox state.

The antioxidant activities of SOD and catalase neutralize the ROS species, O2− and H2O2, without having a direct impact on glutathione redox state. However this frees up glutathione from having to neutralize the ROS species, which it can do so with the glutathione peroxidase enzyme.

star - After the Sun becomes a red giant, will most of the nucleosynthesis still be from proton-proton fusion?

After the Sun becomes a red giant, will it continue to mainly run on the pp-fusion, or some other type of reaction, like the CNO cycle or the triple-alpha process? On that note, do most other stars start to rely on different reactions near their death?

gel electrophoresis - How do Proteins migrate in MES vs. MOPS

Well, to rationalize everyone's comments, I think @leonardo is right.

This is a denaturing SDA PAGE gel. The migration of the SDS Micelles which are negatively charged, depends upon the shielding of the solution around it. The difference in mobility is because the SDS micelles will experience a slightly different field at pH ~6.2 (MES) vs 7.2 (MOPS).

The thought that these have the same charge would be right at exactly the pH corresponding to the pKa. For these two ions the proportions will not be the same when running 0.8 pH units from their respective pKas. the ion concentration goes as +/- log ([BH]/[B-]) and the charge environment of the buffer should not be the same. I think that the higher pH for MOPS running will tend to create more negative charge in the solution from MOPS- but also OH- in solution, slowing the mobility of the micelles and favoring the resolution of the larger proteins with MOPS.

This is all given even if the buffering ion concentration is the same.

I'm sure the gory details are buried in the musty tomes of some physical biochemistry journals deep in the library, but this is how it sounds to me.

genetics - Is there a gene that starts meiosis 2?

Yesterday I thought about a question and asked it to my friend. The question was
which gene is completely the same for a male cell that made meiosis 1 recently.

My answer was the gene that starts meiosis 2, but he said he thought that there is no such gene - only a gene to start meiosis and meiosis 1 and 2 are just parts that scientists named.

My question is is there any gene that starts meiosis 2?

gravity - what is gravitational force?

As a hobbyist, I'll give a limited answer.

Gravitational force is particle or wave?

Until we actually find out what gravity is, the safe answer here is that we don't know, but it's probably a wave and a particle. Most (perhaps all) fundamental particles in quantum physics are waves and particles. Light for example, Electrons too, though we might think of an electron as a particle, it's wave properties are easily demonstrated, and wave properties of protons can be demonstrated as well. There's a ton of stuff that can be googled on the wave-particle duality. Here's one.

It's unclear exactly what fundamental particles "are" so saying a fundamental particle is "this or that" is a bit iffy since we can't see them. We can only study how they behave. Gravity, if it's a fundamental particle, which it probably is, it should be both a wave and a particle, and a field. I know, that's a little hard to think about. We don't get to have many tangible answers in quantum physics.

I know that gravitation depend of the mass and the rotation speed

Gravity depends on mass. Period. Rotation speed can creates a centrifugal force opposing gravity and making things lighter, but that isn't gravity. The gravity is still 100% governed by mass.

what is the transmission medium of this wave the same like
transmission medium of the light ?

Light (as far as I know) doesn't require a transmission medium. Gravity shouldn't either. Sound waves require a medium but light and gravity can pass through empty space.

If you have more specific questions, you might have better luck on Physics rather than Astronomy. Also, my 2 cents, Stan Liou has made some intelligent comments well worth reading.

Two black holes photo interpretation

The image is about a merger of two black holes, before the actual merger takes place.
The two largest of the dark regions are slightly distorted images of two the black holes. Distortion is due to light being bended asymmetrically by the combined gravity of the two black holes.

The two smaller dark areas show the respective other black hole in a strongly distorted version, since the light - or missing light - of the other black hole is bended around the respective less distorted appearing black hole by about 180 degrees, somewhat similar to an arrangement of uneven mirrors.

You may interprete the phenomenon of the smaller dark areas as an extreme version of Einstein rings.

Aren't there more naked-eye-visible stars in the Milky Way plane?

You can tell a lot about Galactic structure by just looking. The ~5000 stars that can be seen with the naked eye have a roughly "lognormal" distribution of distance. I show plots below which were generated from the most recent version of the Hipparcos parallax catalogue. Fig.1 shows results for all stars with $5.5<V<6.5$ (i.e. very faint naked eye stars). The Hipparcos catalogue is almost complete for these stars, though some stars are so far away that the distance is highly uncertain - nevertheless, the general picture should be ok.

The median distance is about 440 light years. But the premise of your question is I think, that you dispute that this is far enough for the non-spherical distribution of stars in our Galaxy to become apparent. The answer is actually that it is, but only just. The Sun is very close to the plane of the disk of our Galaxy. The scale height of stars above this disk varies depending on stellar age and mass. Very approximately, the exponential scale height is 300-500 light years for most of the stars in our Galaxy.

This is just small enough that if we look at the demographics of stars in two Galactic latitude regions we do see a difference. Fig. 2 below shows stars with $5.5<V<6.5$ at low Galactic latitudes (within 15 degrees of the plane in green) and more than 45 degrees from the plane (in blue). There is a clear and significant difference. More stars can be seen at greater distances towards the plane, betraying the non spherically-symmetric nature of stars in our Galaxy.

In other words the structure of the Galaxy is not too small to affect the distribution of naked eye stars.

When we look at the Milky Way we are seeing millions of unresolved stars that are in general more distant still. The effects of the disk-like nature of the Galaxy become more apparent as one goes to greater distances. In particular, at high Galactic latitudes one simply runs out of stars and hence there is no uniformly illuminated sky - leading to Olber's paradox, as correctly stated in another answer. But at low latitudes, there are sufficient stars that in general, within the resolution of our eyes there are many stars whose light sums to provide a visual stimulus. This picture is interrupted by dust. Dust in the Galaxy is even more concentrated towards the plane than the stars. It is for this reason that beyond a few thousand light years, dust plays a major role in shaping the Milky Way structures that we can see, effectively blocking light at very low Galactic latitudes.

Fig 1: Probability distribution of distance for naked eye stars in the Hipparcos catalogue

Fig 2: Probability distribution of distance for naked eye stars divided into low and high Galactic latitude regions (i.e. towards and away from the Galactic plane).

IMPORTANT EDIT:

After the answer was accepted I did a bit of double checking. I split the Hipparcos sample into bright ($V<6$, 5000 stars) and very bright ($V<3$, 173 stars) and looked at the distribution of stars per unit area on the sky as a function of Galactic latitude (Fig. 3). The results are shown below and more obviously make the point. It turns out that the asymmetry between in an out of the plane is clearly seen, even in the brighter sample. The reason is that the very bright stars are not all that much closer than the bright stars. Perhaps a median of 200 light years vs 350 light years. Thus the scale height of the Galactic disc must be small enough that the difference from spherical symmetry is already appearent at these kinds of distances.

Fig. 3: Normalised plot of number of stars per unit area versus Galactic latitude for a sample of bright and very bright stars. Note the concentration towards the Galactic plane.

FURTHER EDIT: You can even see from the plot above that there is also a slight concentration towards negative latitudes; the peak density is around -5 to -10 degrees in both cases. This is likely because the Sun is above the Galactic plane at present (though I also wonder if dust plays a role). The Sun is currently 60 light years above the plane and heading upwards (see How far is the Earth/Sun above/below the galactic plane, and is it heading toward/away from it? ). The combination of the results I have shown may be sufficient to conclude that the Galactic disc has a "characteristic thickness* not more than a few hundred light years, but more than 60 light years!

orbit - Best approximation for Sun's trajectory around galactic center?

The Sun orbits in the Galactic potential. The motion is complex; it takes about 230 million years to make a circuit with an orbital speed of around 220 km/s, but at the same time it oscillates up and down with respect to the Galactic plane every $sim 70$ million years and also wobbles in and out every $sim 150$ million years (this called epicyclic motion). The spatial amplitudes of these oscillations are around 100 pc vertically and 300 pc in the radial direction inwards and outwards around an average orbital radius (I am unable to locate a precise figure for the latter).

This is established by measuring how the Sun moves with respect to the average motion of the stars in the solar vicinity - the so-called local standard of rest. Estimates of the Sun's motion with respect to the LSR vary a little. According to Dehnen & Binney (1998) the Sun currently moves at 10 km/s inwards, at about 5 km/s faster than the average star tangentially to the Galactic centre and at about 7 km/s upwards out of the Galactic plane. Though as I mentioned previously, these motions are of an oscillatory nature around the mean value. The Sun is currently about 8.5 kpc from the Galactic centre and about 20 pc "north" of the Galactic plane.

I leave it as an exercise for the reader to visualise this orbit. From a side-view it would look like a pseudo-sinusoidal oscillation. From above the disk it would look like a slightly eccentric ellipse with a very large precession.

All of the above assumes that the Sun moves in a smooth Galactic potential. In fact the presence of spiral arms, a Galactic bar and the clumpy presence of giant molecular clouds can and will perturb the Sun's Galactic orbit around the regular behaviour described above, so predicting the Galactic location of the Sun in a billion years or so is not necessarily going to be very accurate.

star formation - Could a "burping" supermassive black hole be responsible for a spiral galaxy's look?

Does the supermassive black holes inside the Milky Way still "burp" or
it is already cured

Technically, black holes don't burp it's the material that falls into a black hole, as it gets tightly squeezed and under enormous spiral and magnetic forces it shoots out gamma rays at the poles perpendicular to the accretion disk. More details here.

The super-massive black hole (not holes as there's just one) in the center of the milky way isn't currently "burping" but the next time it eats a star it's expected to "burp" again. I remember reading that this may happen sometime in the next 10,000 years or so, but I couldn't find the article just now.

"Burping" or Jets of material are more active when galaxies are young. The quasars observed by large telescope operate in that way.

I like to think that a supermassive black hole that "burps" lies in
the core of every spiral galaxies. Am I mistaken?

This is essentially correct. Quoting this article

Astronomers believe that supermassive black holes lie at the center of
virtually all large galaxies

Black holes are surprisingly good at shooting material back into space, strange as that may sound. Source.

From which country or area is the new moon visible first?

The duration of one lunation (the period between one new moon to the next one) isn't neither constant as the Moon rotates around the Earth and it around the Sun (changes between 29.272 and 29.833 days due to the perturbing effects of the Sun's gravity on the Moon's eccentric orbit), nor integer divisible by 24 hours or one Earth's rotation around its axis. So this position of the Moon on the skies where the next new moon as the first of its lunar phases will be first observable constantly changes.

   

     ^{Phases of the Moon, as seen looking southward from the Northern Hemisphere. The Southern Hemisphere will see each phase
     rotated through 180°. (Source: Wikipedia on Lunar phase)}

Saying it differently, by the time the Moon completes one lunation (or it's synodic period) and starts the next one, it won't be positioned exactly above the same Earth's longitude (East to West) as the one it started at.

universe - Is the Oort Cloud actually opaque, or is the "cloud" used euphemistically because minor planets inside of it are too dim to be seen?

Well, given that we can see the stars and that the Oort "cloud" is closer than the nearest star, then the answer to your main question is obviously that the "cloud" is not opaque.

I think it is called a cloud because it consists of many individual, small "particles" that don't interact with each other. Also, the term cloud avoids giving the impression that the Oort objects are arranged in any disk-like configuration - they should be much more spherically symmetric. The word cloud also avoids giving the impression that the objects fall between two relatively tight orbital radius limits. i.e. The Oort cloud is not like the Kuiper belt.

evolution - What did pangolin scales evolve from?

The pangolin scale is a horny derivative of the epidermis. It is
complex in structure and is divisible into three distinct regions. The
dorsal plate forms approximately one-sixth of the scale thickness. It
is composed of flattened solid keratinized cells without basophilic
nuclear remnants. This region tends to fray easily. The dorsal plate
contains bound phospholipids and sulphydryl groups but is weak in
disulphide bonds. (On the nature of the horny scales of the
pangolin)

Also,

It is suggested on the basis of histological structure and
dishribution of chemical constituents that pangolin scales are
probably homologous with primate nails.

Sorry, I forgot the second part of your question - were they developed from fish. The article suggests that they did NOT evolve from reptile scales. Since fish are even farther removed from reptiles on the evolutionary scale, I think that pretty much answers your question; pangolin scales do not share a common origin with fish scales.

the sun - When we see the Sun, do we actually see its past?

The speed of light in a vacuum is 299,792,458 meters per second, not infinite. Let's say, for example, particle of a beam of light, the photon, is emitted. It takes ~8 minutes to get to us; when it hits our eyes, we see it. This means that we see a photon that was emitted from the sun 8 minutes ago. We aren't, per se, looking "back in time", but we're looking at a photon that is ~8 minutes old.

How does a telescope measure parallax angle?

No, the telescope doesn't measure the parallax. A sextant or any other angle measuring device fit on the telescope does.

And, we don't(can't) directly measure the parallax angle. Instead, we just track the position of the star/object throughout the year. A little bit of spherical astronomy math shows us that the path of a star in the celestial sphere defined by a fixed reference through the year is an ellipse around it's mean postion, called the parallactic ellipse

Here is a link which does the rough math involved.

The semi-major axis of this ellipse is equal to the parllax angle P, while the semi-minor axis is equal to P*sin(b), where b is the stars ecliptic latitude.

EXTRA NOTE: These parallactic ellipses are often rotated and parameters modified as the abberational ellipse is superposed on it. In practice, things get nigh complicated.

astrometry - Transform pixel coordinates (in FITS file) to equatorial

Assuming that each (square) pixel has the same angular scale (not a given if the field of view is large) of $theta$ degrees/pixel.

Then the declination (in degrees): $delta simeq delta_0 + theta y$

The right ascension (in degrees): $alpha simeq alpha_0 - theta x/cos delta$

where $alpha_0$ and $delta_0$ are the RA and Dec at $x=0$, $y=0$. [The minus sign is there because right ascension increases towards the left of a sky image.]

This approximatimation becomes poor as the field of view gets larger.

A slightly more complex, but accurate, approach is described in http://gtn.sonoma.edu/data_reduction/astrometry.php

black hole - Objects are bigger closer to big masses

I usually heard that on surface of earth time pass slower than on orbit.
As speed of light is constant it will not change based on your position or observed position.

Then if we have big mass that can slow time two times (probably black hole) then should all object close to it look two time bigger?
Image 1 meter rod with laser attached to one end. After laser is turn on it take x = 1/299,792,458 second for laser beam to reach other end.
But for far observer will see that 2x second pass for him when laser hit other end of rod. This mean that rod is two time bigger because light take two time longer to reach other end.

Can planets have orbits other than elliptical or circular orbits?

Orbits are conic sections therefore can be either circular, elliptical, parabolic or hyperbolic.
Of these 4, only first two form a closed curve under 2 body hypothesis, while the later two extend to infinity.
If you talk about planet, by definition it has to orbit a star which would require it to have a closed orbit hence circular or elliptical, with respect to the star. For any other kind of orbit the body will just fly away to infinity never to return back. Infact comets are considered to be parabolic, but in theory they have highly elliptical orbits with their aphelion lying near the edge of the solar system.

But it is possible for a planet to have other kind of orbits if we consider their motion from a different reference frame such as with respect to another planet.
So for an inertial frame of reference a planet will have a circular or elliptical orbit, even Pluto.

cloning - How to DIY preserve pet DNA today so that it can be used in 20 years

For the impatient, I'm going to say that probably if you only have a regular home freezer, its unlikely that you will be able to do this. If you have access to a -80C or liquid nitrogen storage its possible, but also less likely.

Cloning an animal typically starts with the transplantation of a nucleus into an egg cell whose nucleus has been removed. So unless things change drastically in the next 20 years, just purifying DNA and keeping it in a tube of the fridge is not enough - you need to preserve epithelial (skin cells) or some other tissue in a state which can be revived to produce a live and intact cell when you want to do the cloning.

Its possible that we will have machines that can take DNA from a DNA prep and produce a living cell - this is what we hope to do with the mammoth and other extinct animals from fossil tissue some day. But whether that will be the case in 20 years is just a guess, whether it is affordable is also just a guess.

So from what we know now, you will need to preserve a piece of tissue from the animal when it is alive. I'm not an expert on how to do this, but you might be able to get the skin cells to sort of turn into a cell line and then freeze them for not too much money.

If you do this, regular freezers are not cold enough to prevent freezer burn. This is what happens when you put a steak in to the freezer, wrapped in plastic even it will shrivel up and start to dry out as the water in the ice starts to sublime out of the package (the dry air in the freezer basically sucks the water out of the food). If this happens to your animal tissue, its probably not going to revive.

Scientific labs use -80C freezers and liquid nitrogen storage because the water turns into a glass and all biochemical reactions are basically stopped. (besides drying out, the enzymes like DNAse are still nominally functioning in the cells at -20C and even simple bacterial cells don't live for more than a year at -20C, much less mammalian cells). For preserving cell lines, liquid nitrogen is much more preferred. I would say that properly produced cell lines can theoretically revive after indefinite liquid nitrogen storage.

So that's a quick answer. Sorry to be a party pooper - things could change quite a bit in the next 20 years, but we just don't know how much. popping a paw in a baggie or some DNA extract into the freezer might work, but its hard to say for sure.

As far as the choice of where the DNA comes from in the animal, its true that skin cell lines are often producing imperfect animals - the DNA may be modified in the skin in various ways that cause the animal to be smaller, weaker, or even deformed compared to the donor. At this time all the protocols I see (and i could be wrong) are skin cells. I would expect that there is a better tissue to preserve, but that might be just a guess at this point. Its likely that in the next 20 years the choice of cell line from the donor will change quite a bit as well.

stellar evolution - How long does an over contact binary star system last?

I read recently about VFTS 352, an overcontact binary star system where both stars have roughly equal mass. All of the reports I've read (in mass-media type publications) have said that the system has one of two fates: either the two stars will merge, or they'll supernova. But when will this happen?

The wikipedia page for contact binaries says that they have a lifespan of millions to billions of years, but doesn't say if that's different for overcontact binaries. It also says that they're often confused with common envelopes, which have a lifespan of months to years, and I'm not sure where in that spectrum an overcontact lies (or really what the distinction is, since the page for contact binaries says they share an envelope, which sounds the definition of a common envelope). I'm also not sure whether the fact that both stars have roughly equal mass affects the lifespan.

The mass-media articles I've read have implied that the merger-or-supernova is happening soon, but I don't know if this is on a human scale (months) or galactic scale (millions of years).

Is the flat curvature of the universe in the 5th dimension?

Several topics here:

No 5th dimension. Not even a 4th (spatial)

You're on the right track: The Universe being "flat" does not mean "like a table top", just like it being "spherical" and "hyperbolic" don't mean like a ball or a saddle. Those terms are just 2D analogies, whereas the Universe has three spatial dimensions. Like any analogy, they are great for understanding some things, but shouldn't be taken too far.

For instance, the sum of the angles of a triangle, the fate of parallel lines, and the volume of some region in space can be more easily visualized in 2D than in 3D. You can understand that a triangle going North Pole→Congo→Indonesia→North Pole has 270º rather than 180º. But whereas the 2D surface of Earth curves in 3D space, a 4th spatial dimension is not needed for our 3D Universe to curve (not to speak of a 5th). And imaging why a triangle consisting of what seems to be completely straight lines from Earth→GRB090423→EGSY8p7→Earth should not have 180º is difficult, but nonetheless perfectly possible.

The Universe is not a ball with you in the center

When you look around in the Universe, it looks like a spherical ball with you in the center, but that's only because what you see is everything from where light has had the time to reach you in 13.8 Gyr, i.e. since Big Bang. Everything inside that sphere is called the observable Universe, and it's bound by the so-called particle horizon. In the 2D analogy, you can say that Earth looks like a flat disk, but only because that's how far you can see. Again, don't take this analogy too seriously, since the reason here is just the curvature of Earth, and not that Earth has only existed for $10^{-4},mathrm{s}$.

Curvature

The dynamical and geometrical properties of the Universe depend on its expansion rate ($H_0$), the densities of its components (the "Omegas" of baryons, dark matter, dark energy, radiation, etc.), and its "intrinsic" curvature, which can also be written like a density parameter $Omega_k$. The latter is found to be $0.000pm0.005$ (Ade et al. 2015), but as you say, if it's less than $sim10^{-4}$, we may never know (Vardanyan et al. 2009).

Why do the gas giants in the Solar System have comparatively large orbits compared to the inner planets?

When the solar system formed, there was an accretion disc, spinning around the newborn Sun.

The Sun was emitting radiations, which pushed the lighter materials of the accretion disk (gases) further away, and kept the heavier materials (rocks) much closer. This is why gas giants are almost always further away than rocky planets. Almost.

In the case of "hot Jupiters", astronomers think that they simply spiraled their way to their star, "eating" smaller planets on their way. It doesn't happen in our solar system because Saturn holds Jupiter back.

Exact source nowhere to be found on the internet, but everything I wrote here comes from a very interesting movie I watched in "La Cité de L'Espace" (Space City) in Toulouse, France. It is a famous exposition / museum about astronomy and space exploration.

Do black holes have energy?

An isolated black hole is a vacuum solution of general relativity, so in a very direct sense it does not contain any energy anywhere in spacetime. But perhaps somewhat counter-intuitively, that does not imply that such a black hole has no energy.

Defining the total amount of energy is usually very problematic in general relativity, but in some special cases it is possible. In particular, the usual black hole solutions are all asymptotically flat, i.e., spacetime is just the usual flat Minkowski when far away from the black hole.

Here (or in general when we have a prescribed asymptotic form of the spacetime), we can calculate the total energy-momentum, by essentially measuring the gravitational field of the black hole at infinity. The energy just be one component of energy-momentum.

There are actually two relevant different kinds of 'infinity' here: spatial infinity and null (lightlike) infinity, depending on whether we are 'far away' from the black hole in a spacelike or lightlike direction. There's also timelike infinity, but that just corresponds to waiting an arbitrarily long time, so it's not relevant here. The two different infinities beget different definitions of energy-momentum, giving the the ADM energy and the Bondi energy, respectively. In a vacuum, the intuitive difference between the two is that Bondi energy excludes gravitational waves.

So the short answer is 'yes', with the caveat that in a more complicated situation, where we can't attribute everything to the black hole itself, the answer to how much energy is due to the black hole may be ambiguous or ill-defined.

Note that the ADM and Bondi energy-momenta also define their corresponding measures of mass, as the norm of those energy-momenta ($m^2 = E^2-p^2$), but for a black hole we can also define mass more operationally in terms of orbits around the black hole. There are also other alternatives for addressing mass specifically.

astrophotography - Are all the photos of the universe really so colorful or is it just visual effects that are added later?

Many astronomical objects are too dim to be easily seen. There is no place in the universe where M42 (between Orion's legs) would look to the naked eye like it does in photos: it is too dim, and our eyes are not good at detecting colour of dim objects.

Unlike a regular camera, which has pixel detectors for red green and blue light, most coloured astronomical images are made by using a sequence of filters in front of the camera lens. The filters only let a particular colour through. The result is several greyscale images, when several images have been made of the same object, they are mixed together, by assigning a colour to each. So "red" may represent infrared, for example, a colour that cannot be seen by humans. The assignment of image colours to filters is called a pallet, and can make a big difference to the appearance of an image.

The Hubble images of "the pillars of creation" popularised a particular pallet, and Hubble site has some details on how they create the colours in images. The hubble pallet uses "blue" for a mid range filter, yellow green (this colour is produced by oxygen ions), "green" for and orangy red filte: it picks up the light from hydrogen ions, and "red" for a intense red, fading towards the infra-red (sulphur). By using "false colour" details of the distribution of ions can be picked out better, and it makes a more attractive image.

the sun - How to interprete function value of get_sun in astropy

As I know, spring equinox of 2016 is on 2016/3/20 4:30 GMT.

I execute following statements in python:

```

from astropy.time import Time

from astropy.coordinates import get_sun

t=Time('2016-03-20 4:30:00')

s=get_sun(t)

```

I expect to get s.ra.degree and s.dec.degree very close to 0.

But I got s.ra.degree=0.7052336 and s.dec.degree=0.30536325.

Why?

Hope some one can help me. Thanks.

star - Stellar data for data mining

The Sloan Digital Sky Survey website has a good page with lots of links on it to retrieve spectra in different ways, you can access it here. The most useful of these is probably this one. This is just one of many places you can search but its a good starting point if your just curious about analysing spectra and not too picky about the source.

the moon - How does moonrise/moonset azimuth vary with time?

The monthly variations are caused by the change in declination of the Moon as it travels (roughly) along the ecliptic. This is caused by the tilt of the Earth's rotation axis, which results in an angle of about 23° between the equator (declination = 0°) and the ecliptic. Around the constellations of Taurus and Gemini the ecliptic lies at much higher declination than near Saggitarius or Ophiuchus. The same effect causes the change in rise and set times of the Sun during a year.

When you're in the northern hemisphere, a higher declination means that the transit altitude of the Moon or Sun is higher. In fact, their whole daily (apparent - caused by the Earth's rotation) paths lie closer to the North star. Because of that, the points where a daily path intersects with your local horizon lie further north, which you see as a change in rise/set azimuth of the Moon.

All this is true even without the 5° inclination of the Moon's orbit w.r.t. the ecliptic. That inclination only mitigates the effect; the Moons declination does not exactly vary between -23 and +23° every month, but these extreme values themselves can lie between +/-18° or +/-28°, depending on whether the inclination effect increases (23+5°) or decreases (23-5°) the effect of the motion along the ecliptic.

Lastly, the rise/set azimuths do not correlate to lunar phase, as do the rise/set times. Both phases and rise/set times are defined based on the Sun's position; we roughly define "noon" as the moment when the Sun is transiting in the south, and call the Moon "New" when it's in the same direction as the Sun, so there's no wonder that the New Moon transits around noon. This is not true for the effects described above, and hence the corresponding periods are different. The mean lunar period between twice the same equinox point (e.g. Aries) is called the Tropical month and lasts ~27.3d (this is roughly the same as the siderial month, but influenced by precession). The effect of the inclination of the Moon's orbit has a mean period of one Draconic month, with a period of ~27.2d. Instead, the Synodic month, the mean period between two cases of New Moon, lasts 29.5d, about 2.2d longer. Hence, the declination and rise/set azimuths of the Moon do not correlate with the phases.

human biology - Why do we age? or Do we have a theory of senescence?

There is a pretty good discussion on this topic in chapter 2 of Geriactric Medicine - An Evidence Based Approach (4th ed) by Cassel. This is the main reference for the info below which can hopefully add something to the answers already given.

In terms of views on ageing, there's evidence to support both:

• general principles that may apply to it; and


• it being a consequence of a collection of degenerative processes (this is apparently the more supported view).

Since almost all biological systems in the body degenerates with age and this happens seemingly at random, it's been difficult to identify particular catalysts that cause this. Consequently, biologists apparently steer away from a general theory or mechanism.

However, there are two classes of theories that have been floating around. That is 'loose cannon' and 'weak link'.

Loose Cannon encompasses theories that support the 'wear and tear' proposition. Two popular theories under this banner are free radicals and glucose.

Weak Link suggests that particular physiological systems are vulnerable during senescence and if a system fails, the whole body begins to decline. It's suggested that the neuroendocrine and immune systems are particularly vulnerable.

There is also a limit on the ability of cells to replicate - this is called the Hayflick Phenomenon (or limit). The reduction of the enzyme Telomerase, which lengthens telomeres during mitosis, is implicated in limiting a cell's ability to replicate indefinitely.

earth - Can I look at the sky and find the day of the week?

Edit (massive rewrite in response to comments)

There's nothing "out there" that will tell you what day of the week it is.
The concept of a week is determined by religion or culture but there's no objective reason why a week should have seven days. From 1793-1805 the French Republic had a calendar with a 10-day week.

If you wanted to try to extrapolate our idea of a week to other planets, be aware that the planets in our solar system have very different day lengths. One Venus day is 243 Earth days, while the planets Jupiter through Neptune all have days that are less than one Earth day.

It doesn't happen in our own solar system, but it's theoretically possible for a planet to have one side always facing its star. Then it would be impossible to define a day in the way that you've intended.

Even if you wanted to know what day of the year it is, that's possible but quite hard. In particular you would have to account for things like time zones, leap years, and daylight savings, which are not empirically observable phenomena. A few hundred years ago the definition of noon was when the sun was at its highest point in the sky. I think that definition is closer to what you're asking, but that's not the calendar system that we currently use.

Does the genetic expression of specific physical traits ever correlate to behaviour?

Over at skeptics, there were a couple questions asked as to the correlation of specific physical traits in relation to personality/behaviour. For instance, the simian line as well as red hair. Now, I did my best to answer these questions, with what I understand as there being a long history of people attempting to correlate external physical characteristics with behaviour (see the disgraced "art" of phrenology for instance).

However, in answering the simian line question, I did come across some weak correlations. It got me to wondering if this is an area of exploration in terms of perhaps one gene sequence for a physical trait somehow being tied with another gene sequence for behaviour.

So, do any physical characteristics being expressed through genes correlate to behaviour?