flare - What is "emission measure"?

Emission measure is (usually) used in X-ray and EUV astronomy, though I suppose also in cases of optically thin radio emission. It is defined as the square of the number density of free electrons integrated over the volume of the plasma.

The flux of optically thin emission from a plasma (e.g. thermal bremmstrahlung) is directly proportional to the emission measure of the plasma multiplied by a temperature dependent cooling loss law.

In other words, when you measure the flux of X-rays from an unresolved optically thin emitter, there is a degeneracy between the electron number density and the overall plasma volume.

When you fit an X-ray spectrum with an optically thin model, the emission measure (divided by $4pi d^2$, where $d$ is the distance to the object), is a multiplicative free parameter.

Your question about calculation is extremely difficult to answer. Suppose I measure a count-rate of $N$ X-ray counts per second using some X-ray telescope (I can only assume that's what you mean by a "X-ray light curve".).

The count-rate received at the telescope depends on: the emission measure (as defined above) multiplied by a term that depends on the temperature (or temperatures) of the source, the chemical composition of the source and the adopted emission process (is it free-free thermal bremsstrahlung, a thermal plasma or something else). It is then attenuated by any intrinsic absorption in the source and any absorption between us and the source and by the distance to the source (assuming the radiation is isotropic). Finally what is detected is determined by the response of the X-ray detector to X-ray photons as a function of energy.

Are there equal number of planets, stars, galaxies etc in observable universe spinning in both directions?

Just because we observed that our milky way galaxy is spinning in a certain direction therefore we assume it is applicable to all other galaxies, I am curious to find out if hypothetically most of the celestial objects such as natural satellites, planets, stars even galaxies within our observable universe are spinning/rotating in the same direction as the milky way what kind of implications can we say about the condition in the early universe? or is it just an coincident?

light - Sunsets: Mars/Earth - Astronomy

As you know, but other readers maybe don't, on Earth the sunlight is scattered by Rayleigh scattering on the molecules of the atmosphere. This has quite a strong wavelength dependence, with blue light being scattered much more efficiently than red light.

On Mars, there is almost no atmosphere. Instead, the Sun's light is scattered by the fine, red dust swirled up in the (thin) air. In the visible spectrum, the scattering properties of this dust happens to have an opposite wavelength dependence than that of the Earth's atmosphere, with a rather low scattering cross section in the blue, rising steeply toward longer wavelengths (e.g. Ockert-Bell et al. 1997).

Thus, the red light is scattered over the Martian sky, creating a red sky, while the blue light passes almost unhindered through, giving the blue Sun.

In addition, the asymmetry parameter of the dust is rather large, meaning that the blue light that is scattered has a preference for being scattered in the forward direction, i.e. toward the observer of the sunset (e.g. Vincendon et al. 2007). This creates a blue halo around the Sun as it sets.

PCR primer in highly repetitive region

This paper describes some PCR strategies with LINE and SINE PCR identification (Shedlock and Okada. SINE Insertions: powerful tools for molecular systematics. BioEssays (2000) 22:148-160.).

I have no experience with PCR amplification of SINEs or LINEs, however I can think of two strategies right now.

1) You may be able to find a unique 18-20 nt region flanking those regions. If you can, great. If not, perhaps there is a unique site somewhere in the middle of these regions, then you can amplify from the middle outward to both upstream and downstream regions.

2) If there are no unique sites that you can exploit, that would imply a highly regular (ie, repetitive) pattern, but you can take advantage of this too. You can pick primers such that they are on the extremes of the repetitive subunit within these structures. For example, if your sequence looks like this:

A------>BA------>BA------>BA------>BA------>B

Then you can pick primers A and B for PCR use. You will have non-specific primers, and therefore when you amplify the DNA, you will get all possible permutations of DNA that these primers will produce. Specifically, you will get:

A------>B

A------>BA------>B

A------>BA------>BA------>B

A------>BA------>BA------>BA------>B

A------>BA------>BA------>BA------>BA------>B

         A------>BA------>BA------>BA------>B

                  A------>BA------>BA------>B

         A------>BA------>BA------>B

and so forth.

This last approach is obviously inefficient, because your possible products scale exponentially, and will be produced with equal likelihood, minimizing the DNA yield of any particular segment.

fundamental astronomy - Converting ecliptical to equatorial coordinates

When looking for a formula to convert polar ecliptic geocentric coordinates of an object to equatorial coordinates I find various sources that give these formulae (like Wikipedia):

Declination $δ = arcsin(cos ε * sin β + sin ε * cos β * sin λ)$
Right ascension $α = arctan((cos ε * sin λ - sin ε * tan β) / cos λ)$

Where
β = ecliptic geocentric latitude
λ = ecliptic geocentric longitude
ε = obliquity of the ecliptic

But when applying these formulae I get results like in the following list where β = 0° and ε = 23.4°:

 λ       δ        α
  0   0.0000   0.0000
 45  16.3095  42.5443
 90  23.4000  90.0000
135  16.3095 -42.5443
180   0.0000  -0.0000
225 -16.3095  42.5443
270 -23.4000  90.0000
315 -16.3095 -42.5443
360  -0.0000  -0.0000

The values for declination seem good, but right ascension values seem to lack some sort of adjustment to the quadrant of the full circle (just a guess). But nowhere did I find any mentioning of this. Can you help? Thanks.

genomics - What are the limiting factors for gene length and number of exons?

This question drops firmly into the lap of molecular evolution and the constraints that are placed upon genes by the forces of mutation, selection, drift and recombination.

There are numerous situations, particularly gene duplication, that can result in a gene that is free from the selective constraints of it's parent, many of which will accumulate so many deleterious mutations as a result of stochastic processes that they will become non-functional e.g. psuedogenes. Some can be altered and rearranged, accumulating exons and introns, and if they infer a fitness benefit on the organism, may be moved to fixation within a population.

Evolution is a population genetics process, and there are many variables which can effect the outcome, not least the difference in populations size. The genomes of larger populations (such as those of bacteria) appear to have much smaller genomes, and of course no (at least not spliceosomal) introns, perhaps as a result of increased fitness due to the decreased generation time of an organism with a more slender genome. It would be a good idea to read The Origins of Genome Architecture by Michael Lynch, as I think he answers your questions, better than I can.

Many of the genes you retrieve from EnsEMBL will of course have experimental evidence to support them. The genes that are predicted in the pipeline can be looked upon with less confidence, but you can of course look at the alignments with closely related species to see if you think the introns/exons are indeed viable. An example of a gene with 79 exons is the Dystrophin (DMD) gene, the longest annotated gene at 2,217,347bp (see Roberts et al, 1993 and Nishio et al, 1994).

What is the minimum distance from city that allow to see Milky Way plane?

The biggest factor defining the minimum distance is light pollution from the particular city you are talking about. You can find light pollution maps here and here. You can also find charts here that describe the quality of viewing based on a combination of cloud cover, haze, turbulence and wind, temperature, light pollution, and other factors. They look like this:

the sun - relation between azimuth angle of the sun and angle of the shodow

Your diagram is correct, your maths isn't.

The bearing of the object's shadow is, as your diagram shows, away from the sun. If the bearing of the shadow is 75°, then the sun must have an azimuth of of 75°+180°, or 255°.

Similarly if the bearing of the shadow was 300° (pointing towards the northwest) the sun would be at an azimuth of 300 - 180°, or 120° (in the southeast)

spectroscopy - How do people derive ionized gas mass from optical emission lines in a galaxy spectrum?

Suppose you had an optical spectrum of a galaxy, and you unambiguously detected emission lines from cooling ionized gas -- both forbidden lines like [O II] 3727AA and [O III] 4959,5007AA, as well as recombination lines like H$beta$. So you have measured things like the integrated flux, equivalent width ($equiv$ integrated flux / local continuum flux density), FWHM, etc. for your emission lines. Now, you want to know how much warm ionized gas is in the galaxy (or at least in the region in which your spectrum was taken) -- i.e., you want to go from your aforementioned Gaussian parameters like integrated flux to a gas mass in solar masses. How do people typically do this, and what assumptions do you have to make? Any references would be greatly appreciated!

astrophysics - Is phys.org/space-news reliable source?

I found this page: http://phys.org/space-news/ . It seems to me pretty good page with interesting news and so on and I like that it is free. But I noticed in comments people arguing about the validity of the information and so on. Are they just "trolls" or is this page really bad source of information, if that is so, how much can I trust it?
Thanks in advance!

star - How can we hear the sound of the Sun?

Recently NASA has revealed that they have recorded the sound of Sun. They say that it produces a sound like "Om". I can't understand how they can hear it.

It's not that recent (it was 2010), it wasn't NASA (it was researchers at the University of Sheffield who used data from a NASA satellite), it wasn't sound per se (it was instead sonified data), and neither NASA nor those researchers said that the sound produced was "Om".

Data sonification is a scientific tool of growing importance. It is the sound equivalent of visualization techniques that have been used for centuries. (Graphs are a rather old visualization technique.)

In this case, researchers at the University of Sheffield used sonification techniques to translate imagery of solar coronal loops into sound. A faithful representation of that generated sound would not have been audible; the frequency would have been too low for a human to hear it. Those researchers had to speed up the playback to make the generated sound audible.

That's not to say that those sounds are a complete fiction. The surface of the Sun is not a vacuum, nor is its interior. There truly are low frequency sounds associated with events on the surface of the Sun. The Sun rings with sound (low frequency sound). Studying the sounds produced by the Sun gives scientists insights into what is happening inside the Sun.

There's a small downside in making scientific data public: Non-scientists who have a penchant to read/see/hear things into that scientific data will do just that. To me, the people who say that this is proof that the universe does indeed chant the sacred "Om" are no different that those who see cities on the Moon or Mars in NASA photography.

evolution - Is there a comprehensive database of fossils (with images) online?

No. Museums are the traditional repository for fossils, and the process of "digitizing" their collections is slow and labor intensive. Often, the museums only aim to digitize what me might call the "meta-data" associated with the fossil, as was done here:
http://ucmpdb.berkeley.edu/cgi/ucmp_query2?admin=&query_src=ucmp_index&table=ucmp2&spec_id=V8111&one=T

A truly comprehensive database is not feasible in the near future. A single photograph likely would not be sufficient to characterize the fossil -- the interesting components of fossils are often microscopic. For example:

http://www.pnas.org/content/99/14/9117.full.pdf

Even taking a single photograph can be very labor intensive, and fossils can be fragile. Proper photo-documentation would probably require multiple images. More generally, a comprehensive database would probably need to include non-photographic data relating to the fossil -- such as chemical composition of non-visual imaging techniques (X-ray, IR, UV, etc).

For the foreseeable future, "comprehensive" collections will be housed in museums without full digital representation. The only way to know how comprehensive these collections are is to ask the museum curator, who will be aware of the scope and limitations of the collection.

Is Planet Nine shepherding the Kuiper Belt?

Is it possible Planet Nine is shepherding the Kuiper Belt similar to the way Jupiter shepherds the asteroid belt?

I understand that very little is actually known about Planet Nine at this point, but has it been postulated that it has control over the Kuiper Belt? I don't exactly intend to read any theoretical papers on the existence of Planet Nine because I am sure it will be way over my head, but I understand that it is from the position of several Trans-Neptunian objects that its existence has any merit at all. So, I am wondering if part of that theory includes the Kuiper belt as a whole?

universe - What was "space" like before big bang?

This is a common question that comes up - one I've (at least tried to) answer a few times.

Let's try to understand what the Big Bang is. The Big Bang Theory is a model, and scientists came up with it when they observed the universe expanding - and thought that since the universe is now expanding it must have been smaller in the past. Running the clock backwards eventually means all of the universe was squeezed into a "singularity". Our physical models of the universe break down at this point - so we are not too sure of what the universe was like at the Big Bang.

Notice that I said the universe was squeezed into a singularity - not the stuff inside the universe. This means space-time was also compressed into a single point. The universe is not expanding out into empty space - it is space-time itself that is expanding. Hence, there was no "before" the Big Bang - time (as well as space) was created when the Big Bang happened. There is no outside the universe! This means there is no outside the Big Bang!

You can't go back in time and watch the Big Bang - since there is no universe or space (or time) outside the singularity. This can be very confusing, but I hope this helps!

For a few places to look for more info:

https://en.wikipedia.org/wiki/Planck_epoch

https://en.wikipedia.org/wiki/Big_Bang#Singularity

amateur observing - Astronomical telescope making

The first question anyone asks about a telescope is "what is the magnification?" It is almost always not the most important thing. Any telescope can magnify a million times, given a short enough eyepiece - the problem is, how good the image is.

For observing planets, the main thing is resolving power - the ability of the telescope to discern tiny details. The resolving power is limited by aperture, or the diameter of the primary lens or mirror. If the primary lens diameter is measured in mm, and the resolving power in arcseconds, then the limit is:

resolving power = 100 / aperture

Your 50 mm diameter lens would have a resolving power limited to 2 arcseconds in ideal conditions. Any details smaller than that would be blurry, no matter how much magnification you put into that thing. For comparison, Jupiter's apparent diameter is between 30 and 50 arcsec, Mars' between 3.5 and 25 arcsec, Saturn's between 14 and 20 (without the rings).

With that aperture you would see the rings of Saturn, and a couple equatorial belts on Jupiter. Venus shows a crescent when it's far from the Sun. Mars is a little orange disk when it's closest to Earth. I've looked at planets through 50 mm of aperture, and you can see quite a few things that way.

The Moon also looks interesting, you can see craters and mountains, plus it's available every month.

---

Because resolving power is limited by aperture, as you give it more and more magnification, at some point the image is just huge and bloated; big, but blurry. So there's a limit to the useful magnification - again, this depends on aperture. If the aperture is measured in mm, the formula is:

maximum magnification = 2 * aperture

Your 50 mm aperture would support up to 100x magnification before things get too blurry.

So how do you calculate magnification for a given instrument? It is the ratio between the focal length of the primary lens (or mirror) and the focal length of the eyepiece (the lens close to your eye):

M = F / f

For your 1000 mm focal length primary, you achieve 100x magnification with an eyepiece with 10 mm focal length. Anything shorter than that would just bloat the image uselessly. Eyepiece focal lengths that would make sense in your instrument would be between 10 mm and 120 mm.

---

Words of advice:

Make sure the lenses are perfectly aligned. When you build the instrument, have some way to adjust the direction of the primary lens, so as to align it with the main axis of the instrument. Some tweaking will be required - tilt the primary a fraction of mm, check the results, tilt again, repeat until the image looks best. This is called collimation and it's very important for the overall performance of the instrument.

Have some way for the eyepiece to move back and forth until you achieve perfect focus. This will have to be re-adjusted every time you observe. The adjustment is quite sensitive. A real focuser would help a lot, but this could be achieved also with two tubes sliding into each other, held in place by friction, like in an old pirate captain's telescope.

For astronomy, when the scope is focused at infinity, the distance between primary lens and eyepiece is the sum of their focal lengths. E.g. with a 1000 mm primary and a 10 mm eyepiece, the distance would be 1010 mm. This assumes a convergent eyepiece.

The telescope will not work well hand-held. The image will be jumping around too much. You will need some kind of mount. At the very least put the top of the tube on a fence or something. But astronomical instruments work best when they are on a rock solid mount. Even a broomstick hammered into the ground, with a loop on top for the tube, is better than nothing.

Whichever way you mount the primary lens, don't squeeze it too hard in the lens cell or tube. It will deform the lens and make the image bad. Some pressure on the edge is okay; lots and lots of pressure not okay. You could glue the edge of the lens on some kind of ring. Just use common sense.

This is totally doable. I've built a telescope just like that, many years ago. It's great to see what you can achieve on your own.

Good luck!

observation - Moon SHAPE calculator

I have seen a lot of information about moon phase calculation, but need an accurate moon shape calculator/algorithm given the observer's position and time. This implies that the illuminated portion may be rotated, and not only have a right-left orientation as traditionally depicted.

Eclipse Cycle Calculations Needed - Astronomy

NOTE: I am using a "geocentric" frame of reference, where both the
moons and the sun orbit the planet, and am creating an arbitrary xy
coordinate system.

We note from @Hohmannfan's answer that (answering your questions out
of order for simplicity):

• Moon B will eclipse the sun every $frac{10385}{304}$ (~ 34.16)
  days. In this time period, the sun completes $frac{31}{304}$th of
  an orbit and Moon B completes $1frac{31}{304}$ orbits, lapping the
  sun once.




• Moon A will eclipse the sun every $frac{26130}{257}$ (~ 101.67)
  days. The sun will complete $frac{78}{257}$ of an orbit, and Moon A
  will lap it by completing $1frac{78}{257}$ orbits.




• Moon B will overlap Moon A once every $frac{2418}{47}$ (~ 51.44)
  days, in which Moon A will complete $frac{31}{47}$ of an orbit and
  Moon B will lap it by completing $1frac{31}{47}$ orbits.

However, as @Hohmannfan notes, there's no guarantee that the moons
will be full when they overlap.

There's also no guarantee that the two moons will ever both eclipse
the sun at the exact same time, although they will get arbitrarily
close to doing so:

In the $frac{2418}{47}$ days between two successive lunar overlaps, the sun
moves $frac{2418}{47} times frac{1}{335}$ of an orbit.

As above, the moons have advanced $frac{31}{47}$ of an orbit.

Thus, compared to the sun, the moons have advanced $frac{31}{47} -
frac{2418}{47} times frac{1}{335}$ or $frac{7967}{15745}$ of an
orbit (this number is surprisingly close to $frac{1}{2}$ but that's
just a coincidence).

This happens between every pair of overlaps, so the sun's angular
distance (in orbits) from the overlapping moons is $frac{7967
n}{15745}+r$ where $r$ is the angular distance at a specific overlap
and $n$ is any integer.

For the overlapping moons to eclipse the sun $frac{7967 n}{15745}+r$
must be an integer. If $r$ is irrational, this can never happen.

However, the angular distance can get arbitrarily small, even to the
point where an observer wouldn't realize the double moon eclipse isn't
100% perfect.

By a similar argument, you can show the two full moons will get
arbitrarily close to overlapping.

NOW, if we make the simplifying assumption that both moons are
eclipsing the sun at year 0 (perhaps your astronomer-priests have
decided this unusual occurence is a good time to start numbering the
years, and believe zero (not one) is a good first year), we can make
some other calculations.

Since the moons line up every $frac{2418}{47}$ days and the sun and
Moon B line up every $frac{10385}{304}$ days, all three will line up
(to form a double moon eclipse of the sun) on the least common
multiple of these numbers, or 810,030 days (which would be exactly
2418 of your years, and note that 2418 is the product of the two lunar
orbits in days). In this time:

• Moon A will have completed exactly 10,385 orbits.




• Moon B will have completed exactly 26,130 orbits.




• As above, the sun will have completed exactly 2,418 orbits.

As it turns out, there can never be a perfect double full moon bullseye:

• Moon B will be full on day $frac{10385}{608}$ (~ 17.08), at which
  point it will have completed $frac{335}{608}$ of an orbit and the
  sun will have completed $frac{31}{608}$ of an orbit, so Moon B will
  have gained half an orbit on the sun, which is required for a full
  moon. After that, the moon will be full every $frac{10385}{304}$
  days, the period of time it takes the sun to complete
  $frac{31}{304}$ orbits, and Moon B to complete $1frac{31}{304}$
  orbits.




• By similar calculation, Moon A will be full on day
  $frac{13065}{257}$ (~ 50.84) and every $frac{26130}{257}$ days
  thereafter.




• To find when they're both full at the same time, we solve this
  linear Diophantine equation:

$frac{10385 n}{304}+frac{10385}{608}=frac{26130 m}{257}+frac{13065}{257}$

where n and m are integers. This reduces to:

$nto frac{47424 m+15745}{15934}$

Unfortunately, $47424 m$ is always even, so $47424 m+15745$ is always
odd. Since the denominator ($15934$) is even, you are dividing an odd
number by an even number, and the result can never be an integer.

However, this doesn't tell the full story. For example, if we compute
the positions on day $frac{34987576465283}{92766720}$ (~ 377156.55),
we find:

• Moon B is at 122.5656 degrees.




• Moon A is at 122.5581 degrees, only ~ 27 arcseconds away.




• The sun is at 302.5658 degrees, 179.9998 degrees from moon B, and
  179.9924 degrees from moon A (~ 28 arcseconds from opposition).

In other words, this is pretty close to a double full moon, even
though it's not exact.

In a similar vein, even though double solar eclipses only occur once
every 810,030 days, there are several close calls:

$
begin{array}{cc}
text{Day} & text{Sep (')} \
-810030.00000 & 0.00 \
-754313.10860 & 0.91 \
-698596.21710 & 1.82 \
-642879.32570 & 2.73 \
-587162.43420 & 3.64 \
-531445.54280 & 4.55 \
-475728.65130 & 5.47 \
-445735.13160 & 7.29 \
-420011.75990 & 6.38 \
-390018.24010 & 6.38 \
-364294.86840 & 7.29 \
-334301.34870 & 5.47 \
-278584.45720 & 4.55 \
-222867.56580 & 3.64 \
-167150.67430 & 2.73 \
-111433.78290 & 1.82 \
-55716.89145 & 0.91 \
0.00000 & 0.00 \
55716.89145 & 0.91 \
111433.78290 & 1.82 \
167150.67430 & 2.73 \
222867.56580 & 3.64 \
278584.45720 & 4.55 \
334301.34870 & 5.47 \
364294.86840 & 7.29 \
390018.24010 & 6.38 \
420011.75990 & 6.38 \
445735.13160 & 7.29 \
475728.65130 & 5.47 \
531445.54280 & 4.55 \
587162.43420 & 3.64 \
642879.32570 & 2.73 \
698596.21710 & 1.82 \
754313.10860 & 0.91 \
810030.00000 & 0.00 \
end{array}
$

The table above lists all near-eclipses within 7.5 minutes of arc,
where day is the number of days from year 0 (including days before
year 0), and sep is the maximum separation (in minutes of arc) of any
two of Moon A, Moon B, and the sun. Note that days $0$ and $pm
810030$ are perfect eclipses, as expected.

Similarly, the closest we get to double full moons is below. In this
case, sep is (in minutes of arc) the maximum of:

• the angular distance of Moon A from opposition




• the angular distance of Moon B from opposition




• the angular distance between Moon A and Moon B

$
begin{array}{cc}
text{Day} & text{Sep (')} \
-797168.29790 & 10.29 \
-767174.80850 & 8.92 \
-711457.91490 & 7.55 \
-655741.02130 & 6.17 \
-600024.12770 & 4.80 \
-544307.23400 & 3.43 \
-488590.34040 & 2.06 \
-432873.44680 & 0.69 \
-377156.55320 & 0.69 \
-321439.65960 & 2.06 \
-265722.76600 & 3.43 \
-210005.87230 & 4.80 \
-154288.97870 & 6.17 \
-98572.08511 & 7.55 \
-42855.19149 & 8.92 \
-12861.70213 & 10.29 \
12861.70213 & 10.29 \
42855.19149 & 8.92 \
98572.08511 & 7.55 \
154288.97870 & 6.17 \
210005.87230 & 4.80 \
265722.76600 & 3.43 \
321439.65960 & 2.06 \
377156.55320 & 0.69 \
432873.44680 & 0.69 \
488590.34040 & 2.06 \
544307.23400 & 3.43 \
600024.12770 & 4.80 \
655741.02130 & 6.17 \
711457.91490 & 7.55 \
767174.80850 & 8.92 \
797168.29790 & 10.29 \
end{array}
$

Other notes:

• Even though you said this was fiction, note that it's highly
  unlikely that the moons' orbital period will be an exact multiple of
  the planets day. The only exception to this is if the moon(s) are
  tidally locked, in which case the orbital period will equal exactly
  one day.




• Similarly, it's unlikely the planet's orbital period would be an
  exact multiple of its rotation period (ours certainly isn't).

This is an interesting problem in general, and I am writing
https://github.com/barrycarter/bcapps/blob/master/MATHEMATICA/bc-orrery.m
to solve a similar problem:
http://physics.stackexchange.com/questions/197481/

How does a cell sense its size?

This is a question that has been the focus of study for the last century (e.g., Amodel of cell size regulation - Ycas et al, J. Theoret. Biol. (1965) 9, 444-470). Cell size regulation may be in part determined by ribosomal activity (through mTor regulation) and is a critical checkpoint in cell division.

How the cell senses its size, however, is not understood. In 2009, two reports suggested that protein gradients could be responsible for the sensing of cell size. You can read a commentary about those articles in Cell size control: governed by a spatial gradient. - Almeyda and Tyers, Dev. Cell. (2009) 17(1), 3-4:

The phenomenon of cell size homeostasis, whereby cells coordinate
growth and division to maintain a uniform cell size, has been an
outstanding issue in cell biology for many decades. Two recent studies
in Nature in fission yeast demonstrate that a gradient of the polarity
factor Pom1 is a sensor of cell length that determines the onset of
Cdc2 activation and mitosis.

These articles demonstrate one way in which cells may sense their size, but most probably several other mechanisms are also in place.

What types of meteorites can I use for making a wedding ring?

Iron-nickel meteorites are made of metal alloys, either kamacite, or taenite.

But I'm a little sceptical about whether it's a good idea to make a wedding ring of it, since it may oxidize over time, and it may contain toxic elements like cobalt, or it may cause nickel allergy.

It's probably better not to use it for direct contact with skin, or humidity.
However, etching it to reveal Widmanstätten patterns may look good.
Better protect it from skin and air, e.g. by melting it into something like a glass containment. Account for different coefficients of thermal expansion between meteoric material and containment.

Is it possible to get the distance to a star in the IPHAS DR2 catalog?

Only by knowing what kind of a star it is, or by inferring that information from the available colours. From there you use a photometric or spectroscopic parallax calibration. ie. There is a relationship between the absolute magnitude and colours of a main sequence star. Of course there is no guarantee you are looking at a main sequence star.

You can get more idea by cross correlating with the 2MASS JHK catalogue, as the J-H vs H-K colours of giants are usually distinctive.

More specific help needs a more specific question.

gravitational waves - How to compensate the effect of tectonic activity in devices like LIGO?

In late 2015, the LIGO project team announced the detection of gravitational waves. The detector is (very basically) a laser measuring the distance between two mirrors in vacuum, over a large-enough distance to detect very small changes.

The two mirrors are installed on Earth, and a couple kilometers apart.

I believe, perhaps mistakenly, that tectonic activities (if only vibrations) may change the mirror distance over time, so the LIGO project has probably used some model and technology to nullify their effects.

How can that be done? If there is any mistake there, I wonder about how to validate the experiment results.

history - Why was the size of the solar system not defined by Mercury Transits?

It is indeed possible to measure the AU using transits of Mercury, and Edmund Halley tried to do just that in 1677. However, there are two advantages to a transit of Venus. The first is that during the transit, Venus is only 0.28 AU from Earth, whereas Mercury is about 0.7 AU away. This makes the parallactic effect twice as big. The second difficulty is that you need to measure the time between second contact and third contact (i.e., the first and last time that the planet is entirely in front of the Sun). Mercury is so small that it is hard to determine when second and third contact are. (In fact the limiting effect for the measurement of the AU during the transit of Venus was the so-called "black drop effect", which made it impossible to measure the times of second and third contact to a precision less than about ten seconds.)

A former professor of mine from when I was in graduate school has a really excellent writeup of the history of these measurements:

http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit4/venussun.html

supernova - What could be a star-like object that shines at daytime for few seconds? What's the probability to see it?

Several satellites (but most notably Iridium) have large reflective panels. If aligned with the sun properly, it can shine with sufficient brightness to be visible in the daytime.

An Iridium Flare usually lasts only a few seconds. It sounds quite consistent with your description.

It can be hard to tell in a blue sky with no nearby references, but an Iridium satellite would also be moving while fading out.

There are sites that can show you upcoming flares for your area. If you knew the exact date in the past, they could probably even "post-dict" what flares were available then and see if it matched.

epidemiology - What data / tools exist for mapping of disease trends?

You can use official sources such as hospital admissions, prescriptions for drugs fighting the disease you are tracking, sales of over-the-counter medicines.
CDC (cdc.gov/flu/weekly/, cdc.gov/outbreaknet/outbreaks.html, etc), WHO, EuroFlu Weekly Electronic Bulletin map official clinical data, Aurametrix uses these sources.

Several scientific studies have compared GFT, twitter, even facebook with official sources. Johns Hopkins study (pubmed/22230244) used data from local hospitals in addition to CDC.
My sampling studies too showed that social trends are surprisingly good, despite all the noise, but provide good estimates only for highly populated areas.

You might also want to check Sickweather's algorithm, healthmap, usgs disease maps and industry databases providing coverage of pharmaceutical companies and product sales.

Data analysis projects fora newly renovated desktop?

I just renovated a desktop that had been in storage for a while, and I'd like to come up with some big data analysis projects to run on it in the background. Does anyone have any ideas, know of any data sets, or algorithms or processes that could be cool to check out?

Thank you!

cytogenetics - Finding the number of chromosomes of an organism

Answer

An comprehensive online database of the chromosome numbers of all living species most likely doesn't exist. This Wikipedia article is the best and most complete reference comprising animals that I can personally find on the internet.

This source in Spanish, which I've translated with Google Translate reads:

Canedo Delgado (1999) performed the karyotype description of the three species of the genus Pygoscelis , noting that there is a high homology both numerically and morphologically: Pygoscelis antarctica presented 2n = 92, Pygoscelis papua 2n = 94 and Pygoscelis adeliae 2n = 95 in females and 2n = 96 in males.

Also, note that chromosome number cannot be "guessed" or "assumed" for closely-related species. As shown above, the three Pygoscelis species have a high homology, but have very different chromosome numbers.

The other chromosome number you found for the species (38) is actually that of the Emperor penguin, as a simple Google search shows.

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References

• Ledesma, Mario A., T. R. O. Freitas, J. Da Silva, Fernanda Da Silva, and R. J. Gunski. “Descripción Cariotípica De Spheniscus Magellanicus (Spheniscidae).” El Hornero 18, no. 1 (August 2003): 61–64.

redshift - Why Milky Way and Andromeda are being drawn together if there was 'Big Bang'?

The evidence for expansion is that the redshift is proportional to distance.

The redshift of a galaxy can be divided into two components: that due to the cosmological expansion, which stretches the wavelength of light whilst it travels towards us; and a peculiar motion with respect to the cosmological expansion, which causes a straightforward doppler shift.

The former term increases with distance - this is known as Hubble's law. The redshift here is always positive (ie always a stretching of wavelength). It is this that tells us the universe is expanding. The latter term is caused by the gravitational effects of other nearby galaxies and clusters of galaxies on the galaxy in question. It is typically of order a few hundred km/s and can be positive or negative (ie it can cause a redshift or blueshift).

Redshift due to cosmological expansion completely dominates at distances greater than a few hundred million light years. Up till then (and Andromeda is only 2 million light years distant), peculiar motions can result in blueshifts for some galaxies.