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: