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2

It is more likely that the non-detection would be associated with statistics than SNR's not accelerating protons. Fermi-LAT has already shown that $\gamma$-ray emissions from 4 galactic supernova remnants (with molecular clouds nearby them) are coming from proton-proton collisions leading to neutral pions ($pp\to pp\pi^0$, $\pi^0\to2\gamma$): (source, ...


2

I think the following image, which comes from Tomczak et al. (2014) and the so-called ZFOURGE/CANDELS galaxy survey should do the trick. It shows how the galaxy stellar mass function (i.e. the number of galaxies per unit mass per cubic megaparsec that have a certain stellar mass) evolves as a function of redshift. As you might imagine this is not just a ...


10

The most compelling evidence of GR in presence of matter is, in my opinion, in neutron stars. These objects have a surface gravity $SG$ that is (geometric units): $SG_{NS}=GM/c^2 R \simeq 0.1 $ This value is telling us that we can't use Newtonian gravity because we are in the strong field limit. For comparison, the sun has $SG_{SUN}=GM/c^2 R \simeq 10^{-5} ...


0

The field equations of general relativity give rise to a phenomenon called gravitomagnetism, which is related to "monopole" gravity in the same way that magnetism is related to moving electric charges by special relativity. There is conclusive evidence for gravitomagnetism just in the past five years, weakly from the Gravity Probe B mission, and more ...


6

Frame-dragging effects are dependent on the spin of the central object, have been measured by experiments such as Gravity Probe B, and are definitely not dependent on the central metric. Also, any effects on a galactic scale are best quantified in terms of a continuous matter distribution, since the central black hole is a small fraction of the galaxy's ...


1

Its complicated. There wasn't just one Hubble Ultra Deep Field (UDF) observation, but several, taken at different times, with different instruments, but pointed in (almost) the same direction. The first image you refer to was taken with the WFC Infra red camera in 2009, in near infrared bands (1-1.6 microns). The area covered by this camera is $2.4$ ...


4

Your method is correct. When the angle is "small" so that we can ignore curvature, then the rectangular solid angle is just the product of the two side angles. (This doesn't hold for "large" angles). I cannot find a perfect explanation, but one source of this confusion may be that the UDF was imaged by two separate instruments, one optical, one ...


3

The answer is that 41% of the stars have masses below 0.25$M_{\odot}$. To check this I integrated the Kroupa initial mass function. This is that $N(m)$ the number of stars per unit mass is proportional to $m^{-1.3}$ for $0.08<m/M_{\odot}<0.5$ and proportional to $m^{-2.3}$ for higher masses. If I integrate this I find that the ratio of stars with ...


6

The stellar mass distribution is the distribution of numbers of stars within a range of masses in a galaxy (or cluster or what have you), not the mass of the stars. So if you looked at the $\sim10^{11}$ stars in the galaxy, you would observe that about $4\times10^{10}$ of them will have a mass less than 0.25 $M_\odot$, and so on with the rest of the masses. ...


3

According to this source, 100% is the number of stars, not the total mass. Same from another source. The reason is that they usually calculate these pies straight from the H-R diagram. The H-R diagram plots individual stars and shows how stellar mass varies along the main sequence. Actually the mass distribution tends to reverse. Even if larger stars are ...


0

To date there is nothing published (and serious) that makes a confirmed detection of dark matter particles. Thus, the only evidence in favor of its existence remains from indirect methods: calculate the mass that should be there based on visible sources (stars, galactic powder, etc), and use this mass to calculate the speed of stars about the galaxy as the ...


3

There are several commonly used analytic approximations for the initial (birth) mass function (IMF) that cover both stars and brown dwarfs. It is not yet absolutely certain which of these is more correct at the low-mass end, whether there is a lower mass cut-off as one approaches planetary masses, or whether the fraction of brown dwarfs (BDs) to stars varies ...


6

It turns out that it is the distribution of birth stellar masses and most importantly, the lifetimes of stars as a function of mass that are responsible for your result. Let's fix the number of stars at 200 billion. Then let's assume they follow the "Salpeter birth mass function" so that $n(M) \propto M^{-2.3}$ (where $M$ is in solar masses) for $M>0.1$ ...


2

Yes, your qualitative argument is correct and the number of stars brighter than the Sun is almost certainly much smaller than 10 billion. The reason is that the luminosities are hugely variable. Due to the mass-luminosity relation, each doubling of the stellar mass corresponds to increasing the luminosity 10 times. So many if not most of the "stars ...


0

Age of universe is roughly 13.8 billion years, so anything with longest traveling record can only travel 13.8 billion light years at most (in any direction; not necessarily to Earth) because nothing can travel faster than light. Now, we are at the center of the observable universe (which is spherical with visible radius 13.8 billion years), so longest ...


0

A community Wiki answer recording someone else's (Ben Crowell's) comment that I think is worthy of being a permanent answer. The proper length is zero, because the photon's path is lightlike. If you want to define the photon's "odometer," you can't do it in the photon's frame, because a photon doesn't have a rest frame. The 13.7Gyr is measured on a clock ...



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