# Tag Info

## Hot answers tagged astrophysics

9

There are several reasons to believe that dark matter is a particle. The most widely accepted alternative explanations for the different phenomena that led us to conjecture dark matter in the first place, can collectively be labeled "we don't understand gravity well enough". But no matter what, the effects of dark matter are sort of "localized". The ...

7

Yes, the exact solution is known. The general spherically symmetric metric is $$g=-B(r)\mathrm{d}t^2+A(r)\mathrm{d}r^2+r^2\mathrm{d}\Omega^2.$$ The solution for $A(r)$ is $$A(r)=\left[1-\frac{2G\mathcal{M}(r)}{r}\right]^{-1},\quad\mathcal{M}(r)=\int^r \rho \,\mathrm{d}V=\int_0^r 4\pi r'^2\rho(r')\,\mathrm{d}r.$$ The solution for $B(r)$ is ...

6

I like to explain this using a figure from a talk by Marco Limongi some years ago. Based on a given set of models, the $x$-axis shows the initial mass of the models and the $y$-axis the final mass. The different coloured layers show the composition of the star at the moment of collapse. The mass ejected in the supernova is the difference between the curve ...

6

How opaque is that -- would we be able to see a couple of meters, some kilometers, or nothing at all? The photosphere of our sun is somewhere on the order of 500 km thick. For a quick ballpark, you can imagine an exponential decrease in the transmission of light which about this characteristic thickness. It might be a little less, but it's still ...

5

The combined rest mass of a proton and an electron is less than that of a neutron. Fundamentally then, what you need to start turning a star into a neutron star is that the protons and electrons need kinetic energy as well as rest mass energy. How much energy: Well at a minimum (assuming the neutrino doesn't get much), then an electron interacting with a ...

5

A paper came out this week pointing to them having a banal (if amusing) origin: they are from two 27 year old microwave ovens. When people get impatient and open the door before the timer runs down, a short burst from the ovens' magnetron is released, which appears as a peryton if the telescope is pointed in the right direction. Figure 7. shows the perytons ...

4

I got a translation of the article from the German Wikipedia. Here's an excerpt: Perytons are in radio astronomy short radio signals having a length of a few milliseconds, which probably terrestrial are origin. The Perytons are named after mythical creatures . In radio astronomy, terrestrial are noise is always a problem. A well-known noise signal ...

3

The amount of energy liberated per gram of material per second in the fusion reactions depends on the density, the mass fraction (hydrogen, $X$, helium, $Y$, and all others $Z$) and temperature: $$\epsilon = \epsilon(\rho,X, Y, Z, T)$$ Typically we express the energy generation rate as a power law, $$\epsilon\propto\rho^\alpha T^\delta.$$ though the ...

3

The best paper on I've found on tidal tails is Reshetnikov & Sotnikova (2000). Their simple description of tidal tail formation is: To understand the development of tidal tails, one must recall how the water surface of the oceans get stretched radially by differential gravitational attraction exerted on it by our Moon. The differential forces ...

3

Mass of Sun = 1.989 x 10^30 Kg Mass of 1 lion = 190 Kg Mass of 1 lion in spacesuit = 250 Kg Mass of 1 trillion lions = 190 x 10^12 Kg Mass of 1 trillion lions in spacesuit = 250 x 10^12 Kg Mass of 1 trillion pregnant lioness in spacesuit = 300 X 10^12 Kg Temperature of Sun's surface = 5778 K Any objects would instantly disintegrated at close ...

3

The gas outside the white dwarf (and actually, in the interior of the white dwarf but close to the surface) is non-degenerate. So I don't see what the presence of the white dwarf has to do with the question unless it is significantly photo-ionising the cloud? Anyway for spherically symmetric accretion onto an object, one approach is to make an assumption ...

3

Short answer: it is a combination of (1) the ignition occurring in an electron-degenerate, isothermal core in which the equation of state is independent of temperature; and (2) the extreme temperature dependence of the triple alpha He fusion reaction. Details: The helium flash occurs at the tip of the first ascent red giant branch in stars with masses ...

3

The latter idea you talk about is the Giant Impact Hypothesis. It turns out that you can make a moon in a few easy steps (given the correct conditions): Have a bunch of protoplanets whiz about on semi-chaotic trajectories. Smack two of them together at a 45° angle. Let the bits of the protoplanets that don't merge together be ejected from the resulting ...

3

Helium is chemically inert, but in the conditions present in the core of a star or on the surface of an accreting white dwarf helium is prone to fusion. The helium is degenerate, which means that the structure of the Helium core/white dwarf is not being supported by temperature, which means the energy produced during fusion does not cause the core to expand ...

2

The discrepancy between the predicted big bang nucleosynthetic abundance of Lithium 7 and the measured value can be summarised as follows. If we take what we know about the the baryonic mass density of the universe and the Hubble constant, we get a self-consistent picture between the cosmic microwave background, observations of galaxy recession etc. and the ...

2

The Pauli exclusion principle is being applied here to FREE neutrons. There are always free energy/momentum states for the neutrons to fill, even if they are compressed to ultra-high densities; these free states just have higher and higher energies (and momenta). One way of thiking about this is in terms of the uncertainty principle. Each quantum state ...

2

Well, I can give you a definitive answer to Q1, but my answer to Q2 would only be educated speculation. Perhaps some of the astrophysicists on here can be more help with that one. However, before I tackle Q1, a very important disclaimer: Temperature is a measure of the average kinetic energy of the particles of an object, and cannot be used all by itself ...

2

As Ben Goldacre says, "I'm afraid it's a bit more complicated than that." Phil Plait (Bad Astronomy) has occasionally written about this. Astronomers don't have a firm definition for the 'border' of a galaxy, tho' certainly objects classified as belonging should show some sort of contained orbit. But it gets worse, as they also have rough categories of ...

2

I had the same feeling as you when I watched the video again recently. It seemed like one of the ice giants would get ejected after coming too close to Jupiter. It turns out that there's a name for this: the jumping Jupiter scenario. Outside Wikipedia, it's described in Fassett & Minton (2013) (paywall!) and tangentially in Deienno & Nesvorny (2014). ...

2

No, because we've seen it happen between galaxies, where there's not enough intergalactic material to account for this. Here's an example: the galaxy cluster Abell 2744: Then there's Einstein's cross: You need something extremely massive to account for the lensing shown here. Gravitational lensing can accurately predict these images, and the ...

2

For a conventional planet (i.e. one that is self gravitationally round), conservation of the planet's angular momentum makes this impossible (except for the trivial case of the axis perpendicular to the orbital plane. A non-spherical planet with a tilted axis will precess under the influence of tidal forces. It takes the earth about 26,000 years to go ...

1

How then, can they collapse, without violating the Pauli Exclusion Principle. At a certain point does it no longer apply? No. The Pauli Exclusion provides a "degeneracy pressure" as mentioned in the article. That degeneracy pressure is not great enough to stop the collapse in the case of a black hole. This isn't violating the Pauli Exclusion ...

1

This paper describes them. http://www.ursi.org/proceedings/procGA11/ursi/GP2-41.pdf They were apparently given a new name because their origin was uncertain.

1

This might not be a good explanation, but the gist of it is, as stars get hotter, even though they get less dense over time, the extra heat speeds up the fusion process. The helium fusion can only happen at about 100 million degrees. The temperature at the core of our sun is 15 million degrees - new energy is created all the time by hydrogen and ...

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