# Tag Info

11

Great question. 1) There is indirect (and circumstantial) evidence that they do merge. While there are some famous examples of apparently 'binary' (or more accurately 'dual') AGN (e.g. Komossa+2003, or Rodriguez+2006) there seems to be a very conspicuous dearth of such systems --- suggesting that they don't spend very long at observable separations. Note ...

1

I'd like to know what would happen if Venus was flung into a highly eccentric orbit like Sedna (except maybe with its current perihelion) with an orbital period measured in thousands of years. It's kind of a weird question but the first thing to consider is whether the orbit crosses any other planetary orbits, cause if it does, the biggest effect of ...

0

Can we transfer burn to another planet at any time? Yes... if we have a big enough rocket. The problem of figuring an orbit that gets you from point A to point B in a certain time is called Lambert's problem, and it turns out that there is a solution that is relatively easy to calculate—by which I mean that it does require solving equations ...

0

The quantity $$\ell=\frac{1}{n_{\mathrm e}\sigma_{sc}}.\tag1$$ is called the mean free path. It is the average distance a photon travels between two scattering events. To understand this definition, consider a photon travelling in the medium. When the photon travels a distance $x$, it will interact with a scatterer of cross-section $\sigma_{sc}$ if this ...

1

The probability of light getting to an optical depth $\tau$ is $\exp(-\tau)$. So the probability of it being (singly) scattered would $1 - \exp(-\tau)$.

0

I'm trying to determine the accuracy of a simulator (Universe Sandbox 2) with theoretical velocities The problem is that, excepting the situation of a natural or artificial satellite orbiting a celestial body, the theoretical velocity of the satellite has a quite complicated expression or an analytical solution does not exist which means that only the ...

2

The neutron star crust is separated into outer and inner regions. The outer is a crust of neutron-rich nuclei surrounded by degenerate electrons. The inner is similar, but the nuclei are even more neutron-rich and there are degenerate neutrons too. The (qualitative) answer to your question looks at the ratio of electrostatic (Coulomb) energy to the thermal ...

0

Zeroth Order Approximation In the simplest approximation, cold implies that $T_{e} = T_{i} = 0$, where $T_{s}$ is the average temperature of species $s$. There is an entire branch of plasma theory based upon this assumption. It is another way of saying that you assume the plasma is initially at rest with no thermal fluctuations. It also implies the ...

0

I've taught a few classes on stellar evolution and stellar modelling, and here are some of the resources I've recommended there. These all lean on the theoretical stellar structure side, rather than observational characteristics of certain types of star and so on. Free lecture notes Onno Pols (Utrecht) Jørgen Christensen-Dalsgaard (Aarhus) Both these ...

0

Speed of sound usually refers to the propagation velocity of a low amplitude vibration in a medium in equilibrium. A "sound" can travel far faster than the speed of sound if it is, for example, a shock wave

0

it's likely that when you google the term in general, most pages treat about the "speed of sound" at the common human meaning (sound in air :-) ), while its generalization to all the various circumpstances of fluids in the universe might more often been spelt "sound speed". But here it's more linguistic than physics, and it wouldn't be physically incorrect ...

27

They do! There's an entire class of galaxy, called a 'satellite galaxy' which is defined entirely based on them orbiting a larger galaxy (which would be called a 'central galaxy'). Our own milky-way is known to have many orbiting satellite galaxies, or at least 'dwarf-galaxies'. If dwarf-galaxies aren't enough, the milky-way itself is gravitationally ...

90

There are plenty of satellite galaxies orbiting larger galaxies. The question is how long are you willing to wait for an orbit? The Milky Way has a mass $M$ of something like $6\times10^{11}$ solar masses, or $10^{42}\ \mathrm{kg}$. The small Magellanic Cloud is at a distance $R$ of $2\times10^5$ light years, or $2\times10^{21}\ \mathrm{m}$. A test mass ...

0

You are correct, the surface of last scattering is different for different observers. Its distance is approximately 14,000 Mpc. For a given feature, The change in angle from one position to another is given by parallax, and for small angles is approximately $\Delta \theta=d/R$, where R is the distance to the surface and d the distance between vantage ...

0

The New Horizons spacecraft has an internal heat source. It is a radioactive material - plutonium. The best known celestial body is Earth. Its internal heat source are radioactive elements. Pluto is not as large and not as dense as Earth. But it also has a lot of rocks. Those rocks contain radioactive elements which generate heat.

4

Although not a complete answer, one place to start is with the coldest naturally occurring place in the universe, which is the Boomerang Nebula, a planetary nebula that is around 1 K. As best as I can tell, this cooled below the CMB temperature simply by adiabatic expansion, and is insulated in its interior from CMB heating. Is this a feasible way to get to ...

0

Look at the "bar" region of, e.g., NGC1300 and wonder whether you are seeing a neutrino star. If so it could be of greater mass than the visible galaxy so accounting for the linear g:r relationship in the bar region. If it is a Fermi condensate it could have a "liquid" surface at the tip of the bar with a lesser density tapering off in the spiral arms ...

1

I'm going to add another way to break up a neutron star. Shoot antimatter at it. The difficulty with breaking up a neutron stars is that, once they undergo the compression to become Neutron stars, their gravitation tends to keep them there. The minimum size for a Neutron star to form is about 1.2-1.5 solar masses, but once it's shrunk down, the mass it ...

2

You can get a rough idea from the virial theorem. This tells us that for a gravitationally bound system the kinetic energy $T$ and the potential energy $V$ are related by: $$2T = -V$$ or obviously: $$T = -\tfrac{1}{2}V$$ Suppose we start with our dust cloud particles at infinity with $T = V = 0$ and let the system collapse until the potential energy ...

0

The only likely mechanism by which a neutron star can break up is through a collision with another neutron star, in particular in binary neutron star mergers.

1

The neutron star is still "regular enough mater" for that it would react to anything a normal object would react. To me the point is more "since its center is not far to collapsing to blackhole, is it possible to shake (or breakup) a neutron star without making it collapse".

4

Do you mean anything in the real universe or just theoretically? If the latter, then I can think of a few phenomena: Heat: Just heat it up until the thermal velocity at the surface is greater than the escape volcity. Then neutrons will just fly off and it will evaporate (sublimate?). Spin: Wind it up until the tangential velocity at the equator reaches ...

3

For a uniform, spherical distribution of mass (cloud of gas and dust) of radius $R$ and mass $M$ in absence of magnetic, radiation fields etc, we have $dm = 4\pi \rho r^2 dr$ and the potential energy of a spherical shell of inner radius $r$ and outer $r + d r$ is $dU = -G\frac{m(r)dm}{r}$, $m(r) = \frac{4}{3}\rho r^3$, and a simple integration yields, ...

36

The answer lies in something called the virial theorem. You are correct, a cloud that is in equilibrium will have a relationship between the temperature and pressure in its interior and the gravitational "weight" pressing inwards. This relationship is encapsulated in the virial theorem, which says (ignoring complications like rotation and magnetic fields) ...

1

The secret is to evacuate the heat, mainly by radiation. But for this you need dust or "metals", since H and He alone radiates very unefficiently. Paradoxically it is not so easy to collapse completely enough. ( BTW for dark mater there is no possible radiation to dissipate energy, which keeps it fuzzy and a lot less concentrated than ordinary mater.)

13

As gas clouds collapse, they increase in internal energy (measured by temperature). This is part of what causes their pressure to increase. As they increase in temperature, though, they also increase the amount of radiation they emit. As they emit radiation, their internal energy decreases and thus their pressure also decreases, allowing for further ...

0

In agreement with the above answer, a good zero order "motivation" can be found in: http://www2.astro.psu.edu/~caryl/a480/lecture7_10.pdf. Since Tully-Fischer boils down to the mutual interaction between the baryonic mass of galaxies and the dark mass of their halos (star formation, angular momentum exchange, ...) there is no "derivation" in a textbook ...

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