Suppose that you have a probe orbting a $10M_{\odot}$ star in the final moments before gravitational collapse. In a time $t$ the collapse event occurs. So do you really would see all the matter "falling" abruptly? In other words, suppose you have a probe orbiting very close to the star, in the time $t$ (the time of the collapse event) would be possible to maneuverout the probe in sufficient time to leave the collapse or the event is so "fast" and abrupt that this wouldn't possible?

  • $\begingroup$ In Newtonian gravity, gravitational collapse of a ten-solar-mass star takes five hours, assuming no other forces are resisting. I don’t think it is significantly faster in General Relativity. $\endgroup$ – G. Smith Dec 26 '18 at 1:21
  • $\begingroup$ This might be better for astronomy.SE. $\endgroup$ – user4552 Dec 26 '18 at 1:23
  • $\begingroup$ Even if the collapse were extremely fast, and formed a black hole, if the probe was outside the star before the collapse, it would be far outside the horizon of the hole and could escape. $\endgroup$ – G. Smith Dec 26 '18 at 1:25
  • $\begingroup$ @G.Smith could you provide the specific reference of this calculation? $\endgroup$ – M.N.Raia Dec 26 '18 at 1:30
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    $\begingroup$ I was wrong about the five hours. It's actually 30 minutes, if the star has the same density as the Sun. (I knew that the Sun takes 30 minutes, and stupidly originally multiplied by 10. But all that matters is the density, not the mass or the radius.) $\endgroup$ – G. Smith Dec 26 '18 at 1:46

The collapse of the core of a $10 M_{\odot}$ star is triggered by electron capture in the core - which can be modelled as a high density sphere of ionised iron, with mass $\sim 1.2 M_{\odot}$, radius a less than the Earth's and a mean density of around $10^{12}$ kg/m$^{3}$.

The electron capture removes electrons (obviously), but these were the particles that contributed the degeneracy pressure that was mostly responsible for the support of the core. As a result, the collapse occurs in a time only slightly longer than the "gravitational free-fall" timescale of $(3\pi/32G\rho)^{1/2}$ in Newtonian mechanics (General Relativistic corrections are small, see Free fall time of collapsing non-rotating star in GR according to a distant observer ).

Thus the collapse timescale is less than a second.

However, note the density dependence. The collapse timescale is longer for less dense shells of material at larger radii. The net effect is an "inside-out" collapse, such that the core "decouples" from the envelope. Other than a severe burst of neutrinos and perhaps gravitational waves an observer outside the star would notice nothing going on for several hours. At that point a blast wave will send the outer layers outwards at 10% of the speed of light.

If you want a probe to survive it needs to have (a) a neutrino detector and (b) a very rapid turn of speed.


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