I've seen in a documentary that when a star collapses and becomes a black hole, it starts to eat the planets around.
But it has the same mass, so how does its gravitational field strength increase?
Astrophysics – Does a Star’s Gravitational Field Get Stronger when It Becomes a Black Hole?
black-holesgravitystarsstellar-evolution
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No. And maybe, but probably not.
General Relativity tells us that time passes more slowly in an intense gravitational field (from the point of view of someone in a less intense region of the field). This has been verified experimentally on a small scale.
Imagine you're near (but not too near) a black hole, in communication with a probe that's dropping into it. As you observe an analog clock on the probe, you'll see its second hand moving more and more slowly as the probe approaches the event horizon, coming closer and closer to a dead stop.
You'll never actually see the probe reach the event horizon; time on the probe, as seen from an outside vantage point, will come arbitrarily close to the moment it reaches it, but it will never actually get there.
The same thing happens to any matter falling into a black hole. It will quickly reach a point where you can no longer see it, but it will never quite reach the actual event horizon.
Inside the event horizon, time passes infinitely slowly (again, this is all from an outside point of view). So once the event horizon forms during the initial collapse that forms the black hole, nothing further happens inside it. Everything inside the event horizon is frozen in time, and cannot collapse any further.
On the other hand, from the point of view of the probe itself (say, if you foolishly volunteered to ride along), local time continues to pass at its normal rate of 1 second per second. You'll pass through the event horizon, and if the black hole is massive enough for the tidal stress at that point to be manageable, you might not even notice. You might see events, including further collapse, continue to occur after you're inside. But if you look back as you're falling, you'll see time in the outside universe pass more and more quickly, and just as you cross the event horizon you'll see eternity pass in a finite amount of time. So from the point of view of a hypothetical observer who's fallen into a black hole, yes, the body that formed the black hole can continue to collapse -- but any such observer might as well be outside the universe, since there's no way to communicate with them.
All this is based on relativity, ignoring quantum mechanics. Current theory (see Hawking radiation) says that quantum effects cause black holes to evaporate. All information about anything that fell into the black hole is lost (i.e., this isn't a way for you to escape after you've fallen in), but all the mass/energy will eventually come out as random radiation. This happens in a finite amount of time from the point of view of an outside observer -- which means that if you're riding the probe, the black hole will evaporate just as you're crossing the event horizon. (You will not survive the experience.)
Disclaimer: I am not an astrophysicist. I hope that one will come along soon and explain how I've gotten this wrong.
There is a test of the collapse of a magnetized neutron star (case 3) in this preprint: http://arxiv.org/abs/arXiv:1208.3487.
Here is some of the text and figures from that paper that describe how the magnetic field is expelled:
- Magnetized Collapse to a Black Hole
Our final and most comprehensive test is represented by the collapse to a BH of a magnetized nonrotating star. This is more than a purely numerical test as it simulates a process that is expected to take place in astrophysically realistic conditions, such as those accompanying the merger of a binary system of magnetized neutron stars [26, 27], or of an accreting magnetized neutron star. The interest in this process lays in that the collapse will not only be a strong source of gravitational waves, but also of electromagnetic radiation, that could be potentially detectable (either directly or as processed signal). The magnetized plasma and electromagnetic fields that surround the star, in fact, will react dynamically to the rapidly changing and strong gravitational fields of the collapsing star and respond by emitting electromagnetic radiation. Of course, no gravitational waves can be emitted in the case considered here of a nonrotating star, but we can nevertheless explore with unprecedented accuracy the electromagnetic emission and assess, in particular, the efficiency of the process and thus estimate how much of the available binding energy is actually radiated in electromagnetic waves. Our setup also allows us to investigate the dynamics of the electromagnetic fields once a BH is formed and hence to assess the validity of the no-hair theorem, which predicts the exponential decay of any electromagnetic field in terms of Quasi Normal Mode (QNM) emission from the BH.
...
As the collapse proceeds, the restmass density in the center and the curvature of the spacetime increase until an appararent horizon is found at t = 0.57 ms and is marked with a thin red line in Fig. 12. As the stellar matter is accreted onto the BH (the rest-mass outside the horizon $M_{b, out} = 0$ is zero by $t \geq 0.62$ ms), the external magnetic field which was anchored on the stellar surface becomes disconnected, forming closed magneticfield loopswhich carry away the electromagnetic energy in the form of dipolar radiation. This process, which has been described through a simplified non-relativistic analytical model in Ref. [52], predicts the presence of regions where |E| > |B| as the toroidal electric field propagates outwards as a wave. This process can be observed very clearly in Fig. 13, which displays the same three bottom panels of Fig. 12 on a smaller scale of only 15 km to highlight the dynamics near the horizon. In particular, it is now very clear that a closed set of magnetic field lines is built just outside the horizon at t = 1.0 ms, that is radiated away. Note also that our choice of gauges (which are the same used in [61]) allows us to model without problems also the solution inside the apparent horizon. While the left panel of Fig. 13 shows thatmost of the rest-mass is dissipated away already by t = 0.65 ms (see discussion in [62] about why this happens), some of the matter remains on the grid near the singularity, anchoring there the magnetic field which slowly evolves as shown in the middle and right panels.
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Actually, it doesn't have the same mass, it has significantly less mass than its precursor star. Something like 90% of the star is blown off in the supernova event (Type II) that causes the black holes.
The Schwarzschild radius is the radius at which, if an object's mass where compressed to a sphere of that size, the escape velocity at the surface would be the speed of light $c$; this is given by $$ r_s=\frac{2Gm}{c^2} $$ For a 3-solar mass black hole, this amounts to about 10 km. If we measure the gravitational acceleration from this point, $$ g_{BH}=\frac{Gm_{BH}}{r_s^2}\simeq10^{13}\,{\rm m/s^2} $$ and compare this to the acceleration due to the precursor 20 solar mass star with radius of $r_\star=5R_\odot\simeq7\times10^8$ m, we have $$ g_{M_\star}=\frac{Gm_\star}{r_\star^2}\simeq10^3\,{\rm m/s^2} $$ Note that this is the acceleration due to gravity at the surface of the object, and not at some distance away. If we measure the gravitational acceleration of the smaller black hole at the distance of the original star's radius, you'll find it is a lot smaller (by a factor of about 7).