# Why isn't the star that created the black hole a black hole?

If the mass of a black hole is creating so much gravity that light cannot escape, why isn't the mass of the star that created the black hole (before it went supernova) trapping light as well?

By all accounts, that pre-supernova star should have boatloads more mass than the black hole post supernova, right? Doesn't the star lose most of it's mass when going supernova?

You are correct in saying that a star loses a lot of its mass in a supernova. However, there is a reason why the star still becomes a black hole. Actually, I suppose the question here is "Why doesn't a star become a black hole before it even undergoes a supernova?"

There is a reason for a supernova (I'll assume you're talking about type II supernovae, which result from incredibly massive stars). Stars undergo nuclear fusion, and this leads to "thermal pressure", which counteracts the force of gravity. Without this pressure, gravity would indeed make a sufficiently large star collapse upon itself. Gravitational collapse occurs when there is not enough pressure to counteract gravity; the result is a spectacular supernova. So stars only become black holes (or other compact objects, such as neutron stars) when they cannot produce enough energy to counteract the force of gravity due to their own mass.

As for the first part of your question (sorry for answering in reverse), light in the area of a black hole cannot escape if it is inside its event horizon or on a trajectory towards it. The radius of the event horizon for a non-rotating black hole is its Schwarzschild radius, which is proportional to the mass of the black hole. The reason this is non-applicable in stars is because the Schwarzschild radius in stars is deep inside its interior, and there is not a strong enough gravitational field to produce an event horizon to trap any light near it.

Thermal pressure reference: https://en.wikipedia.org/wiki/Gravitational_collapse

I hope this helps.

• Why would the gravitational field be any different for a black hole than for a massive star? If the star has more mass than the black hole, shouldn't the field be stronger? Or is there some aspect of density that affects gravity as well? Sep 18, 2014 at 23:17
• You're right; I meant that there are forces countering gravitational collapse, and the net force is not enough to produce a black hole. Sep 18, 2014 at 23:18
• @Scottie The strength of gravity depends upon the distance to the center of mass. When inside the star you only experience gravity from material closer to the center than you are. Black holes etc. are much more compact than a main sequence star, so you can get much closer to the center before being inside, and so can experience stronger gravity. Sep 19, 2014 at 8:36
• I think @zibadawatimmy's comment is what the OP is actually asking. Sep 19, 2014 at 10:21
• Right. It was answered well. I was just clarifying my question since you asked. Thank you for the answer! Sep 21, 2014 at 20:58

A supernova may actually be necessary in the creation of a stellar black hole.

At the ends of their lives the cores of massive stars are made mostly of iron-peak nuclei from which you cannot extract more fusion energy. To support their weight, these stars rely on electron degeneracy pressure - the pressure caused by the Pauli exclusion principle allowing no more than one electron to share the same quantum state.

In principle a star might be supported by degeneracy pressure forever as it gradually cools - this is the fate of most white dwarfs.

However, the core of a massive star is just too big for that to work. The density increases until all the electron are moving at close-to the speed of light and that's as high as the degeneracy pressure can get. If the core exceeds the Chandrasekhar mass, it will collapse and as it does so, the rest of the star collapses with it (a little more slowly).

The collapse is triggered by the removal of electrons by electron capture into nuclei to form neutrons. At some point enough neutrons are produced for neutron degeneracy pressure to halt or at least slow the collapse. This and the release of a lot of gravitational potential energy are ultimately what power a supernova explosion. But if the collapse is not halted then even neutron degeneracy pressure will not support the star and collapse to a black hole becomes inevitable. A black hole status is reached once a proportion of its mass is compressed inside its Schwarzschild radius $r_s = 2GM/c^2$. i.e. once its density achieves $$\rho > \frac{3M}{4\pi r_s^{3}}$$ i.e. when a central mass $M$ has a density that exceeds $$\rho > \frac{3}{32\pi} \frac{c^6}{G^3 M^2} = 1.8\times10^{19} \left(\frac{M}{M_{\odot}}\right)^{-2}\ {\rm kg/m}^3$$ This is a ball park figure and assumes spherical symmetry and neglects any detailed GR treatment, but is more or less correct - a few times higher than typical neutron star densities.

In other words it is the density of the material that largely determines whether something becomes a black hole. The mass is only an indirect parameter.

Black holes are created because the core of the star becomes very dense, not just because the star is massive. Prior to the creation of the black hole, the core is able to create enough outward pressure to prevent the core from gravitationally collapsing to the density needing to create a black hole.

The star before it turns into a black hole has what is known as radiation pressure, i.e. the fusing of elements that creates nuclear explosions. This outward radiation pressure counter balances the inward force of gravity but when the star finally runs out of fuel that radiation pressure stops. Thus the only remaining force is gravity. Gravity then attracts what is left of the star after the super nova inwards into a deep dense core which then forms the black hole.

Whether an object is a black hole is not just determined by its mass. It's determined by whether that mass is entirely within its Schwarzschild radius.

In principle, any object can be a black hole if all its mass is concentrated into a sufficiently small volume.

For example, the Sun's Schwarzschild radius is about 3.0 km -- but its actual radius is about 700,000 km. It could become a black hole only if it were compressed down to a radius of 3.0 km.

A much easier way to think about this is to consider clouds in the sky. They contain hundreds to thousands of gallons of water molecules but they are very spread out. Same as a hydrogen cloud before it creates a star. When you compact molecules in a tighter space you get a stronger magnetic field in the near vicinity of the object. The amount of molecules in a specific space dictate the "strength" of the magnetic field. When a star explodes it does lose mass but what remains is compacted into an infinitely small space to create a stronger magnetic field. A Swarzchild radius is calculated by using existing mass of a star and seeing how much space do we need to cram that amount of mass into to overcome the speed of light but it does not consider how much mass remains after a supernova explosion. And as for "forever and ever"... Our ideas are constantly changing. Remember, we used to think the planet was flat...I hope this helps, aloha.