What is the difference between Black Holes and Black Dwarf?

Before we go deep down to understand the difference between Black Hole and Black Dwarf, let us understand the two terms with short definitions. Black Hole: It is an astrophysical object which has a radius less than its own Schwarzschild radius - that is, a large enough mass in a small enough volume such that, at its surface, not even light can escape the pull of its gravity. Black Dwarf: It is another type of astrophysical object that consists of the matter left behind when a low mass star finishes carrying out nuclear fusion in its core. When the “nuclear fuel" is all used up, the star stops shining and contracts but the gravity is small enough such that the physical structure of the star is enough to prevent further collapse. What is left is a dark, spherical cooling mass forever gliding through the vastness of space.

The Difference

So, the difference is clear from the definitions discussed above. But still we will discuss the difference between them in two perspectives of size and density.

Size

A black dwarf is a smallish star which simply burned until it had used all its fuel, then faded into darkness. As it cooled it contracted, but it reached a point at which it became effectively solid and stopped. A black hole is a largish star which, after many dramatic events including probably going supernova, still had at least two or three solar masses of burned out fuel left when it had exhausted all new fuel resources. Without new energy being created, it also cooled. But the force of gravity was so strong that it couldn’t stop at solid. The pressure was so high that nothing could prevent it collapsing. And when it did so it reached a point at which, in a hand waving sort of way, the escape velocity reached the speed of light and time, as seen from outside, stopped. What is called an Event Horizon formed, and we have no idea what happened inside it. It became effectively separated from our normal universe.

Density

The density of matter and energy in a black hole is so high that not even light can escape. In fact, its density is thought to be infinite in GR, which it cannot be due to Heisenberg's uncertainty principle, but for all practical purposes it is. A black dwarf however is black only because there are no processes left that could yield energy, i. e. electromagnetic radiation. It is a burnt out star that has lost all remaining heat and finds itself in thermal equilibrium with space (currently 2.71 K). It consists of degenerate matter, but still atomic matter, and if you managed to install a big torch on its surface, it could be seen from above. This is eventually the fate of all stars that are not big enough to end as neutron stars or black holes, including our sun, but it will take trillions of years to reach that state, so there's probably not a single black dwarf in our cosmic neighborhood at the moment.

The End Notes

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