Stellar Evolution Page 11

Black Holes, a worst case scenario

If the core remnant of a collapsed massive star is more massive than about 3 Msun, even neutron degeneracy pressure cannot withstand the force of gravity. Actually, nothing in the Universe can withstand such forces. The star collapses and continues to collapse. It forms a black hole. Matter disappears from contact with the rest of the Universe in a gravitational warp in space.

A black hole does not have a surface in the usual sense of the word. There is a region, or boundary, in space around a black hole beyond which we cannot see. This boundary is called the event horizon. It is an imaginary spherical surface surrounding a black hole, with radius equal to the so-called Schwarzschild radius (escape velocity equal speed of light), within which no event can be seen, heard, or known about by an outside observer.

Anything that passes beyond the event horizon is doomed to be crushed as it descends ever deeper into the gravitational well of the black hole. No visible light, nor X-rays, nor any other form of electromagnetic radiation, nor any particle, no matter how energetic, can escape. The radius of the event horizon (proportional to the mass) is very small, only about 30 kilometres for a non-spinning black hole with the mass of 10 Msun.

Black Hole warping space around it.
Artist impression.

The type of Black hole that results from the collapse of a single star is called a Stellar Black hole. The mass of a black hole will increase when it captures mass from objects that come too close. The radius of the event horizon will increase by about 3 kilometres for every solar mass that it swallows. A black hole in the centre of a galaxy, where stars are densely packed, may grow to the mass of a billion Solar masses and become what is known as a Super massive Black Hole.


Observing black holes

A black hole itself is invisible but its presence can be revealed through interaction with other matter. The movement of a group of stars that orbit a region in space which looks empty can reveal a black hole.

Here is a reconstruction of the observed motion of a number of stellar objects near the centre of the Milky Way galaxy. The period of these orbits together with their size, reveal the presence of a super massive black hole of  
3.7 million Solar masses.


Max-Planck-Institut für extraterrestrische Physik