The Ohio State University

onCampus

• Help
• Buckeye Link
• Map
• Find People
• Webmail
• Search

  Search Ohio State Go

Paul Martini, Department of Astronomy

Posted on | June 2, 2011 | 931 views | Comments Off

What is a black hole?

A black hole is an object whose gravitational pull is so strong that not even light can escape. The black hole’s event horizon describes the boundary where the escape velocity is equal to the speed of light, which is more than 2,500 times faster than the escape velocity from the surface of the Earth. Nothing can escape from inside the event horizon because nothing can exceed the speed of light. This is why a black hole is “black” and how the event horizon gets its name — we are unable to see events beyond this horizon. To create such a strong gravitational pull requires an enormous amount of mass in a very small area. For example, if one could squeeze the entire mass of the sun into a region the size of OSU’s main campus, this would create a black hole. I do not recommend trying this.

How do we know black holes exist?

Black holes can not be detected directly because nothing can escape from the event horizon. We only know that black holes exist because of their gravitational pull on other objects, such as stars and gas in their immediate vicinity. Some of the best evidence for black holes in our galaxy comes from the extremely rapid orbital motions of stars around points in space where we see absolutely nothing, yet these motions imply the presence of such a massive object it could only be a very bright star or a black hole. The earliest evidence that black holes exist came from the discovery of quasars in the 1960s. These objects emit more light than a trillion suns from a region the size of our Solar System. The only viable explanation for this enormous emission is the release of energy as interstellar gas falls into a black hole.

Are there any new developments in the research of black holes?

One of the fundamental characteristics of black holes is that they are such simple objects that they can be completely described with only three numbers: their mass, their spin and their electric charge. This is called the “no hair” theorem, as it states that black holes are free of complicated properties like hair (and ears, eyes, etc.). While black hole masses have been estimated for well over a decade, an exciting new development is that there are now spin estimates for many black holes in our galaxy. These estimates are derived from temperature measurements with X-ray telescopes of the hottest gas orbiting around the black hole. Based on Einstein’s General Theory of Relativity, we expect that the faster a black hole is spinning, the further out the innermost stable orbit for this gas will be, and consequently the lower its temperature. These spin estimates are really interesting because they tell us about how black holes continue to grow with time, as well as give us new insights into how they may have formed from the collapse of an extremely massive star.

Comments