MadSci Network: Astronomy
Query:

Re: How many black holes are there in our galaxy and the universe?

Date: Thu Jun 1 18:07:32 2000
Posted By: Matt Tilley, Undergraduate, Physics and Mathematics double major, Arkansas State University
Area of science: Astronomy
ID: 958742952.As
Message:

Megan--
This is a very good question that still presents a problem in the study of modern astronomy. It is unknown exactly how many black holes exist in the universe, and even our galaxy.

The problem is in detecting and identifying black holes. Most objects in the sky are found by detecting radiation they give off in the electromagnetic spectrum (i.e., radio, visible light, infrared, ultraviolet, X-rays, etc.) A black hole is so massive that its own gravity does not allow even light to escape. So a black hole itself emits no light that could be seen by our telescopes.

There are, however, ways to detect black holes that involve looking for its effects on nearby objects. If a black hole is close enough to a nearby star, material from the star can fall onto the black hole. As it does so, the matter will heat up as it is accelerated toward the black hole. The material can get hot enough that it will emit X- and gamma-rays, and sometimes radio waves, that can be detected by our telescopes; astronomers consider many X- and gamma-ray sources to be possible black holes. Also, sometimes not all of the material will fall into the black hole; rather, through processes we do not yet understand fully, some of the material can be funnelled into jets of super heated matter. The detection of jet is often taken as an indication of a possible black hole.

As the study of black holes continues, the methods of detection will certainly become more reliable, and perhaps we will be able to map all of the black holes in our galaxy and the known universe.

[We do have a number of extremely good candidates for black holes. Perhaps the top three are the sources Cygnus X-1, Sgr A*, and the object at the center of the galaxy NGC 4258. Cygnus X-1 is an X-ray source which is composed of a star orbited by a dark object about 10 times more massive than the Sun. Sgr A* is an dark object at the center of the Milky Way Galaxy that is influencing the motion of stars around it and has a mass of about 2 million times that of the Sun. At the center of NGC 4258, matter moves as if there is a dark object with a mass about 1 billion times that of the Sun. Moderator]


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