Black Holes: Gravity's Relentless Pull

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Black Holes: Gravity's Relentless Pull
OBJECT: M33 (Extragalactic binaries)
DISTANCE: 2.1 million lightyears
BLACK HOLE? Yes (stellar-mass)

This galaxy is number 33 in a famous 18th century catalog of extended objects compiled by Messier, a French comet-hunter. It is also called the Triangulum (Triangle) galaxy, after the constellation in which it resides. It is the third largest nearby galaxy, after Andromeda and the Milky Way. The visible light image shows that the stars are distributed in a spiral pattern. The radio image shows glowing cold hydrogen gas between the stars.

During the life of a galaxy, billions of stars are born. Most of the heaviest stars have already died and have become white dwarfs, neutron stars, or stellar-mass black holes (depending on their birth mass). In isolation, such compact stellar remnants generally cannot be detected. But a very small fraction resides in a binary system and produces bright X-rays by pulling in gas from a normal companion star. The X-ray image of M33 shows that we can detect such binaries also outside of our own galaxy (extragalactic). Each dot in the image is an X-ray binary. It is difficult to tell what type of compact object powers each binary, but we can be certain that some are powered by black holes. Every galaxy has millions of stellar-mass black holes, although most of them are invisible to us.

Unlike many other large galaxies, such as the Milky Way and Andromeda, M33 does not have a supermassive black hole in its center. Astronomers don't yet fully understood why some galaxies grow supermassive black holes in their centers while others don't.

X-Ray Image of: M33 (Extragalactic binaries)
Each dot is a binary system that shines in X-rays. A compact object, in some cases a black hole, pulls matter of a normal star.