in the constellation Cetus
15 million times the mass of the Sun
Slightly smaller than Mercury's orbit around the Sun
M77 is one of the most intensively studied galaxies in the sky. It was among the first galaxies for which astronomers measured a large redshift, which indicated that the galaxy is moving away from us at high speed as part of the general expansion of the universe.
Later observations of the galaxy's core showed strong spectral "emission lines" -- specific patterns of wavelengths that are generated when atoms of hydrogen, oxygen, or nitrogen are bombarded by high-energy radiation. Furthermore, these emission lines were very broad, indicating that the gas clouds that contain the atoms were moving at hundreds of thousands of miles per hour. Strong radio signals also were detected from a tiny point in M77's nucleus.
From all these lines of evidence, astronomers suspected that a large power source must lie in the core of this galaxy -- probably a supermassive black hole.
The currently accepted hypothesis is that the galaxy's "central engine" is a black hole about 15 million times the mass of our Sun. Clouds of dust and gas are drawn towards the black hole, and as they spiral inwards, they heat to temperatures of millions of degrees and glow brightly, releasing high-energy X-rays and gamma rays. Even though M77's black hole is not as massive as those in many other galaxies, it is consuming gas and dust at a much faster rate, so it shines brighter, and is therefore classified as an "active galactic nucleus," or AGN.
With all these gas and dust clouds surrounding the center of M77, it's difficult to get a clear view of what's going on near the black hole. The dust clouds block almost all of the visible light, so astronomers can't see stars orbiting close to the black hole, as they can in many other galaxies.
Fortunately, the gas clouds provide an alternative way to measure orbital speeds. Using a worldwide network of radio telescopes, astronomers have detected the chemical "fingerprint" of water molecules that are orbiting within a couple of light-years of the black hole. By measuring the redshift or blueshift of the water molecules -- whether they are moving toward or away from us, and high fast -- at different locations near the black hole, astronomers can estimate the mass of the black hole: about 15 million solar masses.
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This document was last modified: March 15, 2012.