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Black Holes Encyclopedia


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Alternate Names

NGC 224




in the constellation Andromeda

Finder Chart


2.5 million light-years (0.78 megaparsecs)


30 million times the mass of the Sun


Diameter roughly equal to the orbit of Venus.

Discovery Methods



Our home galaxy, the Milky Way, is part of a collection of about three dozen galaxies known as the Local Group. These galaxies move through space as a single unit bound together by their mutual gravitational pull.

The largest member of the Local Group is the Andromeda Galaxy, M31. Like the Milky Way, Andromeda is a giant spiral galaxy, so it looks like a pinwheel spinning through space. It spans about 125,000 light-years, and contains several hundred billion stars. At a distance of about 2.4 million light-years, it is the closest large galaxy to the Milky Way, and the only one that is visible to the unaided eye.

Using telescopes on the ground and in space, astronomers discovered two large clumps of stars that appear to form a "double nucleus" in M31. The clumps are actually the brightest regions of a disk of stars that completely encircles a supermassive black hole. From measuring the speeds at which these stars orbit the center of the galaxy, astronomers deduce that the black hole is at least 30 million times as massive as the Sun and roughly 10 times as massive as the central black hole in the Milky Way.

In 2006, the disk of superhot gas around the black hole flared up, producing 100 times more X-ray energy than it had before. It soon settled down, but continued to produce 10 times more X-ray energy than before the outburst. Astronomers say the outburst may have been generated by the disk's powerful magnetic field. Lines of magnetic force tangled then snapped, sending energy and charged particles into space in a process similar to the one that produces flares and other outbursts on the surface of the Sun.

M31's black hole was the second supermassive black hole discovered in the heart of a galaxy. The first was found in M32, a small satellite galaxy to M31.


M31 Black Hole Gets Frisky (StarDate)
Giant eruptions mimic processes on the Sun.



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This document was last modified: April 30, 2012.


Ground-Based Photo

Space-Based Photo

M31 Before and After
Space-Based Photo



Mysterious disk of blue stars around M31's black hole