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Imagine yourself under a dark autumn or winter northern hemisphere sky. You're looking towards the 'W" of Cassiopeia, and notice a hazy patch between Cassiopeia and the constellation Andromeda. That is the Andromeda Galaxy (M31). Here are some fascinating facts about this stunning object.

Andromeda Galaxy (M31) Fascinating Facts

In 1923 Edwin Hubble found a special variable star in M31. It's called V1 and is Cepheid variable, a star that can be used to measure large distances in space. It helped Hubble show that that the Andromeda Galaxy was not an object in the Milky Way, but that it was located well beyond our Galaxy. To those who had thought our Galaxy was all there was, the Universe got suddenly bigger.

In 2010 the Hubble Space Telescope made these observations of V1.

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Andromeda Galaxy and the Milky Way are members of the group of galaxies called (not-very-imaginatively) the Local Group. The two large spiral galaxies dominate the group and there is a third spiral that's fairly small, the Triangulum Galaxy (M33). Astronomers don't know exactly how many dwarf galaxies there, but there are at least fifty.

In 3.75 billion years - a bit after my time - this might be a stage in the merger between Andromeda and the Milky Way. Andromeda on the left fills the field of view and begins to distort the Milky Way with tidal pull.

Credit: NASA

Here's wide-field composite view of our neighboring Andromeda Galaxy. It was taken at the Warner and Swansey Observatory on Kitt Peak in Arizon, with additional detail from astrophotographer Vicent Peris. But the inset is X-ray data data from NASA's Chandra X-ray Observatory and shows black hole candidates. A team of researchers looking over 13 years worth of Chandra data found 26 of them to add to nine already known.

There are more black hole candidates in the central bulge of Andromeda than there are in the central area of the Milky Way. Each one represents the death of a massive star.

(We call them black hole candidates because although they appear to be black holes, there isn't enough data for cautious astronomers to confidently proclaim them to be black holes.)

Credits: X-ray: NASA/CXC/SAO/R. Barnard, Z. Lee et al.; Optical: NOAO/AURA/NSF/REU Program/B. Schoening, V. Harvey and Descubre Foundation/CAHA/OAUV/DSA/V. Peris