This
Astrobin Debut Object is a galaxy group of 4 elliptical galaxies, called Hickson 81, located in the constellation of Hercules at a declination of +13 degrees.
This is a true cluster - the 4 galaxies are all about 0.7 billion light years away. Each galaxy is small – 40,000 light years in diameter or smaller. These galaxies, especially 81a and 81b, don’t look like typical spherical ellipticals. Their shading and asymmetry look odd to me – perhaps there is some interaction going on here.
The Hickson catalog is a collection of 100 galaxy groups, established by Paul Hickson in 1982. Hickson compact galaxy groups are strictly defined by 3 criteria: number of galaxies, total magnitude, and isolation from other galaxies. A typical Hickson group has 4 galaxies, but some have up to 8. You can see more about Hickson galaxy groups in my
Astrobin Hickson Collection.
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