Hickson Compact Groups

While the Abell catalog focuses on massive "metropolises" of galaxies, the Hickson Compact Groups (HCG) database focuses on "urban neighborhoods." These are small, incredibly dense associations of galaxies that are physically crowded together in space.

For the modern observer, a "Hickson" represents the ultimate test of both telescope optics and atmospheric clarity.

History & Who Made It

The database was compiled by Paul Hickson, an astronomer at the University of British Columbia. In 1982, he published the "Atlas of Compact Groups of Galaxies."

Hickson didn't use a telescope for his initial search; instead, he meticulously scanned the red-sensitive photographic plates of the Palomar Observatory Sky Survey (POSS). He was looking for groups that met very strict mathematical criteria for "compactness"—meaning the galaxies had to be very close together and relatively isolated from other nearby stars or groups.

The Purpose: Studying Galactic Cannibalism

The purpose of the Hickson database was to provide a "laboratory" for studying how galaxies interact when they are forced to live in tight quarters.

• • Gravitational Interaction: Because these galaxies are so close, they are often in the process of tearing each other apart.

  • Evolutionary Speed: Galaxies in Hickson groups age faster because they constantly collide and merge.

  • Dark Matter: Hickson used these groups to study "missing mass," as the speed at which these galaxies orbit each other suggests there is much more gravity present than what we can see.

What Objects Can Be Found In It?

The catalog contains exactly 100 groups. While it is called a "galaxy cluster" catalog, these are better described as "compact groups," usually containing between 4 and 10 galaxies each.

Hickson Database Breakdown

Catalog Name: Hickson Compact Groups (HCG)

Creator: Paul Hickson

Year Published: 1982

Total Groups: 100 (containing 462 individual galaxies)

Primary Gear: 10-inch to 18-inch telescopes (for visual observation)

Group Composition:

• Most groups contain 4 to 5 visible members.

• Majority are located in the Northern Hemisphere (due to the use of Palomar plates).

• Features a high percentage of "Peculiar" and "Interacting" galaxies.

The "Big Three" Hicksons for Amateurs:

1. HCG 92 (Stephan’s Quintet): The most famous compact group in the sky. Five galaxies in Pegasus, four of which are locked in a violent cosmic dance.

2. HCG 44: A bright, beautiful group in Leo that is accessible to smaller (8-inch) telescopes.

3. HCG 68: A very bright group in Canes Venatici containing three spirals and two ellipticals; one of the "easiest" to see.

Visualizing the Hicksons

To the eye, a Hickson group often looks like a single "fuzzy" patch at low power. As you increase the magnification, that patch begins to "split" into individual stellar cores, much like resolving a tight double star.

The Observer's Challenge

The Hickson catalog is the "Black Belt" of galaxy hunting. Because the galaxies are so close together, the light from the brightest member often "washes out" the fainter ones. Observing all 100 Hicksons is a legendary feat among amateur astronomers, often requiring years of travel to high-altitude, dark-sky sites.

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