Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

September 6, 1999
See Explanation.  Clicking on the picture will download 
 the highest resolution version available.

HCG 87: A Small Group of Galaxies
Credit: Sally Hunsberger (Lowell Obs.), Jane Charlton (Penn State), et al.
& the Hubble Heritage Team (AURA/ STScI/ NASA)

Explanation: Sometimes galaxies form groups. For example, our own Milky Way Galaxy is part of the Local Group of Galaxies. Small, compact groups, like Hickson Compact Group 87 (HCG 87) shown above, are interesting partly because they slowly self-destruct. Indeed, the galaxies of HCG 87 are gravitationally stretching each other during their 100-million year orbits around a common center. The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers. HCG 87 is composed of a large edge-on spiral galaxy visible on the lower left, an elliptical galaxy visible on the lower right, and a spiral galaxy visible near the top. The small spiral near the center might be far in the distance. Several stars from our Galaxy are also visible in the foreground. The above picture was taken in July by the Hubble Space Telescope's Wide Field Planetary Camera 2. Studying groups like HCG 87 allows insight into how all galaxies form and evolve.

Tomorrow's picture: Water From Space

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