Free Astronomy Magazine September-October 2015

left to right) NGC 839, NGC 838, NGC 835, and NGC 833 — four of the sev- en galaxies that make up the entire group. They shine brightly with their glowing golden centres and wispy tails of gas, set against a back- ground dotted with much more distant galaxies. (A tidal tail is a thin, elongated region of stars and inter- stellar gas that extends into space from a galaxy. They are a result of the strong gravitational forces around interacting galaxies.) Compact groups represent some of the densest concentrations of gal- axies known in the Universe, making them perfect laboratories for study- ing weird and wonderful phenom- ena. Hickson Compact Groups in par- ticular, as classified by astronomer Paul Hickson in the 1980s, are surpris- ingly numerous, and are thought to contain an unusually high number of galaxies with strange properties and behaviours. HCG 16 is certainly no exception. The galaxies within it Hubble views a bizarre cosmic quartet by NASA T his new NASA/ESA Hubble Space Telescope image shows a gathering of four cosmic com- panions. This quartet forms part of a group of galaxies known as the Hickson Compact Group 16, or HCG 16 — a galaxy group bursting with dramatic star formation, tidal tails, galactic mergers and black holes. This quartet is composed of (from F our of the seven members of galaxy group HCG 16.This quar- tet is composed of (from left to right) NGC 839, NGC 838, NGC 835, and NGC 833. This new image uses observations from Hubble's Wide Field Planetary Camera 2 combined with data from the ESO Multi-Mode Instrument, installed on the European Southern Obser- vatory's New Technology Tele- scope in Chile. [NASA, ESA, ESO]