The Mice — Galaxy in Coma Berenices

NGC 4676 is the cataloged name for a pair of interacting galaxies in the constellation Coma Berenices, popularly known as 'The Mice' (or Arp 242) for the long straight tidal tails that trail behind them like mouse tails. NGC 4676A and NGC 4676B are about 290 million light-years away and are caught mid-collision: a close passage several hundred million years ago sent streams of stars and gas arcing thousands of light-years into intergalactic space. Computer simulations show the pair is not flying apart but is in a long, slow, final merger that will eventually produce a single elliptical galaxy.

A demanding target. The pair is faint (combined magnitude around 14) and small — the bright cores span less than an arcminute, and the tails are essentially invisible visually. A 10-inch telescope at 200x shows two close fuzzy stars of roughly equal brightness; a 16-inch or larger under dark skies begins to hint at elongation on the A component. The Mice lie in the Coma-Virgo cluster region, about 1 degree northeast of the bright edge-on galaxy NGC 4565 (the Needle Galaxy / C38), which is an easier nearby target for calibration. Best observed March through July.

Discovered by William Herschel on March 13, 1785, from his observatory at Slough. The dramatic tidal tails were not recognized until the galaxies were photographed at large professional observatories in the mid-20th century. Halton Arp included the pair as entry 242 in his 1966 Atlas of Peculiar Galaxies. Alar and Juri Toomre's 1972 simulation of The Mice was a landmark moment in theoretical astrophysics — the first computer model to convincingly reproduce observed galaxy tidal features, cementing the idea that many 'peculiar' galaxies are simply in the act of merging.