Messier 94, NGC 4736

Messier 94 (NGC 4736) is a spiral galaxy in Canes Venatici, and one of the nearest beyond our Local Group of Galaxies. M 94 was discovered by Pierre Mechain in 1781; Charles Messier observed and cataloged it later that year.

With a visual magnitude of 8.2, Messier 94 is an impressive object. It has a brilliant core, brightening to a nonstellar nucleus and is surrounded by an elongated, diffuse 7' x 3' disk. A series of knots on the east and southeast sides suggest a spiral pattern. It has tightly wound spiral arms, making it appear nearly spherical in a small telescope.

M 94 is about 16 million light years distant, has a diameter of approximately 33,000 light-years, and appears to be receding from us at 210 miles per second. M 94 is one of the brightest galaxies in the Canes Venatici I Galaxy Group (also called the M 94 Group), a small group of galaxies near M 94 that appear to form a gravitationally bound system. This group is one of many that lies within the Virgo Supercluster.

Messier 94 is also notable in that it has two ring structures. The inner ring is a site of strong star formation activity, and is sometimes referred to as a starburst ring. It is traced by young, blue star clusters in color images, which sharply separates it from a much fainter population of an older, yellowish stars. In the outskirts, this region ends again, in a ring with moderate star formation activity, so that M 94 is one of the relatively rare galaxies in which two "waves" of stellar formation can be observed. In very long exposures, a further very faint ring, about 15' across, becomes visible.

These rings appear to form at resonance locations within the disk of the galaxy. Several possible events could have led to the origin of M 94's ring structures, including the accretion of a satellite galaxy, or gravitational interaction with a nearby system. However, further research has found problems with each of these scenarios.

In 2008, a study analyzed the rotation curves of the galaxy's stars, and appeared to show that M 94 has very little dark matter present, i.e. that ordinary luminous matter appeared to account for all of its mass. This result was unusual and somewhat controversial, as current models have difficulty explaining how a galaxy could form without a dark matter halo, or how a galaxy could lose its dark matter altogether.