Messier 90, NGC 4569

Messier 90 (M90, NGC 4569) is one of the brighter spiral galaxies in the Coma-Virgo Galaxy Cluster, and is one of the few Coma-Virgo galaxies that show a blue shift in their spectra.

M 90 was discovered by Charles Messier on March 18th, 1781, the same night he discovered M 84, 85, 86, 87, 88, and 89. M 90 was included by Halton Arp as no. 76 in his Atlas of Peculiar Galaxies as a "Spiral with a High Surface Brightness Companion": 14th-magnitude IC 3583, which is visible in wide-field views of M 90.

Visually, M 90 is an impressive magnitude 9.5 spiral galaxy about one 1° north of M 89 and 1°.5 away from the subgroup centered on Messier 87. M 90 measures 9.5' x 4.5' in size. Its outer disk is very faint but well defined, and its western side is noticeably bulged. Within the disk are faint spiral arms attached to the ends of an extended, mottled core. The arms are tightly coiled counterclockwise around the core. In small telescopes, M 90 appears as little more than as an elongated, mottled disk.

The center of the Coma-Virgo cluster is 65 million light years distant. If M 90 is 50 million light years away, its absolute magnitude is -21.4, it has a luminosity of 30 billion Suns, and its true diameter is at least 150,000 light years. As it is approaching us at 383 km/sec, M 90 has a very high velocity 1500 km/sec relative to the Virgo cluster, and is possibly escaping the cluster. Only one Messier galaxy, M 86, is approaching us faster.

Although M 90 is conspicuous and bright, studies indicate quite a low value for its mass, which implies that the galaxy has a very low star density. It has tightly-wound, smooth spiral arms, rather than knotted arms like galaxies with extended star formation, which appear to be completely "fossil", meaning that no star formation appears to take place in them currently. This is a consequence of the galaxy's fast motion through the intergalactic medium. As a result this "ram pressure stripping", the galaxy has lost much of its interstellar gas and star-forming matter.

The only exception is the inner disk region, near the darker dust lanes. Multiple supernovae in the nucleus have produced "superwinds" that are blowing the galaxy's interstellar matter outward into intergalactic space.