M90 (NGC 4569) is an intermediate spiral galaxy that contains around a trillion stars, and hosts roughly 1,000 globular star clusters, making it is one of the biggest members of the 2,000 strong group of galaxies called the Virgo Cluster. By way of comparison, the Milky Way has only 200-400 billion stars, and around 150 or so globulars. At an absolute magnitude of -22, which is brighter than the Andromeda Galaxy (Messier 31), M90 is also one of the brightest members of the Virgo Cluster, and can be found only 1.5 degrees from a subgroup of the cluster that is centered on the supergiant elliptical galaxy M87. While M90 does have a satellite galaxy (IC 3583), the two structures are too far apart to be interacting.
• Constellation: Virgo
• Coordinates: RA 12h 36m 49.8s | Dec. +13° 09′ 46″
• Distance: 58.7 million light years
• Object Type: Intermediate spiral galaxy
• Redshift: -0.000784 ± 0.000013
• Effective Diameter: 165,000 light years
• Apparent Magnitude: +10.26
• Other Designations: NGC 4569, UGC 7786, Arp 76
M90 is situated in the zodiac constellation of Virgo, with the best time to view the galaxy from northern hemisphere being during the spring and summer months. However, M90 can be difficult to spot using binoculars and small telescopes in less than dark skies, hence the chart provided to aid observers in finding the right galaxy. Look for M90 among the cluster of galaxies in the constellation right on the border between Virgo and Coma Berenices. Bear in mind that an amateur telescope smaller than 8-inch aperture will not reveal much, if any detail of this faint +10.26 magnitude galaxy.
One striking feature of M90 is the smooth, uniform appearance of its spiral arms. This is the result of the violent interactions between it and the intergalactic medium in the Virgo Cluster that has stripped much, if not most of the interstellar medium from M90. In turn, this has deprived the spiral galaxy of the material required for star formation in a process known as “ram-pressure stripping”. Recent observations have located much of this material outside of the M90 in long “tails” of stripped-out ionized gas, some of which are up to 260,000 light years long, which is longer than the galaxy is wide.
The smooth outer parts of the galaxy is evidence of the fact that active star formation had been interrupted; however, star formation in the central regions of the galaxy is continuing apace, with around 5×104 O, and B-type stars having formed around 5.5 million years or so ago. This central star forming region is encircled by a large number of Type A supergiant stars that formed in previous bursts of star formation ranging from 15 million to about 30 million years ago. As a result of the different rate of star formation in different parts of the galaxy, some researchers refer to M90 as an example of an “anemic” galaxy, while others describe M90 as an example of a “passive” spiral galaxy, since many of M90’s features are similar to most other galaxies that have very high red shifts.
A further feature of M90 is the two collimated jets of interstellar material that are being blown out of the galaxy as the result of multiple supernova events that have created so-called super-winds. These winds are driving the material out from the galactic poles, and the distorted shape of one of these jets is the result of the jet colliding with the intracluster medium as M90 moves through the cluster.
Contrary to the spectra of most other galaxies that show a distinct red shift, the spectrum of M90 is blue-shifted, which means that the galaxy is approaching us. While the rate of approach is only about 383 km/s, M90 is moving through the Virgo Cluster at a rate of about 1,500 km/sec, and recent studies suggest that M90 may be leaving the cluster, possibly as a result of being expelled through tidal interactions with the rest of the cluster.
However, while the blue-shift of the galaxy’s spectrum was at first taken to be evidence of the fact that M90 was only a foreground object relative to the Virgo Cluster proper, new studies suggest that the galaxy’s blue-shifted spectrum is more likely to be the result of the large range of relative motions and velocities of objects that occur within the Virgo Cluster itself.