Messier 99: Spiral Galaxy in Coma Berenices

M99 is a spiral galaxy that we see nearly face-on, in the Virgo Cluster on the Coma Berenices side. In most telescopes it is a modest round glow, but it has a place in the history of astronomy as one of the first galaxies in which a spiral pattern was ever seen.

What it is

M99 lies about 55 million light-years away. Its 5.4′ disk corresponds to roughly 85,000 light-years, a little smaller than the Milky Way. It is a lopsided spiral: one arm is much longer and looser than the others. Astronomers think a close pass by another galaxy, or the galaxy's fast plunge through the hot gas of the Virgo Cluster, stretched it this way. The disturbance also seems to have stirred up star formation along the arms. It shares the area with M98 and M100.

When to see it

M99 is a spring object, highest around midnight in late March and in mid to late evening through April and May. From about 40°N it climbs to roughly 64° in the south. Being just north of the celestial equator, it is visible from most of the Southern Hemisphere too, low in the northern sky; from 35°S it reaches about 40°.

How to find it

Go to Denebola, the tail star of Leo, and move about 6.6° east to 6 Comae Berenices, a fifth-magnitude star. M99 lies about 0.8° southeast of 6 Comae. M98 is on the other side of the star, 0.6° west, and M100 is about 1.7° north-northeast of M99, so you can visit all three in a few minutes.

What you will see

Binoculars show little or nothing except under excellent skies, where large binoculars may catch a faint spot. A 70–100 mm telescope shows a small, round, evenly faint glow with a slightly brighter center; suburban light makes it hard. A 200 mm or larger telescope under a dark sky shows a brighter core and, with averted vision, an uneven disk; a 250 mm or larger instrument on a good night may hint at the long, lopsided arm. It is a galaxy, so filters do not help.

History

Pierre Méchain found M99 on March 15, 1781, and Messier measured its position the next month. In 1846 Lord Rosse, using his 72-inch reflector in Ireland, drew spiral structure in it, making it the second galaxy (after M51) in which spiral arms were recognized. Several supernovae have been recorded in M99 since the 1960s.