Messier 100: Grand-Design Spiral Galaxy in Coma Berenices

M100 is one of the largest and brightest spiral galaxies in the Virgo Cluster, seen almost exactly face-on. Photographs show two strong, symmetrical arms wrapped around a bright core. At the eyepiece it is subtler: a soft round glow with a starlike center, best under a dark spring sky.

What it is

M100 is a "grand-design" spiral, a galaxy whose arms are long, well defined and continuous rather than broken into fragments. It lies about 56 million light-years away, and its 7′ disk spans roughly 110,000 light-years, similar to the Milky Way. The arms are lined with pink star-forming regions and blue clusters of young stars, and a ring of star formation circles the core. In the 1990s the Hubble Space Telescope picked out Cepheid variable stars in M100, which gave one of the first reliable distances to a Virgo Cluster galaxy and helped pin down the expansion rate of the universe. Nearby in the sky are M98, M99 and, a few degrees south, the dense core of the Virgo Cluster around M84 and M86.

When to see it

M100 is a spring galaxy, highest around midnight in late March and placed well in the evening through April and May. From about 40°N it rises to roughly 66° in the south. It is just north of the celestial equator, so it is visible from the Southern Hemisphere as well, low in the north; from 35°S it reaches about 39°.

How to find it

From Denebola, the tail star of Leo, move about 6.6° east to the fifth-magnitude star 6 Comae Berenices. M100 lies about 1.8° northeast of 6 Comae. If you have already found M99, 0.8° southeast of 6 Comae, M100 is about 1.7° north-northeast of it. In total, M100 sits a little over 8° east of Denebola.

What you will see

In binoculars under dark skies, M100 is a faint, small round smudge; from suburban skies it is usually invisible. A 70–100 mm telescope shows a round glow several arcminutes across with a small, brighter core and a nearly starlike nucleus. The outer disk has low surface brightness and fades quickly under light pollution. A 200 mm or larger telescope on a dark night shows the bright core, a mottled halo and, with averted vision, hints of the two main arms. Filters do not help.

History

Pierre Méchain discovered M100 on March 15, 1781, and Messier measured its position in April. By 1850 Lord Rosse had listed it among the first handful of "spiral nebulae" seen through his giant reflector in Ireland, long before anyone knew these were separate galaxies. Several supernovae have been recorded in it since, including SN 1979C.