Messier 50: Heart-Shaped Cluster in Monoceros

M50 is a bright, rich open cluster in the faint winter constellation Monoceros, the Unicorn, just north of the border with Canis Major. It is easy to find from Sirius and rewarding in any telescope.

What it is

M50 is an open cluster about 3,120 light-years away. Its 16-arcminute span works out to about 15 light-years across, with a denser core in the middle. Plates taken in 1893 suggested about 200 stars in its main body. The cluster's age is estimated at about 78 million years, and most of its bright members are blue-white stars. A red giant lies a few arcminutes south of the center and stands out by color against its neighbors. Observers often describe the overall shape as a heart.

When to see it

M50 is a winter object. It is highest around midnight in early January and well placed in the evening from January through March. From about 40°N it climbs to about 42° above the southern horizon. Its declination of −8° puts it well up in the sky from the Southern Hemisphere too.

How to find it

Draw a line from Sirius to Procyon, the bright star of Canis Minor. M50 lies a little more than a third of the way along it, about 9½° north-northeast of Sirius. A closer guide is Theta Canis Majoris, the 4th-magnitude star at the top of the Great Dog's head, a few degrees northeast of Sirius: M50 is about 4° north-northeast of Theta. The cluster shows up in binoculars or a finder as a small hazy patch in a rich Milky Way field.

What you will see

Binoculars show M50 as a small round glow with a few stars sparkling in it. A small telescope at low to medium power resolves a few dozen stars in a rounded, slightly heart-shaped group, with the reddish star easy to pick out. A larger telescope shows many more faint members filling the field and makes the color contrast stronger. The cluster sits in a busy part of the Milky Way, so a dark sky helps it stand out from the background, but it is still clearly visible from the suburbs.

History

Charles Messier found M50 on April 5, 1772. A remark in a book by Jacques Cassini, published in 1740, suggests that his father, Giovanni Domenico Cassini, may have seen the cluster before 1711, so it may have been known earlier.