Messier 47: Open Cluster in Puppis

M47 is a scattered handful of bright stars in the northern part of Puppis, sitting right next to its very different neighbor M46. Under a dark sky you can spot it without optical aid as a faint knot of light, and in binoculars it is one of the easiest open clusters of the winter sky.

What it is

M47 is a young open cluster about 1,530 light-years away. Its stars spread over half a degree of sky, about the width of the full Moon, which works out to roughly 13 light-years across. It is a coarse cluster with only about 50 main members, many of them hot blue-white stars, plus a couple of orange giants. Its age is estimated at about 78 million years, so it is a little younger than the Pleiades (M45), which it resembles in make-up. Near its center is the attractive double star Struve 1121, two matched stars close together.

When to see it

M47 is a winter object. It reaches its highest point around midnight in mid-January and is well placed in the evening from late January through early March. From about 40°N it climbs to roughly 35° above the southern horizon. From the Southern Hemisphere it passes high overhead in the summer evening sky.

How to find it

Start at Sirius, the brightest star in the sky. Look about 12½° due east of it, a bit more than the width of your fist at arm's length. Sweep there with binoculars to find two clusters about 1.3° apart: M47 is the western one with a few bright stars; M46, to the east, is a faint, even glow of many dim stars. Another check: M47 lies about 5° south-southwest of Alpha Monocerotis.

What you will see

In binoculars M47 shows a dozen or more stars, several bright enough to stand out at once, in a loose group with M46 glowing next to it in the same field. A small telescope at low power shows 30 or more stars and splits Struve 1121 easily at moderate power. A larger telescope adds fainter members but the cluster stays sparse, so keep to low power. Because its stars are bright, M47 holds up well under suburban skies.

History

Giovanni Battista Hodierna recorded M47 before 1654 as a nebulous patch between the two dogs, but his book remained unknown until 1984. Charles Messier found it on his own on February 19, 1771. He made a sign error when working out its position, so for nearly two centuries "M47" pointed at empty sky. Caroline Herschel saw the cluster in 1783, and William Herschel cataloged it again in 1785. T. F. Morris finally matched M47 with NGC 2422 in 1959.