Messier 73: Four-Star Asterism in Aquarius

Messier 73 is the odd one out in Messier's catalog: not a cluster, nebula or galaxy, but four faint stars in a tight little Y. It lies in western Aquarius, a short hop east of the globular cluster M72.

What it is

M73 is an asterism, a chance pattern of stars. Its four stars range from about magnitude 10.5 to 12 and fit within about 3 arcminutes. They are not a real cluster. A 2002 spectroscopic study by Odenkirchen and Soubiran found that the four stars move at different speeds along our line of sight and lie at quite different distances, and their motions across the sky do not match either. That is why this page gives no single distance; SEDS lists the stars at somewhere between about 1,000 and 2,300 light-years.

The question was argued for decades. Some astronomers suspected the four stars might be a tiny real cluster, and a study in 2000 suggested they could be the remains of an old open cluster. The later spectra favored the plain explanation: four stars that happen to line up from our point of view.

When to see it

Like M72, it is best on late summer and early autumn evenings, crossing the meridian around 10 p.m. in early September. From 40°N it climbs only to about 37° in the south. Southern Hemisphere observers see it high in the sky.

How to find it

First find M72. Start at Algedi (Alpha Capricorni), the wide double star at the northwest corner of Capricornus, and move about 8½° due east to M72; or go about 3½° south-southeast from Epsilon Aquarii. Then move about 1⅓° east, staying at almost the same declination.

What you will see

Binoculars cannot split the group; at most it is a faint, star-like point. In a 70–100 mm telescope at medium power you see three stars in a tight triangle with a fourth, fainter one beside them, together forming a small Y. At low power the stars blend into a dim knot that does look a little like a fuzzy patch, which explains how it got into the catalog. A 200 mm telescope shows the four stars sharply, along with their slightly different colors. It shows up fine from the suburbs.

History

Charles Messier recorded M73 on October 4, 1780, while measuring the position of M72. He described a cluster of three or four small stars that looks like a nebula at first glance. Some later versions of his catalog dropped it as unimportant, but John Herschel listed it in his General Catalogue, and J.L.E. Dreyer included it in the New General Catalogue as NGC 6994.