Messier 93: Open Cluster in Puppis

M93 is a bright, compact star cluster in Puppis, the stern of the old ship constellation Argo, southeast of Sirius. From mid-northern latitudes it never climbs high, but it is bright enough to be worth catching low in the south on winter evenings.

What it is

M93 is an open cluster, a loose family of stars born together from the same cloud of gas. It lies about 3,170 light-years away, based on Gaia measurements, and its 22′ extent corresponds to roughly 20 light-years across. It is young by star standards, around 100 million years old, and its brightest members are hot blue-white stars. Its core is fairly concentrated, with a roughly wedge-shaped or triangular outline that many observers notice.

For other winter clusters in the same part of the sky, try M46 and M47 to its north, and M41 just south of Sirius.

When to see it

M93 is a winter object. From about 40°N it is highest around midnight in late January and around 10 p.m. standard time in late February, but even then it reaches only about 26° above the southern horizon, so you need a clear view to the south. From the Southern Hemisphere it is a summer-evening object that passes high overhead; from around 24°S it crosses the zenith.

How to find it

Start at Sirius and look about 17° to the east-southeast for Xi Puppis, a 3rd-magnitude star that stands out in a fairly empty area. M93 lies about 1.5° northwest of Xi Puppis, close enough that both appear in the same binocular field. If you know M46 and M47, M93 is about 9° due south of M46.

What you will see

In 10×50 binoculars M93 is a bright, slightly grainy patch, and under a dark sky a few of its brightest stars resolve. A 70–100 mm telescope at low power shows several dozen stars in a tight, wedge-shaped group, with a few brighter blue-white stars and a sprinkling of fainter ones. A 200 mm telescope resolves it fully into a rich cluster of stars of mixed brightness. Because it is bright, M93 holds up well from the suburbs, but low altitude and haze dim it for northern observers. No filter is needed.

History

Charles Messier discovered M93 on March 20, 1781. It was the last object in his catalog that he discovered himself; the later entries were found by Pierre Méchain or added by others. Messier described it as a cluster of small stars without nebulosity. Modern Gaia data have refined its distance and membership.