Amateur astronomers actually spend very little time looking at individual stars through their telescopes, but star clusters are another matter. The sight of hundreds of stars glittering against the blackness of the night crowding into the eyepiece is a delight. And at this time of year there are clusters of all types across the sky. The best known of them all is the Pleiades, aka the Seven Sisters, close to the Moon tonight. And you can enjoy the sight with ordinary binoculars, so when the Moon has moved away from them take a look.
That little fuzzy patch, showing maybe six or seven stars to the naked eye, is immediately transformed. You can now count dozens, but the cluster actually has more than 1000 members.
The Pleiades have attracted eyes since the dawn of history. They are mentioned in the Bible, and are thought to be depicted on a bronze star disc dating from 1600 BC. Like all star clusters, they were formed within a cloud of gas known as a nebula, like the Orion Nebula, which has long since disappeared having yielded up its gas to form stars. In the case of the Pleiades this happened around 100 million years ago.
As with many clusters, its brightest members are hot, blue stars because the brightest stars in a young cluster such as the Pleiades are the most massive. They have comparatively short lifetimes, using up their fuel quickly compared with the less massive stars such as the Sun. (Stars shine because they convert hydrogen into helium, releasing energy in the process.) Eventually the stars of a cluster such as the Pleiades will drift away from one another, and all that will be left is a widely separated group of fainter stars as the most massive ones have ended their lives, so the cluster id no longer obvious.
Professional astronomers have learned a lot from star clusters as they provide a set of stars at a similar distance and age, so they can compare the way different sizes opf stars evolve.
Other clusters in the February sky
See if you can find the clusters in the map below. Starting from Jupiter, look overhead for the bright star Capella, then for the pentagon of stars known as the constallation of Auriga. Strung out through Auriga is a line of three clusters known as M36, M37 and M38. Each is slightly different from the others, with M36 the most condensed and M38 the least.
Then find a pair of stars between Mars and Jupiter, which are part of Gemini (marking Castor’s left foot) and look just beyond them to find M35.
The clusters were first catalogued by French observer Charles Messier in the 18th century, hence the M in front of their names. Nothing to do with motorways.
For the clusters below, first find the W-shape of Cassiopeia, almost overhead, and then the stars of Perseus to its lower left. The Double Cluster is one of the most beautiful clusters in the heavens, and consists of two nearby clusters some 7000 light years away. Also look for M34, which is closer but with fewer stars, the smaller NGC 663 in Cassiopeia, and NGC 752, a large cluster but with faint stars that might be a challenge from a city area or when the Moon is nearby.
