A guide to M45, also known as the Pleiades, and images of the famous open star cluster.

By Patrick Moore

Published: Thursday, 26 October 2023 at 11:07 AM

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The Pleiades is probably the most famous star cluster in the sky, prominent with the naked eye and difficult to mistake.

Located 440 lightyears away, the Pleiades is an open star cluster contained in Charles Messier’s catalogue of clusters and nebulae, the Messier Catalogue, drawn up in 1781, as M45.

In a way this is surprising, because Messier was a comet-hunter and catalogued clusters and nebulae as ‘objects to avoid’ – and there is no chance of confusing the Pleiades with a comet.

The nine brightest stars in the Pleiades are concentrated in a field just over one degree across, so they are splendidly shown in binoculars or in a telescope equipped with a wide-angle eyepiece.

With higher magnification only part of the cluster can be shown at any one time, so that the full beauty of the spectacle is lost.

Greek mythology

The Pleiades has been known from very early times and there is no shortage of legends about it.

Also known as the ‘Seven Sisters’, in Greek mythology they were the seven daughters of the giant Atlas and Pleione, daughter of the sea god Oceanus.

Alcyone, Electra, Merope, Maia, Taygete, Calaeno and Asterope (or Sterope). They were walking along, quite peacefully, when they were seen by the huntsman Orion who promptly gave chase.

Zeus, ruler of Olympus, decided to intervene, so he changed the sisters into stars and swung them into the sky, where they remain to this day.

In 1767, the English clergyman John Michell – a brilliant researcher, too often neglected by modern scientific historians – calculated that the probability of a chance alignment of so many bright stars was only about one in half a million.

When studies were first made of the proper motions of the stars in the Pleiades, it was found that they were all moving across the sky in the same direction at the same rate.

Yet the cluster will not survive indefinitely. Like all galactic clusters it will eventually be dispersed because of the gravitational pull of non-cluster stars.

It should remain identifiable for at least 250 million years, however, so we need be in no hurry to observe it before it merges into the general background.

Although there are only six stars easily distinguishable with the naked eye by people of average sight, the cluster has traditionally been known as the Seven Sisters.

This has led to the legend of the ‘lost Pleiad’, which faded below naked-eye visibility.

What type of stars are in the Pleiades?

The main cluster is about 12 lightyears in diameter and contains around 500 stars. The total mass contained is estimated to be about 800 times that of the Sun.

Hot bluish-white stars are dominant. There are no red giants, but there are a number of brown dwarfs – that is to say, objects with less than 8% the mass of the Sun, whose cores have never become sufficiently hot to trigger off nuclear reactions.

There are several white dwarfs, and there is obviously nebulosity, which is brightest in the region of the star Merope.

The nebulosity was discovered in 1859 by the German astronomer Ernst Tempel, using a 4-inch (10cm) telescope, who described it as “a faint stain like a breath on a mirror”.

The nebula is not particularly elusive – I saw it easily with my five-inch (12cm) refractor – and it is a favourite target for astrophotographers (see our gallery below).

Rather naturally, it was assumed to have been formed at the same time as the Pleiades themselves – but this cannot be so.

The cluster is of the order of 100 million years old, and a reflection nebulae of this kind would long since have been dissipated.

In fact, the explanation is that the stars are simply passing through a dust cloud which happens to lie in their path.

How to find the Pleiades

First look for the very familiar constellation of Orion with its two leaders, the orange-red Betelgeuse and the even brighter white Rigel.

The three stars of Orion’s Belt point upwards to Aldebaran, which like Betelgeuse is orange-red; slightly higher up, you will see the Pleiades.

At first they may look like a misty patch, but if conditions are even reasonably good it rapidly becomes evident that they are individual stars, close together.

Pictures of the Pleiades

Below is a selection of images of the Pleiades captured by BBC Sky at Night Magazine readers and astrophotographers from around the globe.

For advice on imaging the night sky, read our guide to astrophotography or our tutorial on how to photograph stars.

And don’t forget to send us your images or share them with us via Facebook, Twitter and Instagram.

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