The Pleiades star cluster, or Seven Sisters, is the most well-known star cluster in the night sky. It’s what’s known as an ‘open cluster’, which is a group of stars that form from the same huge cloud of dust and gas. As the cloud collapses under gravity, temperatures rise and stars begin to take shape, becoming loosely bound by their mutual gravitational attraction. The Pleiades star cluster contains some 3,000 stars, all less than 100 million years old, making them mere babies compared to our Sun’s 4.6 billion years.
Also known as the “Seven Sisters” and Messier 45, the object derives its English name from Greek legend. The Pleiades star cluster are the seven daughters of the Titan god Atlas and the ocean nymph Pleione. During an ancient war, Atlas rebelled against Zeus, the king of the gods, who sentenced his foe to forever hold up the heavens on his shoulders. The sisters were so sad that Zeus allowed them a place in the sky in order to be close to their father.
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Located 440 lightyears away, the Pleiades star cluster is an open star cluster contained in Charles Messier’s catalogue of clusters and nebulae, the Messier Catalogue, drawn up in 1781, as M45.
The Pleiades star cluster are an example of an open star cluster — a group of stars that were all born around the same time from a gigantic cloud of gas and dust. The brightest stars in the formation glow a hot blue and formed within the last 100 million years.
By pointing Kepler at the Pleiades star cluster, researchers confirmed that six of the Seven Sisters — Alcyone, Atlas, Electra, Merope, Taygete and Pleione — are slowly pulsating type B stars, which change in brightness over the course of one day. The seventh star, named Maia, has a brightness that fluctuates over a longer period of 10 days.
When studies were first made of the proper motions of the stars in the Pleiades star cluster, it was found that they were all moving across the sky in the same direction at the same rate. 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.
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.
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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.
To find the Pleiades star cluster, first locate the three stars in Orion’s Belt. During November, look above the eastern horizon from around 10pm. Draw an imaginary line going through the belt from left to right, and continue this line through Orion’s bow. This will direct you to the brightest star in Taurus: Aldebaran.
If you’re familiar with the famous constellation Orion, it can help you be sure you’ve found the Pleiades star cluster. See the three stars in a row in Orion? That’s Orion’s Belt. Draw a line through these stars to the V-shaped pattern of stars with a bright star in its midst. The V-shaped pattern is the Face of Taurus the Bull. The bright star in the V – called Aldebaran – depicts the Bull’s Eye. A bit past Aldebaran, you’ll see the Pleiades star cluster, which marks the Bull’s Shoulder.
In the night sky, the Pleiades star cluster sits within the constellation of Taurus. It’s actually possible to see up to 14 of the stars with the naked eye in areas with no light pollution. You can see the Pleiades star cluster between October and April, but the best month to look for it is November, when it can be seen for the entire night.
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Keep following the same line from Orion’s belt, and you’ll run into a fairly dim cluster of blue stars. These are the Pleiades star cluster, also called the Seven Sisters — although most people can only see six or fewer, and telescopes can see many more. The Pleiades are an “asterism,” a star pattern much smaller than a constellation.
The Pleiades star cluster are part of what astronomers call an open star cluster, a group of stars all born around the same time. Telescopes have identified more than 800 stars in the region, though most humans can spot only about six on a clear, dark night.
In our Northern Hemispheres skies, the Pleiades star cluster is associated with the coming winter season. It’s easy to imagine this misty patch of icy-blue suns as hoarfrost clinging to the dome of night. Frosty November is the month of the Pleiades star cluster, because it’s at this time that the Pleiades star cluster shine from dusk until dawn. But you can see the Pleiades star cluster in the evening sky well into April.
The Pleiades star cluster rise in the southeast after dusk and travel west during the night. During their peak in November, they climb high in the sky and disappear in the northwest before dawn. In late winter and early spring, they will only be visible for a few hours, traveling east to west across the southern part of the sky
As a general rule, the Pleiades star cluster rises into the eastern sky before Aldebaran rises, and sets in the west before Aldebaran sets. The star name Aldebaran comes from an Arabic word for follower. It’s thought to be a reference to this star’s forever chasing the Pleiades star cluster across the heavens.
Historically, the Pleiades star cluster have served as a calendar for many civilizations. The Greek name Pleiades star cluster probably comes from a word meaning to sail. In the ancient Mediterranean world, the day that the Pleiades star cluster first appeared in the morning sky before sunrise announced the opening of the navigation season.
The modern-day festival of Halloween originates from an old Druid rite that coincided with the midnight culmination of the Pleiades star cluster. People believed the veil dividing the living from the dead is at its thinnest when the Pleiades star cluster culminates – reaches its highest point in the sky – at midnight.
