The Pleiades (M45), also known as the Seven Sisters, is one of the easiest and most recognizable star clusters in the night sky. Visible to the naked eye as a tight group of bright stars in the constellation Taurus, it is actually a young open cluster containing more than 1,000 stars about 444 light-years from Earth.
The Pleiades is also one of the best star clusters for beginner stargazers. You can find it without a telescope, see many more of its stars through binoculars, and even glimpse the blue reflection nebulosity surrounding some of its brightest members in photographs. Its distinctive appearance has also inspired stories and names across cultures for thousands of years.
Here are some of the most interesting facts about the Pleiades, including what its stars are, why it is called the Seven Sisters, and how to find it in the night sky.
Pleiades Quick Facts
- Object type: Open star cluster
- Constellation: Taurus
- Distance: About 444 light-years
- Age: About 100 million years
- Apparent magnitude: 1.6
- Number of stars: More than 1,000 known members
- Coordinates: RA 03h 47m 24s, Dec +24° 07′
- Other names: Messier 45 (M45), Seven Sisters, Melotte 22
The Pleiades Is a Young Family of Stars
The Pleiades is an open star cluster, a group of stars that formed together from the same giant cloud of gas and dust and are loosely bound together by gravity. It is extremely young by astronomical standards, particularly compared with our 4.6-billion-year-old Sun.
More than 1,000 stars have been identified as members of the traditional Pleiades cluster. They include stars of many different masses, but the hot, luminous blue-white stars are the ones that dominate its familiar appearance in the night sky. The cluster also contains numerous faint, low-mass stars and brown dwarfs that cannot be seen with the naked eye.
Over hundreds of millions of years, interactions between its stars and the gravitational influence of the Milky Way will gradually pull the Pleiades apart, with its members eventually dispersing into the galaxy.
Why Is It Called the Seven Sisters?
Despite being a large star cluster, the Pleiades has been known as the Seven Sisters since ancient times. Most people can distinguish about six stars with the naked eye under good conditions, although observers with particularly sharp eyesight and dark skies may see more.
The brightest member is Alcyone, with an apparent magnitude of about 2.9. Other prominent stars include Atlas, Electra, Maia, Merope, Taygeta, Pleione, Celaeno and Asterope. Many of these names come from Greek mythology, where the Pleiades were the seven daughters of Atlas and Pleione.
Why a group usually seen as six stars became associated with seven sisters is an old puzzle. One Greek tradition explains the missing seventh sister as Merope, whose light was said to have faded after she married a mortal. The real explanation may simply involve differences in eyesight, observing conditions and the brightness and close spacing of the cluster’s stars.
A telescope reveals a very different picture. When Galileo Galilei observed the Pleiades in 1610, he recorded dozens of stars around the familiar naked-eye group, an early demonstration that the Seven Sisters were only the brightest members of a much larger cluster.
The Blue Haze Is a Reflection Nebula
Long-exposure images of the Pleiades reveal a striking blue haze surrounding several of its brightest stars, particularly Merope. This is a reflection nebula: interstellar dust scatters the stars’ light, with shorter blue wavelengths scattered more efficiently than red ones.
It would be natural to assume that this dust is material left over from the formation of the Pleiades, but the cluster is far too old for its original birth cloud to have survived around it. Instead, the Pleiades is passing through an unrelated region of interstellar dust, temporarily illuminating the material as its bright stars move through it.
The nebulosity is prominent in photographs but much harder to see with the eye. Under very dark skies, experienced observers may detect some of the haze around Merope through a telescope, but for most stargazers the stars themselves will dominate the view.
How to Find the Pleiades in the Night Sky
The Pleiades lies in the constellation Taurus and is easy to recognize once you know what to look for. To the naked eye it appears as a small, compact group of stars, often described as looking like a tiny version of the Big Dipper.
One of the easiest ways to find it is to start with the three stars of Orion’s Belt. Follow the line of the belt northwest toward the bright orange star Aldebaran, then continue in roughly the same direction until you reach the much tighter grouping of the Pleiades.
When and How to See the Pleiades
For Northern Hemisphere observers, the Pleiades is best seen during autumn and winter, when it rises in the evening and climbs high in the night sky. It remains visible into spring before disappearing into the Sun’s glare. In the Southern Hemisphere, it is also a prominent evening target during the warmer months.
No equipment is needed to enjoy the Pleiades. Under a reasonably dark sky, most people can make out several individual stars, while the cluster may look more like a small misty patch from light-polluted locations.
Binoculars are one of the best ways to view the Pleiades, combining low magnification with a wide field of view so that dozens of stars can be seen while the entire cluster remains in view. A telescope can reveal still fainter members, but higher magnification can spread the stars beyond the field of view and make the cluster’s distinctive overall shape harder to appreciate.
Humans Have Watched the Pleiades for Thousands of Years
Few groups of stars have attracted as much attention across different cultures as the Pleiades. Its compact, easily recognized appearance and seasonal return to the sky made it useful for marking times of year as well as a natural subject for stories and traditions.
In Japan, the cluster is known as Subaru, a name meaning “to come together.” In Māori tradition in New Zealand, it is known as Matariki, whose reappearance in the predawn sky during midwinter is associated with the beginning of the Māori New Year.
Similar traditions developed in many other parts of the world. The Pleiades has been given different names and meanings in cultures across Asia, Europe and the Pacific, showing just how readily this small group of stars caught the attention of people looking at the night sky long before modern astronomy.
The Pleiades and Hyades Are Different Star Clusters
The Pleiades is not the only prominent open star cluster in Taurus. The Hyades forms the broad V-shaped pattern around the face of the Bull, while the Pleiades appears nearby as a much smaller and more compact group.
Despite appearing close together in the sky, the two clusters are physically separate. The Hyades is much nearer to Earth, at about 150 light-years away, and is also considerably older than the Pleiades. Its stars consequently appear spread across a much larger area of the sky, while the more distant Pleiades forms the tighter grouping we recognize so easily.
The bright orange star Aldebaran makes the Hyades particularly easy to recognize, but it is not actually a member of the cluster. Aldebaran lies much closer to Earth and simply happens to appear along the same line of sight.