The sky — astronomy

The Pleiades: The Seven Sisters

The Pleiades (Messier 45, or M45) are a young open star cluster in the constellation Taurus, about 440 light-years away. Most people can see 6 or 7 stars with the naked eye, yet the cluster holds around a thousand. That is why it is also called the Seven Sisters.

An open cluster in Taurus

The Pleiades, listed as Messier 45 (M45) in Charles Messier's catalogue, are one of the nearest and brightest star clusters to Earth. They lie in the constellation Taurus (see Taurus, the zodiac sign) roughly 440 light-years away. An open cluster is a loose group of stars born together from the same cloud of gas and dust, which still travel through space as a loosely bound family.

The cluster is young by cosmic standards. Its age is estimated at about 100 million years, while the Sun is roughly 4.6 billion years old. The brightest members are hot blue stars, which give the cluster its cool, bluish glow.

There are around a thousand stars in the group, but they are packed into a small patch of sky about 2° across, roughly four times the width of the full Moon. Over time the stars will drift apart. Open clusters are not permanent and usually disperse over hundreds of millions of years.

The dust around the stars

Long-exposure photographs show a blue haze around the brightest stars. This is a reflection nebula: a dusty cloud that does not shine on its own but reflects the light of nearby stars. It was once thought to be leftover material from the cluster's birth. More recent studies suggest the cloud is more likely a chance encounter, a dust cloud the cluster is passing through on its journey across the Galaxy.

How many stars can you see?

The cluster is nicknamed the Seven Sisters, but what you see depends on your eyesight and the sky. Under good conditions most people can pick out 6 or 7 stars, and observers under very dark skies may see a few more. From a city you may notice only 3 or 4, or just a faint smudge.

The brightest stars carry names from Greek mythology: Alcyone, Atlas, Electra, Maia, Merope, Taygete, Pleione, Celaeno and Sterope. That makes nine named stars, the parents Atlas and Pleione plus their daughters.

To find the cluster, follow the line of Orion's Belt (see Orion) upward through the orange star Aldebaran, the eye of the Bull, and continue about the same distance again. You will reach a tight little group of stars. Aldebaran is not part of the cluster: it lies much closer to us and only happens to sit along the same line of sight. People often mistake the Pleiades for a tiny Little Dipper because of the cluster's compact cup shape.

From the Northern Hemisphere the cluster is best seen on evenings from autumn to early spring. For general tips, see how to look at the sky.

The Pleiades in myths around the world

The Pleiades are among the most widely mentioned objects in the night sky, probably because they are easy to recognise and appear at a particular time of year.

• Greece. In Greek mythology they are the seven daughters of the Titan Atlas and the sea nymph Pleione. The poet Hesiod, in Works and Days, links their rising and setting to farming tasks.
• Bulgaria. In folk tradition the cluster is the Kvachka, a mother hen with her chicks, a nod to the tight huddle of small stars.
• Japan. The cluster is called Subaru, meaning gathered together. The Japanese car maker took its name from it, and its logo shows six stars.
• New Zealand. For the Maori, the Pleiades are Matariki, and their reappearance in the winter sky marks the start of a new year.
• Hawaii. They are known as Makali‘i.
• Australia. In many Aboriginal Australian stories they are a group of sisters pursued across the sky.

These are cultural stories, not scientific claims. Their charm is that people on different continents looked at the same stars and imagined similar pictures: a family, a flock, a gathering.

Observing with binoculars

The Pleiades are one of the best targets for binoculars. Because the cluster spans about 2°, a high-power telescope often cuts it into pieces, whereas binoculars frame the whole scene. Standard binoculars such as 7×50 or 10×50 reveal dozens of stars arranged in chains and arcs, with the brightest shining blue-white.

Tips:

1. Move away from bright lights and give your eyes 10 to 15 minutes to adapt to the dark.
2. Brace your elbows or use a tripod to steady the view.
3. Look slightly to the side of the cluster. Averted vision helps faint stars appear.
4. Do not expect the blue nebulosity from photographs. It is too faint for binoculars.

If you are unsure what to buy, read binoculars or telescope.

Why scientists study them

Because the cluster is close and rich, astronomers use the Pleiades as a benchmark. Stars in a cluster share nearly the same age and composition, so comparing them helps test theories of how stars evolve. The exact distance to the cluster was once the subject of a long scientific debate between different measurement methods, and newer astrometric data have helped to settle it. So the Pleiades are both a beautiful evening sight and an important laboratory for science.

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