Seven Sisters of the Pleiades

What makes this famous cluster of stars in the night sky so interesting?

The Pleiades or Seven Sisters has been observed for millennia, with its proximity to Earth and large stars making it observable to the naked eye and also an important tool for astronomers. Over 1,000 stars are contained within the cluster, and its characteristics have made it a stepping stone to understanding other clusters and stars in the universe.

Some of the earliest known depictions of Pleiades date back to 1600 BC, but it was not until 1771 AD that Charles Messier measured the position of the cluster and classified it as M45. Its nine brightest stars are named after the Seven Sisters of Greek mythology along with their parents. In order of brightness they are Alcyone, Atlas, Electra, Maia, Merope, Taygeta, Pleione, Celaeno and Sterope.

The Pleiades is between 390 and 460 light years away, making it one of the most easily viewable star clusters from Earth. This has seen it become an important calibration tool for calculating the distance to other stars in the universe.

The core cluster of stars within Pleiades have a radius of about eight light years. The cluster is dominated by bright hot blue stars, 14 of which are visible to the naked eye. These stars are typically between 15 and 90 times the mass of our own Sun with a luminosity at least 30,000 times greater and as much as 1,000,000 times more. However, owing to their large mass, they burn through their hydrogen fuel very quickly. Whereas our Sun is thought to have a lifetime of ten billion years, a hot blue star will live for just tens of millions of years.

A quarter of the Pleiades cluster is made up of brown stars, stars with less than eight per cent of the Sun’s mass that are not heavy enough to ignite nuclear fusion in their cores. The relative proximity of Pleiades to the Earth has made it an excellent location to study these dim and diffuse stars, as in older clusters further away they are much more difficult to observe.

The cluster itself seems to bear some signs of being a reflection nebula. Dust in Pleiades reflects the light of the hotter blue stars, which may be a remnant of when the cluster first formed. However, its age would suggest that most of the dust from its formation should now have dispersed, suggesting that the nebula elements may merely be the result of the Pleiades passing through an especially dusty region of interstellar space.

The Pleiades is estimated to have an age of between 75 and 150 million years. It is an open star cluster, which means it will not stay gravitationally bound forever, and indeed calculations suggest it will disperse within 250 million years after interaction either with spiral arms in the Milky Way or the Orion constellation.