THE SEVEN SISTERS

By Richard Hall

‘Seven Sisters – Matariki’

The most celebrated stars in our night sky are the Pleiades. In folklore they are usually known as the ‘Seven Sisters’.  In Japan they are known as ‘Subaru’ (and you thought it was a car) and, in New Zealand they are of course, Matariki.

Historically the Pleiades are very significant. 5,000 years ago their heliacal rising (rising just before the Sun) marked the northern spring Equinox.  This was known as the “Pleiad-month” and marked the beginning of the year.  Their rising and setting divided the year into seasons for sowing and harvest for the Egyptians, Greeks and peoples of Asia. To the Greeks, 3,000 years ago, the heliacal rising of the Pleiades signaled the opening of the sea-lanes.  This was perhaps their most important event of the year because their economy depended upon sea trade  The Pleiades became associated with Athene, the great goddess of wisdom and civilization, and great temples such as the Acropolis, were orientated to their rising.

 The ancestors of the Polynesians came from what is now Indonesia.  At that time it was the tradition in Asia to start the new-year with the heliacal rising of the Pleiades. 4,500 years ago the migrations into the Pacific began and they carried this tradition with them.  To Maori the Pleiades are known as Matariki and to this day their heliacal rising heralds the beginning of the Maori New Year.  

SO, WHAT IS MATARIKI?      

Matariki, the Pleiades is an open star cluster. This is a group of stars that share a common origin and, are relatively close together and gravitationally bound. The number of stars in an open cluster varies from a few dozen to several thousand.  

All stars are formed in clusters but, with the passage of time, the clusters disintegrate.  Few survive 1,000 million years. By comparison our Sun and its system of planets are 4,600 million years old.   Once, long ago, our Sun was part of an open star cluster.  Its siblings have long since vanished and now make their own way through the star-fields of the galaxy.   If you look along the Milky Way with a pair of binoculars you will discover dozens of star clusters.  By their very existence you will know that they are composed of comparatively young stars.

‘The Pleiades as seen from the southern hemisphere.’

Although the Pleiades are called the ‘Seven Sisters’, depending on sky conditions, the average person can see between 5 and 9 stars.  However, the cluster actually contains over a thousand stars. All stars vary in size, mass and luminosity.  When we look at the Pleiades with the unaided eye we see only the brightest members of the cluster.

A young cluster of giant stars

The cluster has a central distance of 443 light-years and a diameter of 35 light-years.   By comparison with the same volume of space surrounding our sun there are 61 stellar systems.  These systems contain 73 stars, of which 57 are red dwarfs, which are by far the most common type of star in the Galaxy. These red dwarfs are so intrinsically faint that not one can be seen without a telescope. The Pleiades would have a similar composition except the density of stars is 14 times greater.  The other difference is that because the Pleiades are relatively young they contain bright massive stars that no longer exist in our region of space.  These bright stars in the Pleiades are the Sisters.

Stardust in the Pleiades, photo by Arnaud Mariat on Unsplash

As already mentioned cluster stars tend to be young and the Pleiades, with an age of about 100 million years, are infants as far as stars go.  The cluster is immersed in a cosmic cloud of dust and ice-crystals that reflects the blue light of the brighter stars.  The wispy blue nebulosity looks fabulous in long exposure photographs but is difficult to see visually without a large telescope. 

In Greek mythology the sisters are seven nymphs, the daughters of the Titans Atlas and Pleione.  Atlas, the brighter of the two, is a blue-white giant (class B8 III) that is 594 times brighter than our Sun.  What appears to be a single star is actually a multiple system.  Two bright stars (magnitudes 3.84 and 4.46) orbit around their common center of gravity in a period of 290 days.  The brighter of the two is itself a spectroscopic binary system.  So, what appears to be a single star is in fact a triple system.

Pleione, the mother

‘Pleione (artist (Richard Hall) impression)

Atlas has a distance of 431 light-years. Below Atlas in our photograph, and slightly closer to us is Pleione. She is a blue-white sub-giant (class B8 IV), 190 times brighter than our Sun, and has a distance of 422 light-years.

Actually I’m rather fond of Pleione.  She was the very first star I studied.  And, after many a night in the cold, she finally revealed her light curves.   Pleione is white hot, more than double the surface temperature of our Sun, and has a mass equal to 3.4 suns.  She spins on her axis so fast that she has the shape of a lens.  Due to her rotational speed of 329km/s she spins once on her axis every 11.8 hours.  Our Sun takes 25.3 days.

As Pleione spins she hurls off rings and shells of glowing plasma at irregular intervals.  This causes her to fluctuate unpredictably in brightness.  Within hours she can rival Atlas by more than doubling in brightness.  These fluctuations and eruptions can be observed with a pair of binoculars.     

Pleione is a spectroscopic binary system.  She has a smaller companion star that orbits around her in a period of 218 days. The mean separation is 24 AU. (Uranus is 19 AU from the Sun, Neptune 30 AU).  Like Atlas, Pleione may have once been a triple star system. The bright star could once have been a contact binary system in which the two stars have merged together.   This would explain her tremendous rotation.