North Hawaii News Articles from CFHT

Six Stars, Seven Sisters

Their names are Maia, Electra, Alcyone, Taygete, Asterope, Celaeno and Merope. They are the seven sisters. They are the Pleiades. Their mother is Pleione, the Oceanid. Their father is Atlas, one of the twelve Titans, supporting the world at arms length. That's according to the Greek Mythology, of course! But one day, a long time ago, it is said that the Pleiades were travelling with their mother when they met Orion, the mighty hunter. Orion immediately fell in love with the beautiful women and started chasing them. The chase lasted for several years until Zeus, the king of Heaven and Earth and of all the Olympian Gods, helped them escape by changing them into doves. The doves flew into the sky and became the stellar cluster we see today, the Pleiades, still beautiful at 20 million years old, laced with ice-blue glowing interstellar dust.

The Pleiades are located in the constellation Taurus, the bull, whose head has a dinstinctive V-shape visible with the naked eye and featuring the bright and reddish Aldebaran. As expected, Orion can also be seen nearby, although slightly further East and to the South, still chasing them. (see the night sky map for February in the Jan. 27th issue of NORTH HAWAII NEWS )

Since Antique times, people from various cultures, from all over the world, have assigned myths to the seven stars. Often their appearance is the signal for a change of seasons. For us living in the Northern Hemisphere, Makali'i, the Pleiades, become visible at dusk on the eastern horizon in the fall and signals the arrival of winter. To the opposite, for people living in the Southern Hemisphere, the Pleiades rise at dusk in the spring and signal the proximity of the rain season. To us here and now, and although they have lost a bit of their mythical influence, the Pleiades still signal winter. They also remain one of the most recognized stellar cluster in the sky and one the most beautiful to look at.

The Pleiades can be seen very easily these days, near the zenith (right above your head!) in the evening, at 20h00. The stars are fairly bright and together they look like a small version of the Big Dipper. Six of the stars are clearly visible with the naked eye, seven if you have a very good vision (I don't!). If you are very patient and willing to use binoculars, you could probably see a few more. But this is only the tip of the iceberg. With large telescopes, like those located atop Mauna Kea, the Pleiades were discovered to actually contain hundreds of stars, forming what the astronomers call an open stellar cluster, a group of stars loosely bound together that will eventually disperse. Today, the cluster spreads over many degrees in the sky. The seven sisters themselves covering a smaller area, about half a degree, the size of the full moon.

In Hawaii, the Pleiades are often studied in greater details with the state-of-the-art instruments of the Mauna Kea Observatories. For example, recent observations show that a large fraction of the stars in the cluster are actually twins, binaries. What we thought were mostly single stars turned out to be mostly close pairs, two stars orbiting rapidly around one another, a bit like the Earth and the Moon although with two stars. That's the case for more than half the stars in the Pleiades!

Another discovery that was made by astronomers (or is it really the case?) is that a few members of the Pleiades cluster are also well known for their large variations, their brightness fades and increases by large factors over the years. The ancients obviously knew that before us as they claim that Merope, one of the sisters, was so ashamed to have had a mortal husband, Sisyphus, that she vanished, deserted her sisters. That's why we only see six of the seven Sisters today... without a telescope that is.

Francois Menard
Visiting Scientist
Canada-France-Hawaii Telescope Corporation