The Seven Sisters resemble a tiny dipper of stars, but they struck me differently this past Monday night. Their faint appearance and stealthy climb up the eastern sky reminded me of a fox on the prowl at night, with the cluster a fresh pawprint in the snow. The time was 9:30 p.m. The summer Milky Way held sway overhead, but the Pleiades — the other name for those seven suns — hinted of future frosts.
There are more than 1,000 known open clusters in the Milky Way galaxy — gravitationally-bound groups of stars all born from the same massive clouds of dust and gas cast off by earlier generations of stars. They're called "open" because they're loosely arranged and contain fewer stars compared to globular clusters, which pack stars cheek by jowl inside enormous spherical beehives.
Of the many open clusters visible with the naked eye or telescope, the Seven Sisters is one of the brightest, closest and most recognizable. Most people see five stars arranged in a small dipper. That's especially true right now during the evening hours when it's still low in the sky. But if you wait for the Sisters to rise higher or view it from the countryside it's not too difficult to spot one or two additional members by looking around the cluster rather than directly at it, a technique called averted vision.
Some skywatchers have gone deeper yet under rural skies. On one attempt several years back, I spotted five more faint members for an even dozen, by no means a record. The bright cluster we see represents only the core. All told, the Pleiades comprises as many as 3,000 stars across its span of 13-light-years.
One reason the Seven Sisters shine so brightly is because they're relatively close to Earth, just 444 light-years away. That's still a mind-boggling distance when you consider that the light arriving from the cluster in 2021 left in 1577, 31 years before the invention of the telescope! Don't worry though. Despite the lag, the Pleiades are still around. Most of the bright members we see are blue giant and subgiant stars born 75-100 million years ago. They'll be around for millions more years and continue to shine even after the cluster gets torn apart and dispersed across space over the next 250 million years.
Wait a minute. No more Seven Sisters? Although the cluster's suns exert gravity on one another and hang together as they circle the center of the galaxy, other forces work to tear them apart. Large clouds of gas, the kind that created the Pleiades in the first place, can steal stars away through gravitational interactions. Supernova explosions from its most massive members can disrupt and eject stars, while the larger stars can gravitationally slingshot the smaller ones into the wilds of space.
Even the gravity of the Big Heavy, the Milky Way galaxy itself, gnaws away at clusters and slowly sucks the suns right out of them. And the more stars a cluster loses the less able it is to hold itself together. Life's just not fair.
The Pleiades represent the the seven daughters of the mythological Greek titan Atlas and the sea-nymph Pleione. The name itself may derive from Greek word "plein," which means to sail. The navigation season on the Mediterranean Sea officially began when the group first appeared in the dawn sky before sunrise. The Japanese call it Subaru and emblazon a stylized version of the object on each car in that line. The ancient Celts knew it as Twr Tewdws, which translates to a thick bunch or heap (of stars).
I hope you have a chance to see this pretty heap ascend the eastern sky in the next couple of weeks before the moon returns to light up the night. Take a pair of binoculars along, and you'll add 50 or even 100 stars to the six or seven visible with the unaided eye.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.