Seven Celestial Sisters Rule the Sky

The Pleiades as seen by the Hubble Space Telescope.
Space Telescope Science Institute

 In the story Top 10 Cool Things in the Sky, you get a sneak peek at a little star cluster that is famous the world over. It's called "The Pleiades" and makes its best appearance in the night skies from late November to through March each year. In November, they're up from dusk to dawn.

This star cluster has been observed from nearly every part of our planet, and everyone from amateur astronomers with small telescopes to astronomers using Hubble Space Telescope has taken a shot of it. 

Many of the world's cultures and religions focus on the Pleiades. These stars have had many names and show up on clothing, flats, pottery, and artwork. The name we know these stars by now comes from the ancient Greeks, who saw them as a group of woman who was companions to the goddess Artemis. The seven brightest stars of the Pleiades are named after these women: Maia, Electra, Taygete, Alcyone, Celaeno, Sterope, and Merope.

Pleiades and Astronomers

They make up an open star cluster that lies about 400 light-years away, in the direction of the constellation Taurus, the Bull. Its six brightest stars are relatively easy to see with the naked eye, and folks with very sharp vision and a dark sky sight can see at least 7 stars here. In reality, the Pleiades have more than a thousand stars that formed in the last 150 million years. That makes them relatively young (compared to the Sun, which is about 4.5 billion years old).

Interestingly enough, this cluster also contains many brown dwarfs: objects too hot to be planets but too cold to be stars. As they're not very bright in optical light, astronomers turn to infrared-sensitive instruments to study them. What they learn helps them determine the ages of their brighter cluster neighbors and understand how star formation uses up the available material in a cloud.

The stars in this cluster are hot and blue, and astronomers classify them as B-type stars. Currently, the core of the cluster takes up an area of space about 8 light-years across. The stars are not gravitationally bound to each other, and so in about 250 million years, they will begin to wander away from each other. Each star will travel on its own through the galaxy.

Their stellar birthplace probably looked largely like the Orion Nebula, where hot young stars are forming in a region of space about 1,500 light-years away from us. Eventually, these stars will go their separate ways as the cluster moves through the Milky Way. They'll become what's known as a "moving association" or a "moving cluster". 

The Pleiades appear to be passing through a cloud of gas and dust that astronomers once thought was part of their birth cloud. It turns out this nebula (sometimes called the Maia Nebula) is unrelated to the stars. It does make a pretty sight, though. You can spot it in the nighttime sky pretty easy, and through binoculars or a small telescope, they look spectacular! 

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Petersen, Carolyn Collins. "Seven Celestial Sisters Rule the Sky." ThoughtCo, Feb. 16, 2021, Petersen, Carolyn Collins. (2021, February 16). Seven Celestial Sisters Rule the Sky. Retrieved from Petersen, Carolyn Collins. "Seven Celestial Sisters Rule the Sky." ThoughtCo. (accessed March 29, 2023).