Starry Sisters

Credit: NASA, ESA and AURA/Caltech

High in the win­ter sky, the Pleiades look like a tiny dip­per. Those with sharp eyes can see seven stars in the open star clus­ter M45, or the Seven Sis­ters. Some peo­ple report see­ing up to 14 stars under the best con­di­tions. But most, includ­ing myself, can see six at best. In this image from NASA’s Hub­ble Space Tele­scope, we see hun­dreds of stars.

Ancient Greeks gave us name the Pleiades. But the star clus­ter was known to other ancient cul­tures as well. The Bably­lo­ni­ans referred to them as Mul­Mul or “star of stars.” The Aztecs knew them as Tian­quiztli. To the Maya, they were Tzab-ek. The Chi­nese, Per­sians, Aus­tralian Abo­rig­ines, Native Amer­i­cans and Per­sians all have sto­ries about this promi­nent clus­ter near the shoul­der of Tau­rus, the Bull.

Explore the image. Do you notice the color of the stars? The clus­ter is fairly young and is dom­i­nated by hot, blue stars. Astronomers believe the clus­ter formed about 100 mil­lion years ago. And while clus­ters like this are born out of huge clouds of gas and dust, the misty star cloud around the clus­ter is just a neb­ula that the star clus­ter is pass­ing through. The cloud reflects the blue light of the bright­est stars in the Pleiades. Astronomers call this a reflec­tion nebula.

The Pleiades are just 440 light years away from Earth mak­ing it one of the clos­est star clus­ters to Earth. The core of the clus­ter is packed in an area about 8 light-years across. A light-year is the dis­tance trav­eled by light in one year; about 6 tril­lion miles. The clus­ter is more spread out, how­ever. In a star­ship trav­el­ing at the speed of light, it would take us 43 years to pass through the entire star clus­ter. The Pleiades move through space as a group. In a few thou­sand years they will pass near the feet of Orion as seen from Earth. The stars in the clus­ter also will drift apart and disperse.