The lovely open star cluster, the Pleiades, has been called the Seven Sisters since antiquity. But nobody sees seven stars in this showpiece of the late autumn and winter, not by unaided eyeball and certainly not by telescope.
Like a tiny kite of bright points, in November the Pleiades are visible in the east after sunset and are easily seen until dawn. They lodge on the shoulder of the constellation Taurus the bull. Bruce McClure, in an article posted by EarthSky.org, says November is often called the month of the Pleiades because it's when the cluster can be seen all night, although "you can see the Pleiades cluster in the evening sky well into April. … Most people see 6, not 7, Pleiades stars in a dark country sky."
Ancient Greeks associated this regal star cluster with the mythological seven daughters of Atlas (he who carries the world on his shoulders) and Pleione. Going counter-clockwise from center, the names of the brightest are Alcyone, Merope, Electra, Caleano, Taygeta, Sterope (aka Asterope, a double star) and Maia — oh, and Pleione and Atlas. The latter two stars, named for the parents, are removed a short distance from the main section but really are part of the cluster. So we have nine that shine brightly enough for pre-telescope naming, not seven.
"The actual number of Pleiades stars visible (to the naked eye), however, may be more or less than seven, depending on the darkness of the surrounding sky and the clarity of the observer's eyesight," wrote astrophotographer Marco Lorenzi.
The disconnect between the seven sisters and the actual number in the cluster is much more dramatic when they are studied with modern instruments. NASA has posted separate estimates of over 1,000 and 3,000 stars — with no explanation for the difference.
Open star clusters are bound together by gravity, as the stars formed from the same interstellar cloud of gas and dust; the members slowly move apart until they are no longer associated. Many nebulas are dusty clouds lit up by their stars as the stars form, but this is not so with the Pleiades, even though the group is dominated by wispy streams of a nebula's gas and dust. This material gives the star collection a fuzzy look.
Known also as Messier 45, the Pleiades are about 400 light-years from Earth and stretch across 13 light years. The stars are believed to be youngsters of 100 million years old, writes astronomer Stephen John Gibson of Western Kentucky University, Bowling Green. Their original nebula has been absorbed or dissipated. Gibson adds, "Several Pleiades appear surrounded by intricate blue filaments of light. This nebulosity is the result of starlight scattering (reflecting) off minute grains of interstellar dust in the vicinity." The blue color results from the same effect that gives our sky that tint, light scattered by material in the atmosphere.
"The dust particles are inside a cloud of mostly hydrogen gas that the cluster seems to be plowing into," he says.
Tony Hallas, the famous astrophotographer, writing a description for his Astronomy Picture of the Day hosted by NASA, explains that the stars and the dust cloud are not related, "they just happen to be passing through the same region of space."
We can enjoy the Pleiades even from light-polluted cities, usually by unassisted vision, looking up and generally from southeast to southwest; if that doesn't work, try binoculars.
During the November new-moon period, when that orb's monstrous glare was absent, I tried to photograph M-74, the holiday wreath galaxy, from the front yard of our home in Salt Lake City. The city's light pollution rendered the galaxy invisible. But the Pleiades were so bright that I was able to obtain a view of Maia and the blue nebula. I also got a look at Electra and Celaeno.