M 45
Published on Oct 29, 2006 at 8:48 pm.
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Filed under astrophotography, star clusters.
We had a star party this past weekend. As usually happens, the public got tired and went away before the astronomers. So, I took an opportunity to take a couple of astrophotos. Here is the Pleiades star cluster (Messier number 45) in Taurus. I used a Nikon D70 camera with a 300mm lens riding piggyback on a telescope. The camera was set for 200 ISO, and I used a 1 minute exposure.
You can actually see the Pleiadies with the naked eye. This time of year, it comes up a bit after sunset in the Northeast. It takes a couple hours, though, to get up high enough to appreciate. It looks sort of like a tiny version of the Big Dipper, and often people who don’t know better mistake it for the Little Dipper.
There are actually several hundred stars in this star cluster. 
It is dominated by several very hot, bright, blue stars. This cluster is fairly young, as far as star clusters go, and with a larger telescope and longer exposure time, you can see some nebulosity shining by reflected light from the hotter stars in the cluster, as seen in this photo from the Anglo-Australian Observatory. For years, it was taught that this nebulosity was leftover from the formation of the star cluster, but now we believe that it is unrelated to the cluster’s formation. Any material leftover from formation should have been dissipated by now, and many astronomers now believe that this star cluster is simply moving through a rather denser than average part of the interstellar medium. Besides, we now have measurements that show that the nebulosity and the stars have different motions. They are crossing each other’s paths. The Pleiades are important because it is one of the nearest star clusters, and it is important in calibrating our distance scale for objects beyond our immediate part of the Milky Way. It is about 440 lightyears away and is about 12 lightyears across. In all, the Pleiades has about 800 solar masses worth of stars, including numerous brown dwarfs.
Given that the Pleiades are visible to the naked eye as a little clump of stars, it is no wonder that they have been observed and mentioned in ancient writings. Homer mentions the Pleiades, and they are also mentioned in the books of Job and Amos in the Bible. The Pleiades were important to the Celts and the Aztecs in their calendars and 
various religious celebrations. And, in Japan this group of stars is called Subaru, which I am told means “unite”. In fact, the car company uses the Pleiades as its symbol, as seen in this closeup photo of the back of my car.
So, go out yourself and take a look at the Pleiades.
-Astroprof
