M45: the Pleiades open cluster - the Seven Sisters
At least 500 mostly faint stars belong to the cluster, spread over a 2 degree field. Their density is pretty low, compared to other open clusters, one reason why the life expectancy of the Pleiades cluster is also pretty low. The earliest known references to this cluster are by Homer in his Iliad (about 750 B.C.). The Bible has three references. Their Japanese name is "Subaru", taken to christen the car of same name. The Old English and German name referred to a "Hen with Chicks".
According to Greek mythology, the main, visible stars are named after the seven daughters of "father" Atlas and "mother" Pleione: Alcyone, Asterope (a double star, also sometimes called Sterope), Electra, Maia, Merope, Taygeta and Celaeno. In 1767, Reverend John Michell used the Pleiades to calculate a probability of about 1/496,000 to find such a group of stars in any place in the sky by chance alignment. Therefore, he concluded correctly that the cluster is a real physical group. Charles Messier included the Pleiades as No. 45 in his first list of nebulae and star clusters, published 1771.
The Pleiades are embedded in nebulous material, which reflects their blue light. The brightest of these nebulae, around Merope, is NGC 1435. Around Maia is NGC 1432 or the Maia Nebula. Slipher in 1912 revealed them to be reflection nebulae, because their spectra are exact copies of the spectra of the stars illuminating them.
The age of the cluster is about 100 million years. The Pleiades have an expected future lifetime as a cluster of only about another 250 million years; after that , they will be spread as individual (or multiple) stars along their orbital path. By accurate parallaxes of Pleiades stars, the cluster is at a distance of 440 +/-6 light-years.
Binoculars show more than 100 stars in a field of about 1 1/5 degrees As the Pleiades are situated close to the ecliptic (4 degrees off), occultations of the cluster by the Moon occur quite frequently: the Moon fits into the quadrangle formed by Alcyone, Electra, Merope and Taygeta. Venus, Mars, and Mercury even pass through occasionally to give a wonderful spectacle.
Orion Optics UK AG16 Astrograph: SBIG 11000 CM: Titan Mount
17 x 4 minutes (1 hr 8 minutes total) single shot colour, processed with CCDStack, Photoshop, Pixinsight
Data capture: 10th December 2012
M45PleiadesclusterOrion Optics AG16 astrograph