Messier 36, an Open Star Cluster
Messier 36 is one of the most beautiful open star clusters in the winter sky. It is located at a distance of about 4,200 light years in the constellation "Auriga"
and therefore belongs to the most distant star clusters of the Messier catalog. With an apparent visual magnitude of +6.3mag it is already visible as a nebulous
spot in small binoculars. Because of the arrangement of the bright stars in outward running star chains it is also called "Pinwheel Cluster".
The apparent diameter in the sky is 12' (arcminutes), which corresponds to a true diameter of about 14 lightyears. But the stars are bound to the cluster in a
much larger distance. The so-called "tidal radius" is 10 light-years. Within this range the stars are subject to the attraction of the cluster.
Altogether the cluster contains about 200 stars, newer findings based on the 2nd GAIA release even suggest up to 260 stars. But only 12 of them are brighter
than 10mag. The brightest star with an apparent magnitude of 8.8mag is located near the center of the cluster. This corresponds to a true luminosity of 420
suns. The star has a spectral type of B2 and belongs to the white giant stars. Its surface temperature is in the range of 20,000 Kelvin. In general almost all
bright stars are of spectral type B. Some of these stars seem to rotate very fast and thus could belong to the group of "Be- stars" - stars which are clearly
flattened due to their high rotation velocity. The many stars of spectral type B suggest a young age and in fact Messier 36 was formed only about 20- 25
million years ago. So, when the dinosaurs populated the Earth, this beautiful cluster did not even exist. By the way, what you search in vain in the "Pinwheel
Cluster" are red giant stars.
One star in relation to Messier 36 is of special importance. It is a neutron star with the designation "PSR J0538+2817" in the constellation Taurus. It rotates
once in 139 milliseconds around its own axis and is associated with the supernova remnant "Simeis 147", which was formed about 40,000 years ago. And
what has this pulsar to do with Messier 36? Well, it is assumed that the progenitor star once belonged to Messier 36.
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But now we come to the observation with a 20" telescope:
Star clusters from the Messier catalog are of course a blast in a 50cm Dobson. You are literally overwhelmed by the abundance of stars and sometimes you
don't know what's happening. This is not different with Messier 36. At low magnification of 120x two dozens of stars appear especially bright and whitish.
These are the "white giants" mentioned above. They arrange themselves in star chains leading from the center outwards.
But the cluster appears most beautiful at 210x. The brightest stars appear even brighter against the now pitch black background. This twinkling in the eyepiece
is just wonderful to look at. Around these bright giant stars are 70 to 80 fainter stars, whose number increases towards the center. Especially great is also the
big difference in brightness between the individual stars. What a sight!!
