Sirius Stargazing: Alpha Persei Cluster (Cr 39)

Welcome to Sirius Stargazing. I'm TK and in this episode we're going to look at the brilliant
Open Cluster associated with the star Alpha Persei. Now, the Greek letter alpha tells
us that this star is the brightest in its parent constellation, which in this case is
Perseus, but it also goes by a much more common name of Mirphak. See, Mirphak is very interesting
in its own right. It's a truly huge star, one that we would call a Supergiant, as it
has a diameter around 60 times that of the Sun. It's also much brighter than the Sun,
about 5,000 times, so it's no surprise that even at a distance of590 light-years away,
it's still in the top 40 brightest stars in the whole sky. The cluster that we're focussing
on is also very interesting. Technically, astronomers would refer to it as an 'Association',
because it's so open, and it's stars, which number in about 50, are very loosely bound
by gravity. As it happens, it's not rapidly dispersing, but all of the stars are generally
moving together in the same direction, so sometimes we call it a 'Moving Cluster'. So,
quite a few designations there, and if you're worried about which one to pick, then you
can always use the name Collinder 39, as Per Collinder - a Swedish astronomer - catalogued
it in 1931. So, as is fairly typical with open clusters, most of the stars are very
hot, very big and very blue, and this indicates that they're very young. Studies of some of
the stars have revealed age constraints of around 70 million years; sounds old, but not
much for a star. If we were to compare 70 million years to the life expectancy of the
Sun, then in relative terms, this corresponds to your or I being a 6 month old baby. To
find Collinder 39, you'll need to identify Perseus. Its brightest stars make a very distinctive
shape, like a letter Y, but you'll probably find it easier to find the W shape of Cassiopeia
first, and then use that to guide you to Perseus. Once you find it, Mirphak is the star that
marks the point where the three lines of the Y shape all meet in the centre. Training your
binoculars on it, you'll have absolutely no problem identifying the cluster, as its stars
appear much brighter than the background haze of the Milky Way. It's one of those really
excellent places in the sky, where you can sort of go to town and make your own constellations,
and I'm not going to tell you what I always see when I look at Collinder 39, because I
want to know what it looks like to you. So leave a comment and tell me. Now, when you've
spent some time with the cluster, see if you can make out the difference in colour between
the most distinct blue-white stars, and this one, Sigma Persei. It's actually a few hundred
[light] years closer than the rest of the cluster, and isn't technically a member, but
it's also cooler, and in good conditions you should be able to detect a hint of orange
in it. So see if you can split it from the fainter background star, and this should make
any colour that you detect more apparent. That's all for this episode, so until next
time, clear skies.