Thursday, 19 May 2016

The wealthy charioteer

Auriga is a very rich constellation because the Milky Way runs through it and therefore it houses many star clusters and nebulas. Although it's interesting to note that it lies exactly on the opposite side of the centre of our galaxy! Its Latin name means "charioteer" as it is often identified with the legendary Athenian hero Erichtonios, who was credited with the invention of the four-horse chariot. 

When you take your binoculars and point it at Auriga, you'll easily notice why even ordinary field binos make a perfect instrument for observing the night's sky. Here's a combination of three views through my 100mm binoculars, showing you three of the most famous star clusters: M37 (on the left), M36 (centre) and M38 (on the right). I already posted a zoomed-in sketch of M37 here which was made with my old 18" telescope, so now you can make a comparison between a telescope and binoculars. As you can see, both instruments are perfectly complementary because they're doing a different job: the binos offer a wide field of view and allow you to travel across the heavens, whereas the telescope zooms in on a particular object and shows you the smallest details. 

As you'll remember, M37 is already a fairly old star cluster, with an estimated age between 400 and 550 million years, and yet it is extraordinarily rich with over 500 identified members. Usually star clusters break up as they grow older, torn apart as they are by the tidal forces of our galaxy, and the stars each go their own way. But M37 on the other hand has remained extremely compact, probably because it's such a large and compact cluster and the gravitational pull of the group is strong enough to counter the pull from our galaxy.

M36 is much younger than its neighbour, estimated 25 million years, and it contains a lot less stars as well - about 60. But due to its young age the stars are still much hotter and therefore this smaller cluster, even though it lies at roughly the same distance (4.100 lightyears), looks slightly brighter to us. If it were closer, it would actually very much resemble the famous Pleiades

And then there's M38. As far as age is concerned, it lies exactly between the other two: 220 million years. It contains twice the number of stars of M36 but here our galaxy's gravitational influence is evident and it's structure has become very irregular. Most observers describe it as an oblique cross, rather than it having a standard circular shape. With time it will dissipate ever more until the cluster will be gone completely.

But this isn't all! If you look carefully, the three have yet another companion, albeit a much smaller one, denominated NGC1907. It lies just a tad to the bottom-left of M38... can you spot it? This little cluster contains only 30 members because most of its stars have already gone their own way. It's about the same age as M37 so here you see what usually happens to older star clusters. Scientists have also measured some interaction with nearby M38, although they were born in different regions of our galaxy and obviously have a different age as well. So probably we'll have to conclude that the pair are just having a coincidental fly-by. 

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