We saw the Little Dog Star, Procyon, in the Little Dog constellation, next to
Gemini, so where is the Big Dog?
What's so interesting about this globular cluster is that it's in the wrong part of the sky. All the clusters we've looked at are open clusters. Globular clusters are very different, and this globular cluster is all alone on this side of the sky. The summer sky is just filled with globular clusters -- for some reason all the globular clusters are close to the center of the Milky Way galaxy, and in the summer we are looking toward the center of the galaxy. Now we are looking out toward the edge of the galactic disk, and there's this one lone globular cluster -- why would that be?
Well, in November of 2003, it was discovered that the Milky Way is eating up a smaller galaxy -- it is called the Canis Major galaxy because most of what's left of it is in the direction of the Big Dog. It was caught in the gravity of the Milky Way and now is being pulled apart and is getting sucked into the disk of our galaxy. M79 is a globular cluster that was a part of that little galaxy, not originally part of our galaxy, which is why it's in the wrong place.
If you follow the line of the two stars at the center of the rabbit's body south to the next star, M79 is right next to that star. Instead of being a randomly scattered group of stars, a globular cluster is a well-defined ball, glittering with hundreds of thousands of stars. Globular clusters are rare - only about 150 are known, and they are completely different from open clusters like the Pleiades or M35 in Gemini.
The Pleiades are about 100 million years old, M79 is estimated to be at least 10 BILLION years old, making it so old that it formed before the disk of the Milky Way galaxy formed! While the Pleiades cluster has maybe 500 stars in total, M79 has several hundred thousand stars, maybe a million. The Pleiades are about 400 light years away and about seven light years across, while M79 is 40 thousand light years away and 100 light years across.
These numbers are typical -- globular clusters are much, much bigger than open clusters, and they are much much older - some are nearly as old as the universe!
So we've come full circle. We started with Orion the Hunter and here we are, back with Orion. Our winter sky tour is completed. For the die-hard stalwarts with a particularly high resistance to the night chill, we may continue on to observe some additional deep sky objects, perhaps some of the ones we skipped for the sake of time. Or we might learn a few additional constellations, like Leo which is on the rise about this time. But that's really part of the Spring Sky Tour, and we don't want to start getting ahead of ourselves, particularly since we've covered so much already.
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All content material was graciously provided and used by the permission of Randy Culp