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Galaxies and clusters

April 2, 2013

Space is vast and seems to go on forever. There are so many objects to look at while you're out under the night sky, including the moon, planets, asteroids and comets, which are all within our solar system. Step outside of the solar system, and you get even more objects such as stars, nebulae, oen clusters and globular clusters, old stars and new stars.

Take a step a little bit further, and you have a lot of galaxies to look at. A the approximated 150 globular clusters in our galaxy are found within the spherical halo that surrounds our Milky Way galaxy, some of which as old as 12 billion years.

Our solar system was in the early stages of life 4.6 billion years ago with a gravitational collapse of a molecular cloud - like the formations of stars taking place within emission nebulae like the Orion nebula. The majority of the collapse collected in the center, forming our sun as the rest flattened into a protoplanetary disk that, over time, created the smaller bodies in our solar system.

Article Photos

Figure 1: The open clusters we can see are all on the same side of the Milky Way galaxy as the sun, as seen in these diagrams (made with Where is M13 software).

Some moons within the solar system are believed to be asteroids that have been captured by their planets, such as some of the smaller moons around Jupiter with eccentric orbits. A large majority of the objects within the solar system never formed into planets or were left as asteroids, comets and dwarf planets.

But let's not spend much more time on just the solar system; there are a lot of other interesting things much further away.

All the stars you see when you step out and look up on those clear nights - you know, the ones that form all those familiar constellations - are within our Milky Way galaxy. The best part is that those aren't the only stars within our galaxy. There are so many more that are farther away, or so faint that you need a telescope to see them. There are some stars within our galaxy that we just can't see due to them being on the opposite side, and we can't penetrate through the bright and dusty core of our galaxy to see them.

As I mentioned above, the stars you see, and the solar system we are in, were all formed in some sort of a gravitational collapse of a nebula, so I won't go on more about that. Open clusters all formed from the same molecular clouds, giving all the stars within the cluster roughly the same age. These stars can be disrupted by other clusters and clouds of gas spreading them apart from each other as they migrate towards the main body of the galaxy. All of these nebulae, planets and, as shown in Figure 1, open clusters are what we can see on this side of the galaxy. I'm almost positive the other side would be just as full of objects as this side is, but we may never get a chance to see them.

As I stated earlier, globular clusters are mainly all just outside the disk of our galaxy, and as shown in Figure 2, we can see them whether they are on this side or on the far side because with them outside, we don't have the dust and debris from the galactic core in our field of view.

These old stars are tightly bound together due to gravity which, in turn, balls them up and gives them a denser core of stars in the center and a little looser towards the outside edges. The groupings are larger than the groupings of the less dense open clusters found within the disk of the galaxy.

The dense balls of stars orbit around galaxies much like the planets orbit around the sun, and many other galaxies have been known to possess their own globular clusters, some of which may have been acquired by other galaxies that have a stronger gravitational pull on them.

It's not quite certain what role these clusters play in the galactic evolution, but it is clear that globular clusters come into being as part of the formation of stars of their parent galaxies. Although these objects are far from where we stand within the galaxy, if you look out even farther, you can see more objects; these are galaxies.

Huge galaxies that range in size, shape and type are beyond where we stand and contain many more stars, nebulae, open and globular clusters. The majority of these objects you need a telescope to see, with the exception of a few - if you count the Magellanic clouds in the southern hemisphere, too. Just like snowflakes, galaxies are all different from each other. Some are classified as elliptical, which have an oval shape. Spiral galaxy are disk shaped and have dusty, curving arms. And there are irregular galaxies that have unusual shapes, which is usually caused by a disruption of gravity from a neighboring galaxy. Some galaxies have collided and interacted with one another, and some currently are doing so.

The Whirlpool galaxy, M51, in Canes Venatici can be seen interacting as a smaller galaxy is merging with a larger galaxy. The views don't change from day to day as this is a long process, but with distances such as 23 million light years, the merger of the galaxy could be over and we wouldn't know, since we are looking back in time when viewing objects in space.

The more I learn about space, and the vastness of it, the more impressed I am with nature.

Looking up can be one of the most relaxing things you could do on a clear night. It's fun to sit back and enjoy the stars, and hopefully I was able to provide you with a little more information to think about the next time you're out looking up whether it's with your unaided eye, binoculars or a telescope. Get out and enjoy the sky on a clear night now that the weather is warming up.

Over the summer, if you look south toward the constellation Sagittarius, you are looking at the galactic core, and beyond that is still a mystery to us, even though we can see millions of light years in other directions.


Beyond this column, local astronomers are anxious to share their love of the sky with you. Check out the Adirondack Public Observatory website at for events, listen for Aileen O'Donoghue on North Country Public Radio about once a month during "The Eight O'clock Hour," or email her with any questions at



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