Date: March 14, 2009
Title: Globular Star Clusters: How Old Am I? Where Are You?
Podcaster: Richard Kinne
Organization: AAVSO http://www.aavso.org/
Description: Globular Star Clusters – big, beautiful balls of stars orbiting our Galaxy – have special properties that not only are able to tell us how old they are, but were we, ourselves, are in the Galaxy. Some of them are also easy to find with binoculars. Come along as we take a look at these gems of the heavens and what they’ve taught us about them, and ourselves.
Bio: Richard Kinne, better known to most people as “Doc,” has been studying stars in one way, shape, or form since he was 12. A former Planetarium Operator for the Charles Jerred Planetarium at the State University of New York at Oswego, Doc now works as an Astronomical Technologist at the American Association of Variable Star Observers in Cambridge, MA. Professionally he helps maintain the largest database of variable star observations in the world. His astronomical interests revolve around variable stars, solar observing, and a tiny bit of asteroid hunting.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Harold Goldner for his wife’s birthday today, and for all those nights when she was home alone and he was out stargazing in the middle of the night.
Transcript:
Hello. I’m Richard Kinne, an astronomical technologist from Boston who works with the American Association of Variable Star Observers. Welcome to the today’s edition of IYA’s 365 Days of Astronomy Podcast. I’d like to introduce you to some of the most beautiful, and important, objects in the sky – Globular Star Clusters.
Globular Star Clusters are, basically, what their name describes. They are “balls of stars.” But Globular Star Clusters are not galaxies, rather we find they are the satellites of galaxies. Like our Earth has the Moon for a satellite, and Jupiter has its many, many moons as satellites, galaxies have revolving around them Globular Star Clusters, these big balls of stars. We have discovered 151 of these galactic satellites in orbit around the Milky Way. We believe there may be as many as 180, but the others are currently hidden from our sight by dust clouds between us and the Globular Star Cluster.
Besides being beautiful, these balls of stars are special. Their nature can tell us something about their age. Their position can tell us something about where we are in the Universe. Let’s see how!
Stars are created from collapsing gas clouds. Our own Sun was created from such a collapsing gas cloud billions of years ago. In a Globular Star Cluster, all the stars that you see were created by the same collapsing gas cloud. This means that all the stars in a Globular Star Cluster are roughly the same age. But, if we look closely at them, the stars don’t look the same. Some of them are different colors. Some of them are different sizes. If they are the same age, shouldn’t they call look the same?
Well, not really. The rate at which a star ages is usually determined by how massive it is – how much hydrogen gas it starts out its life with. Stars with more hydrogen – more mass – burn hotter and faster, and so they age faster. Smaller stars use their fuel in a more miserly fashion and go through their lifespans more slowly.
We know how stars age, and what they look like as they age, by plotting them on what is called a Hertzsprung-Russell diagram. That’s a fancy name for a “scatter plot” diagram that you may have encountered in school a few times. Its a diagram of dots scattered about that usually is supposed to show some type of trend in what its showing – the number of cats in the US by year, or something like that. This diagram plots the stars brightness, or apparent magnitude against its color index. The color index is the difference in the brightness of a star looked at through a blue filter as opposed to its brightness looked at through a yellow-green filter. Depending on the temperature of a star they’ll actually look brighter or dimer than usual if you look at them through different colored filters. Subtracting the brightness you get in the yellow-green filter from the brightness you get in the blue filter gives you the star’s color index. Negative numbers means the star is a hot, blue star. Positive numbers tell us its a cool, red star.
When the stars in and around our local stellar neighborhood are plotted, most of them form a rough line that slants from a low point in the lower right of the diagram, to a high point in the upper left, looking a little bit like a backslash on a computer keyboard. This slant is called the Main Sequence and is where a star finds itself most of its life. As a star enters old age it starts to drift upwards and to the right on this chart as it becomes a red giant star. So a normal H-R diagram looks sorta like a backwards “y.”
Now remember, all the stars in a Global Star Cluster were born at roughly the same time. How fast they’re living their lives depends on how massive they are. So, if you plot the stars of a Global Star Cluster in an H-R diagram, you don’t get a backwards “y,” you get something that looks a bit like a “<” sign, as those stars that have started to enter old age because they’re big and fat turn off, as a line, away from the backslash of the diagram. The position of the point in the “<” sign, and how bright the stars are at that point, can tell us the age of all the stars in the Globular Star Cluster.
So, due to the nature of the Globular Star Cluster – all its stars were born at the same time, and they’re all roughly the same distance away from us – and the nature of how stars age and evolve, we can tell the age of Globular Star Clusters!
But there’s more! Globular Star Clusters are also special because they were able to clue us in to where our Solar System lived in the Milky Way Galaxy. Remember I said earlier that Globular Star Clusters are satellites of the Milky Way Galaxy. As satellites they orbit the center of the Galaxy, its center of mass. They orbit around the galactic core. At the very beginning of the 20th century astronomers and everyone else believed that the Earth and the Solar System resided in the center of the Milky Way Galaxy. Harlow Shapley started studying Globular Star Clusters and knew they were satellites of our Milky Way Galaxy. As satellites he knew they’d orbit the galactic center or the core. If that was the case he knew he should see roughly the same number of Globular Star Clusters wherever he looked in the sky. He didn’t. Shapley found many more Globular Star Clusters in the section of the sky around the constellation Sagittarius than around Gemini, at the opposite end of the sky. That meant that our solar system was not in the center of the galaxy, but was actually off to one side, in one of the spiral arms. With that discovery Shapley continued the journey our solar system has had, conceptually speaking, from the center of the universe to an average planet, circling an average star, in an average place, in an average galaxy. The history of discovery in astronomy has shown us again and again, so far, that we are nothing special, and Globular Clusters were one of the things that told us that.
With all their importance, can you actually see one of these objects for yourself? Yes, and they’re beautiful! Most of them require a telescope to see really well, but a couple of them can be glimpsed with the naked eye in a good location, and then training binoculars on them can reveal them and give you a hint of their glory. One of the best in that regard is known as M13 in the northern constellation of Hercules. The asterim, or unofficial star pattern, that Hercules is known by is the Trapazoid. On a dark night, looking carefully, you’ll see a tiny, fuzzy patch just off the east side of the Trapazoid. Train your binoculars, or a low-powered telescope on that patch and enjoy the view.
Globular Star Clusters are one of the many neat types of objects you’ve been hearing about this year as you’ve been listening to 365 Days of Astronomy. They’re special to me because I did part of my graduate work with them. I hope they might become special to you, too.
This has been Richard Kinne for today’s 365 Days of Astronomy hoping that you all keep having a stellar year!
365 Days of Astronomy
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