Date: March 16, 2012
Title: Star Clusters
Podcasters: Mel Blake & Deb Bailey
Organization: UNA Planetarium
Link: http://www.una.edu/planetarium
UNA Planetarium Facebook Page
@unaplanetarium
Bios: Dr. Mel Blake. I am from Newfoundland, Canada. I received a PhD from York University in Toronto, Canada, and I am currently director of the University of North Alabama Planetarium and Observatory. I am an Associate professor in the department of Physics and Earth Science. My research interests include binary stars and star clusters.
Deb Bailey is an undergraduate honors student from Alabama who majors in professional writing and English. She plans to pursue graduate studies in the fall.
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Transcript:
Dr. B. – Hello. I am Dr. Mel Blake.
Deb – And, I’m Deb Bailey.
Dr. B. – And we’re at the University of North Alabama Planetarium and Observatory, and today we’ll be discussing star clusters.
Dr. B. – If you look out in the night sky towards the direction of the stars in Taurus, the constellation of The Bull, you will see the familiar pattern of its V-shaped head, with the bright star Aldebaran making the Bull’s eye. The horns of the Bull stretch off to the east, and in the shoulder of the bull is the little group of stars called the Pleiades or the Seven Sisters.
Deb – The stars that make up most constellations are just chance alignments of stars with their own distances from the Sun, their own ages, and separate motions through the Milky Way galaxy. They all just happen to lie in the same direction and make a picture in our imagination.
Dr. B. – But, the Pleiades are different. They aren’t just a chance alignment of stars; they make up a star cluster. A star cluster is a group of stars that were all born from the same gas cloud and are now traveling through the galaxy together. It turns out we can learn a lot from star clusters.
Deb – Early surveys of the sky turned up lots of tight groupings of stars. Many early catalogs of celestial objects include the various star clusters. It was quickly noticed that the clusters come in two types. The first were the so-called galactic, or open clusters. These clusters tended to be less concentrated and were located in the plane of the Milky Way.
Dr. B. – The second group was called globular clusters. These tended to lie all over the sky and were giant globs of stars, sometimes so concentrated that the stars blended together. There are about 100 globular clusters in the galaxy, and hundreds of open clusters, both big and small.
Deb – Astronomers developed techniques for finding the distances to star clusters. The open clusters had stars whose temperature and brightness could be deduced using the spectra, the rainbow of colors from blue to red in a star’s light.
Dr. B. – Astronomers could measure the distance of nearby stars by measuring the annual movement across the sky back and forth due to the earth’s movement around the Sun. This is called parallax. If a star in an open cluster is the same type as one whose distance was measured this way, then the star’s distance could be found. By measuring many clusters, a three dimensional map of the locations of the objects could be made. When maps of the locations of the open clusters were made, they appeared in regular repeating bands which we now call the spiral arms of the galaxy.
Deb – The globular clusters have stars which pulsate on a regular cycle. As described by Henrietta Leavitt, the brightness of these RR Lyra stars was tied to their pulsation. This enabled the distances to the globular clusters to be found. Harlow Shapely did just that, and he found that the globular clusters were in a large spherical volume that was centered towards the constellation of Sagittarius. He correctly deduced that the center of the Milky Way was in that direction, about 20 thousand light years away.
Dr. B. – To begin to understand star clusters we need to understand a bit about how stars work. Stars are giant thermonuclear engines with atoms being smashed together in their centers to produce heavier elements. The most common stars are converting hydrogen to helium. Massive stars run out of their fuel first, but are very bright.
Deb – The most common way to describe stars is to plot their brightness versus their temperature. Most stars lie in one region, called the main sequence, for 90% of their energy-producing lives. Star clusters are laboratories for studying these processes. All the stars in a star cluster are the same age, composition and distance. They differ only in the mass they have. So we can study how stars of different masses behave and verify our models of stars.
Dr. B. – We expect massive stars to change faster than others and that’s exactly what we see. As a result, we can determine the age of a star cluster. We can calculate how long a given star will be on the main sequence and that will tell us the age of the cluster.
Deb – When these calculations were done for the globular and open clusters, the results were very different. The open clusters had ages that ranged from less than a few million years old to up to several billion years old. The globular clusters, on the other hand, were all around 13 billion years old.
Dr. B. – So, not only were the clusters located in different parts of the Galaxy, but they were also different ages. This tells us something important about the galaxy. The large halo where the globular clusters lie is old and the disk of the galaxy where the open clusters live is younger. The Milky Way must have formed from the inside out!
Deb – Besides the information about the galaxy that we can learn from star clusters, interesting events can also occur in star clusters because stars can interact more closely. Stars in globular clusters are even thought to collide to form larger stars from time to time.
Dr. B. – As we can see, star clusters have many interesting properties. We can use them to map the Milky Way, tell how old it is and we can use them to study the evolution of stars. We can even observe rare events such as stellar mergers and collisions. So the next time we look up at the Pleiades we can imagine what it might be like to live in such a cluster, with many bright, nearby stars surrounding us in all directions.
End of podcast:
365 Days of Astronomy
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