Types of Star Clusters

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  • 0:01 Star Clusters
  • 0:54 Open Clusters
  • 1:30 Globular Clusters
  • 2:13 A Cluster's Age
  • 4:28 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Our lesson will explore what star clusters are, the two types of star clusters, and their characteristics and differences as well as how we can figure out their ages using the H-R diagram.

Star Clusters

When traveling through a park or a neighborhood, some people prefer to go at it alone. And why not? Maybe they're sick and tired of the concrete jungle. But plenty of people like to hang around in groups or travel in groups. It's more interesting and safer.

Something similar happens in our galaxy, the Milky Way. Some stars are loners and travel through it alone. But plenty of stars move together in star clusters, which are groupings of stars that formed together and stick together due to mutual gravitational attraction.

Star clusters are of the same blood; they formed from the same gas cloud at almost the same time as the gas cloud condensed. What's important to remember about star clusters is that, although the stars that are found in them are of different masses, they are of the same age.

Open Clusters

Star clusters can be open clusters, about 10-1,000 relatively young and loosely packed stars moving together in the disk of the galaxy. These clusters have an open, or transparent, appearance to them because they are so loosely packed. The relatively young stars are often very hot and very luminous. Furthermore, open clusters are concentrated in the spiral arms of the galaxy. To help remember this, just think of open clusters greeting you with open arms.

Globular Clusters

Other than open clusters, there are also globular clusters, star clusters with tight concentrations of about 100,000-1,000,000 stars. Globular clusters are located in a halo, a spherical region that surrounds a galaxy's disk. As opposed to open clusters, globular clusters contain very old stars.

Globular cluster are less numerous than open clusters. We know that there are about 150 globular clusters that are associated with our galaxy. Astronomers estimate that there may be 20 other ones we can't really see.

The Age of a Star Cluster

In any case, astronomers can figure out the all-important age of a cluster by making an H-R diagram of the stars found in the cluster itself. An H-R diagram is a diagram that plots a star's luminosity versus its surface temperature.

For example, let's take two open clusters, one called Pleiades, and the other Praesepe (the Beehive cluster). Do you see how the Pleiades cluster is located along this one band, running from the top left to the bottom right? That band is called the main sequence.

An H-R diagram of the Pleiades and Praesepe star clusters

At the very top left of the main sequence are stars that are very large and luminous. Such stars, like a giant car burning through gasoline, do not live for very long. As a result, they do not stay on the main sequence for long, even though they're about the same age as smaller stars on the main sequence, which, like hybrid cars, can keep going for longer.

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