Have you ever taken time to gaze at the stars on a clear night, either with a casual eye or a telescope? If so, you might have seen the famous star cluster, the Pleiades, without even knowing it! Known as the Seven Sisters from Greek mythology, it is a bright and compact group of stars. The Pleiades cluster actually contains about one thousand stars of which the seven brightest ones outshine all the others. This post will introduce you to star clusters like the Pleiades, the subject of a significant part of the IU Department of Astronomy’s research.
“The Seven Sisters.” The name conveniently suggests that star clusters can be considered “families” of stars, as stars are known to be born from shared molecular clouds. These families have to fight against the gravitational pull of the much larger galaxy (and its glamorous city life) to keep its members within its own gravitational hug, but often many family members escape and become part of the general galactic population. Smaller star families with weaker gravitational bonds are often disbanded completely, while larger ones—though they still lose a number of children—are able to survive and orbit the galaxy together. These are the star clusters that we enjoy gazing at, and also the ones that we study as astronomers.
Star clusters are found all over the Milky Way and other galaxies, and come in different shapes and sizes, but are usually grouped into two categories: open clusters and globular clusters. The Pleiades are an example of an open cluster, containing hundreds to a few thousands of stars, and are relatively young (tens to hundreds of millions of years old). Open clusters are named thus because the stars are not tightly packed together.
In contrast, globular clusters have many more stars (up to millions!), are older (10-12 billion years), and are more densely packed into a spherical shape. A well-known example of a globular cluster is Messier 13 (M13, the Great Globular Cluster). In contrast to the Pleiades, M13 is only visible as something like a single fuzzy star, even with binoculars. With a telescope it becomes easier to see what is so special about it; what looks from afar like a single fuzzy star is now seen as many stars, much more numerous and more tightly packed than the Pleiades.
Star clusters are extremely important to the field of astronomy because they conveniently allow a sample of stars that have the same age and are born from the same molecular cloud to be observed together in the same telescope field. With star-birthdate as a control (a constant for comparison in scientific experimentation), astronomers have shown that stars with different masses age at different rates and go through different evolutionary states. This knowledge enables astronomers to calculate cluster ages and distances from the sun, which in turn allows star clusters to be used as probes for studying different locations in the Milky Way.
Researching star clusters is a significant part of IU’s Department of Astronomy. There are research programs to measure the abundance of elements in star clusters, search for interesting stellar objects such as X-ray binaries within globular clusters and use extragalactic globular clusters to study their host galaxies. Graduate students involved in these projects often observe clusters with the WIYN telescopes at Kitt Peak, Arizona, which are partly owned by IU. Other research programs work on simulations of star clusters. Using IU’s Big Red II supercomputer, these simulations can help us understand how star clusters evolve over their lifetimes, following the individual gravitational interactions between each star, as each star also goes through its internal evolution. These simulations can be used to infer the early stages of star cluster development from the properties of clusters we observe today.
Now that you’re more familiar with star clusters, go stargazing and find a star cluster like the Pleiades (remember that all those stars were born together as a family). You’ll also see a lot of single stars who left their family to become part of the galactic population. Even our own sun went through this phase, though we are not sure whether its original family is still together today. Today the star clusters that stay intact help us to learn about our universe, while for centuries their visual appeal and fascination inspired the mythology and folklore of civilizations on Earth!