Open and Globular Clusters

Raghda Abdel Khaleq is an astronomy guide at Sydney Observatory and a physics student at the University of New South Wales. In this post she discusses the different clusters in our night sky.

Alongside the more commonly known planets, stars and black holes, astronomers study other interesting and important objects in the sky, one of them being star clusters. A star cluster is simply a group or collection of stars that are gravitationally bound to one another. There are two main types of stars clusters: Globular and Open.

Open star clusters are relatively young groups of stars, and usually contain less than a few thousand members. Globular clusters are much older than open clusters, and are around the same age as the universe itself. They are usually spherical in shape, and contain 10,000 to a few million stars.

One of the youngest known open clusters is the Jewel Box, which received its name from Sir John Herschel in the 1800s due to its resemblance to a fancy piece of jewellery. It is approximately 14 million years old, and is about 6,400 light years from Earth. As an open cluster, it contains approximately 100 stars, the brightest of which are supergiants. The central members of the cluster form an “A”-shaped asterism. The cluster is often referred to as “Traffic Lights”, as the three middle stars lie in a straight line resemble traffic lights with their varying colours. The Jewel Box is located towards the Southern Cross, which can be seen from most of the southern hemisphere.

The Jewel Box taken by the FORS1 instrument on the ESO Very Large Telescope (VLT) at ESO's Paranal Observatory. Image Credit: ESO

The Jewel Box taken by the FORS1 instrument on the ESO Very Large Telescope (VLT) at ESO’s Paranal Observatory.
Image Credit: ESO

The Sagittarius cluster (M22) however, is an elliptical globular cluster in the constellation of Sagittarius, and was one of the first globular clusters to be discovered in the 1600s. It is approximately 10,600 light years away from Earth, and like many globular clusters contains a large number of stars, approximately 70,000. This cluster is particularly unusual, in that it is one of few globular clusters that contain a planetary nebula. More so, two black holes were discovered in this cluster, which suggests there may be as many as 100 black holes in it as well. Each black hole has a mass 10-20 times the mass of our very own Sun. That’s a lot of mass!

M22 is currently (November) setting in the western sky, however 47 Tucanae is high in the southern sky. Much like M22, 47 Tucanae is a globular cluster, located in the constellation of Tucana 16,700 light years away from Earth. It is one of the largest globular clusters in the Milky Way Galaxy, with millions of stars residing in it. Unlike M22, no evidence of the existence of black holes in this cluster has been found as yet. As one of the brightest globular clusters in the sky right now, you wouldn’t want to miss your chance of viewing this beautiful object through one of our telescopes here at the Observatory- so be sure to visit us soon!

47 Tucanae taken by NASA's Hubble Space Telescope. Image Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

47 Tucanae taken by NASA’s Hubble Space Telescope.
Image Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

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