If you are an astronomer or a person who enjoys watching stars and constellations, you must have come across the words globular and open clusters. According to NASA, “Star clusters are groups of stars which are close together in space, rather than just accidentally lined up one behind the other.”
Globular and open clusters are common and easily identifiable. In the Milky Way, there are around 150 to 158 known globular clusters and some open clusters are visible to the naked eye like Pleiades also referred to as ‘Seven Sisters’. It’s very close to the earth and located in the Taurus (the bull) constellation. Although the ‘sisters’ are visible to the naked eye, it’s intriguing to know that they are 400 light years away.
The major differences between globular and open clusters are classified into number of stars, cluster diameter, age, star separation, total stellar mass and distribution.
Understanding globular clusters
Definition: Globular cluster or globular is a collection of stars that are sphere shaped and tightly held together by gravity. They also have high stellar densities at the center. The central stellar density combined with the high gravity, forces them to take the spherical shape. A globular cluster circles a galactic core like a satellite. This is different to open clusters that are loosely held together by gravity.
Characteristics and composition: Globular clusters are made up of about ten thousand to a million old stars and low-metal. Low-metal also Metallicity is defined as “proportion of an object matter made up of chemical elements other than Helium and Hydrogen”. They have no dust or gas and it is believed that all the dust and gas turned into stars for globular clusters are said to be very old (10 to 20 billion years). In size, a globular cluster can span up to 200 light years in diameter with 1-5 million total stellar mass.
Position: In the earth’s Milky Way, globular clusters are found around the galactic center (Ophiuchus, Sagittarius and Scorpius). Sagittarius has 33, Scorpuis 19 and Ophiuchus 25 globular clusters. They are also distributed in all directions.
Understanding open clusters
Definition: An open cluster is made up of thousands of stars that came from the same molecular cloud. Unlike globular clusters, open clusters are said to be of same age and not tightly held together by gravity. Since they are not strongly held together, open clusters are easily interfered with by other clusters or clouds and they don’t live for long. Long in this case being less than one billion years compared to 10-20 billion years in globular clusters.
Characteristics and composition: Open clusters are not made up of low metal or old stars like globular clusters. They begin when part of a giant molecular cloud collapses. As the cloud collapses, the fragments form large stars which break into small and smaller stars explaining the appearance of an open cluster. The formation of open clusters takes tens of millions of years and an open cluster can have 10 to 10,000 stars with 10-10,000 stellar mass.
Position: In the earth’s galaxy, there are over 1000 identified open clusters distributed along the plane of Milky Way with some being as close as 180 light years. The Pleiades and the Hyades are visible to the naked eye.
NASA.gov – Astronomy Picture of the Day [M45: The Pleiades Star Cluster]
NASA – Ask an Astrophysicist – What are star clusters made of? What forms a star cluster?
Seds.org – Globular Star Clusters
Seds.org – Open Star Clusters
Fippy.net – Open and Globular Clusters – a comparison
WordiQ.com – Metallicity – Definition