A Deep Look into the Omega Centauri and Jewel Box Clusters

This article will give an in-depth look into the two star clusters of Omega Centauri and the Jewel Box. These are two spectacular star clusters seen as stars to the naked eye yet reveal themselves to be star clusters when observed through telescopes.

What they are

Omega Centauri (pictured above) is a globular cluster located 17,000 light years away in the constellation of Centaurus. It is made up of millions of stars. It is visible with the naked eye and is almost the size of the full moon, although a telescope is required for it to be seen as a cluster rather than a single star (Boffin & Rodriguez 2008).

The Jewel Box (above) is a 10 million year old open cluster at a distance of 6400 light years from the Earth and is comprised of over 100 stars. It has a size of approximately 20 light years. It can be observed with binoculars and is located at the Southern Cross constellation, also known as Crux (Nemiroff & Bonnell 2010).

Main features
Omega Centauri is very distinct from other globular clusters. One defining distinguishing characteristic is that it is ten times larger than the typical globular cluster. It is also unique in that instead of containing stars from a single generation, it contains a multitude of stars of varying ages. It is thought that Omega Centauri is actually instead a dwarf galaxy since there have been observations of a black hole at its centre (Dunbar 2010). Omega Centauri is a class VIII globular cluster. Globular clusters are categorised by the Shapley-Sawyer Concentration Class, which ranges from I-XII in descending order of star density (Inglis 2013).

The Jewel Box cluster is different from Omega Centauri in that it is not a globular cluster, but an open cluster. The difference between the two is open clusters are younger and contain significantly fewer stars than globular clusters. One can easily see the individual stars in an open cluster with a telescope. Open clusters are only weakly held together by gravity, as opposed to a globular cluster where the stars are tightly held together. This will eventually result in the stars breaking away from each other. The two cluster types also differ in their location. Open clusters are located on the spiral arms of the galaxy whereas globular clusters can be found in the halo that surrounds the galactic plane (CSIRO n.d).

History of the two star clusters
The Omega Centauri star cluster was once catalogued as a single star by two of the great astronomers of the past Ptolemy and Bayer. Later on in the 19th century John Frederick William Herschel discovered that it is in fact not a single star but a globular cluster. Modern observations of this globular cluster have shown evidence supporting the existence of a black hole at its centre (Boffin & Rodriguez 2008).

Before the era of telescopes, the Jewel Box cluster was thought to just be a single star. This star was given the name Kappa Crucis. When it was first viewed through a handheld telescope by Nicolas de Lacaille in the mid-18th century it was discovered that it was actually a nebulous cluster. As telescopes got larger the Jewel Box was found to be a star cluster with over 300 stars (Ventrudo 2009).

References
DUNBAR, B. 2010. Omega Centauri [Online]. NASA. Available: http://www.nasa.gov/mission_pages/WISE/multimedia/gallery/pia13125.html [Accessed 25/4/2015].

VENTRUDO, B. 2009. The Jewel Box Cluster [Online]. One-Minute Astronomer. Available: http://oneminuteastronomer.com/848/jewel-box-cluster/ [Accessed 25/4/2015].

BOFFIN, H. & RODRIGUEZ, V. 2008. Omega Centauri - The Glittering Giant of the Southern Skies [Online]. ESO. Available: http://www.eso.org/public/australia/news/eso0844/ [Accessed 25/4/2015].

INGLIS, M. 2013. Field Guide to the Deep Sky Objects, Springer London.

NEMIROFF, R. & BONNELL, J. 2010. Astronomy Picture of the Day [Online]. NASA. Available: http://apod.nasa.gov/apod/ap100817.html [Accessed 25/4/2015.

CSIRO. (n.d). Star Clusters [Online]. CSIRO. Available: http://www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_clusters.html [Accessed 26/4/2015].