Super star cluster

A super star cluster (SSC) is a very massive young open cluster that is thought to be the precursor of a globular cluster. These clusters are referred to as "super" due to the fact that they are relatively more luminous and contain more mass than other young star clusters. The SSC, however, does not have to physically be larger that other clusters of lower mass and luminosity. They typically contain a very large number of young, massive stars that ionize a surrounding HII region (however, SSC's do not always have to be inside an HII region), similar to the Milky Way Galaxy's so-called "Ultra dense HII regions (UDHIIs)". An SSC's H II region is in turn surrounded by a cocoon of dust. In many cases, the stars and the HII regions will be invisible to optical observations due to high levels of extinction. As a result, the youngest SSCs are best observed in radio and infrared. SSCs, such as Westerlund 1 (Wd1), have been found in the Milky Way, however, most have been observed in farther regions of the universe.

Specifically, SSCs have been seen to form in the interactions between galaxies and regions of high amounts of star formation with high enough pressures to satisfy the properties needed for a star cluster to form. These regions can include newer galaxies, such as dwarf starburst galaxies, arms of a spiral galaxy that have a high star formation rate, and in the merging of galaxies. In an Astronomical Journal published in 1996, using pictures taken in the UV spectrum by the Hubble Space Telescopes of star-forming rings in five different barred galaxies, numerous star clusters were found in clumps within the rings which had high rates of star formation. It was found that these clusters had masses of about $10^{3}$M_☉ to $10^{5}$M_☉, ages of about 100 Myr, and radii of about 5 pc, and are though to evolve into globular clusters later in their lifetimes. These are the properties of SSC's.

Characteristics and Properties

The typical characteristics of SSCs:

• Large electron densities $n_{e}=10^{3}$–$10^{6}$ cm$^{-3}$ (this is a property of the HII region associated with the SSC)
• Pressures $P/$$k_{b}$$=10^{7}$–$10^{10}$ K cm$^{-3}$. (this is a property of the HII region associated with the SSC)
• Mass $\gtrsim$ $10^{5}$M_☉
• Radius ≈ 5 pc ≈ $10^{19}$ cm
• Age ≈ 100 Myr (Although other sources state that observed SSCs have an age of 1 Gyr)

Hubble Space Telescope Contributions

Given the relatively small size of SSC's compared to their host galaxies, astronomers have had trouble finding them in the past due to the limited resolution of the telescopes at the time. With the introduction of the Hubble Space Telescope (HST) in the 1990's, finding SSC's as well as other astronomical objects became much easier thanks to the higher resolution of the HST (angular resolution of ~1/10 arcsecond). This not only allowed astronomers to see SSC's, but also allowed for them to measure their properties as well as the properties of the individual stars within the SSC. Recently, a massive star, Westerlund 1-26, was discovered in the SSC Westerlund 1 in the Milky Way. The radius of this star is thought to larger than the radius of Jupiter's orbit around the Sun.

List of SSC's

Name	Galaxy	Comments	Notes	Pictures
Westerlund 1 (Wd1)	Milky Way Galaxy	First SSC discovered in the Milky Way Galaxy. This SSC was discovered by the HST.		Westerlund 1
NGC 3603	Milky Way Galaxy			NGC 3603
R136	Large Magellanic Cloud (LMC)	The prototype SSC		R136 (Located in the Tarantula Nebula)
NGC 1569 A1 and A2	NGC 1569	Galaxy contains two SSCs		NGC 1569

External links

• Monster Super Star Cluster Discovered In Milky Way