A super star cluster (SSC) is a very massive young open cluster that is thought to be the precursor of a globular cluster.[1] 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.[2] The SSC, however, does not have to physically be larger than other clusters of lower mass and luminosity.[3] They typically contain a very large number of young, massive stars that ionize a surrounding HII region or a so-called "Ultra dense HII regions (UDHIIs)" in the Milky Way Galaxy[4] as well as in other galaxies (however, SSCs do not always have to be inside an HII region). An SSC's HII region is in turn surrounded by a cocoon of dust. In many cases, the stars and the HII regions will be invisible to observations in certain wavelengths of light, such as the visible spectrum, due to high levels of extinction. As a result, the youngest SSCs are best observed and photographed in radio and infrared.[5] SSCs, such as Westerlund 1 (Wd1), have been found in the Milky Way Galaxy.[6] However, most have been observed in farther regions of the universe. In the galaxy M82 alone, 197 young SSCs have been observed and identified using the Hubble Space Telescope.[7]

Generally, SSCs have been seen to form in the interactions between galaxies and in regions of high amounts of star formation with high enough pressures to satisfy the properties needed for the formation of a star cluster.[2] These regions can include newer galaxies with much new star formation, dwarf starburst galaxies,[8] 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 ultraviolet (UV) spectrum by the Hubble Space Telescope 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. These clusters were found to have masses of about ${\displaystyle 10^{3))$M to ${\displaystyle 10^{5))$M, ages of about 100 Myr, and radii of about 5 pc, and are thought to evolve into globular clusters later in their lifetimes.[9] These properties match those found in SSCs.

## Characteristics and properties

The typical characteristics and properties of SSCs:

• Mass ${\displaystyle \gtrsim }$ ${\displaystyle 10^{5))$M[2]
• Radius ≈ 5 pc ≈ ${\displaystyle 10^{19))$ cm[2]
• Age ≈ 100 Myr[2] (Although other sources state that observed SSCs have an age of 1 Gyr[3])
• Large electron densities ${\displaystyle n_{e}=10^{3))$${\displaystyle 10^{6))$ cm${\displaystyle ^{-3))$ (this is a property of the HII region associated with the SSC)
• Pressures ${\displaystyle P/}$${\displaystyle k_{b))$${\displaystyle =10^{7))$${\displaystyle 10^{10))$ K cm${\displaystyle ^{-3))$.[5] (this is a property of the HII region associated with the SSC)

## 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 ground-based and space telescopes at the time. With the introduction of the Hubble Space Telescope (HST) in the 1990s, 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[10]). This has 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 be larger than the radius of Jupiter's orbit around the Sun.[11] Essentially, the HST searches the night sky, specifically nearby galaxies, for star clusters and "dense stellar objects" to see if any have the properties similar to that of a SSC or an object that would, in its lifetime, evolve into a globular cluster.[3]

## List of SSC's

Name Contained
in Galaxy
Westerlund 1 (Wd1) Milky Way Galaxy First SSC discovered in the Milky Way Galaxy. This SSC was discovered by Bengt Westerlund in 1961. [12]
Westerlund 1
NGC 3603 Milky Way Galaxy Candidate for SSC [13]
NGC 3603
NGC 2070 Large Magellanic Cloud (LMC) Candidate for SSC
NGC 2070
R136 Large Magellanic Cloud (LMC) The prototype SSC, inside NGC 2070 [14]
R136 (Located in the Tarantula Nebula)
NGC 346 Small Magellanic Cloud (SMC) May be a SSC [15]
NGC 346
NGC 1569 A1 and A2 (NGC 1569 A) NGC 1569 Clusters A1 and A2 formed SSC A [16]
NGC 1569
NGC 1569 B NGC 1569 It contains older population of red giants and red supergiants [17]
NGC 1569
NGC 5253's central SSC NGC 5253 It is very dusty
NGC 5253
This list is incomplete; you can help by adding missing items. (February 2011)

## References

1. ^ Gallagher & Grebel (2002). "Extragalactic Star Clusters: Speculations on the Future". Extragalactic Star Clusters, IAU Symposium. 207: 207. arXiv:astro-ph/0109052. Bibcode:2002IAUS..207..745G.
2. Johnson, Kelsey (2001). "The Properties of Super Star Clusters In A Sample of Starburst Galaxies" (PDF). Bibcode:2001PhDT.......182J. S2CID 117321058. Archived from the original (PDF) on 2017-02-27. `((cite journal))`: Cite journal requires `|journal=` (help)
3. ^ a b c de Grijs, Richard. ""Super" Star Clusters" (PDF). `((cite journal))`: Cite journal requires `|journal=` (help)
4. ^ Kobulnicky, Henry A. & Johnson, Kelsey E. (1999). "Signatures of the Youngest Starbursts: Optically Thick Thermal Bremsstrahlung Radio Sources in Henize 2-10". Astrophysical Journal. 527 (1): 154–166. arXiv:astro-ph/9907233. Bibcode:1999ApJ...527..154K. doi:10.1086/308075. S2CID 15431678.
5. ^ a b Johnson (2004). "Extragalactic Ultracompact HII Regions: Probing the Birth Environments of Super Star Clusters". ASP Conference Series. 527: 322. arXiv:astro-ph/0405125. Bibcode:2004ASPC..322..339J.
6. ^ "Super Star Cluster Discovered in Our Own Milky Way - Universe Today". Universe Today. 2005-03-22. Retrieved 2017-02-10.
7. ^ Melo, V. P.; Muñoz-Tuñón, C.; Maíz-Apellániz, J.; Tenorio-Tagle, G. (2005-01-01). "Young Super Star Clusters in the Starburst of M82: The Catalog". The Astrophysical Journal. 619 (1): 270. arXiv:astro-ph/0409750. Bibcode:2005ApJ...619..270M. doi:10.1086/426421. ISSN 0004-637X. S2CID 16452890.
8. ^ Hunter, Deidre A.; O'Connell, Robert W. (2000). "The Star Clusters in the Starburst Irregular Galaxy NGC 1569". The Astronomical Journal. 20 (5): 2383–2401. arXiv:astro-ph/0009280. Bibcode:2000AJ....120.2383H. doi:10.1086/316810. S2CID 6445978.
9. ^ Maoz, D.; Barth, A. J.; Sternberg, A.; Filippenko, A. V.; Ho, L. C.; Macchetto, F. D.; Rix, H.-W.; Schneider, D. P. (1996-06-01). "Hubble Space Telescope Ultraviolet Images of Five Circumnuclear Star-Forming Rings". The Astronomical Journal. 111: 2248. arXiv:astro-ph/9604012. Bibcode:1996AJ....111.2248M. doi:10.1086/117960. ISSN 0004-6256. S2CID 12241545.
10. ^ "FAQ - Frequently Asked Questions". www.spacetelescope.org. Retrieved 2017-03-18.
11. ^ Wallace, Amy (March 10, 2017). "Hubble finds young super star cluster, giant star". UPI.
12. ^ Clark, J. S.; Negueruela, I.; Crowther, P. A.; Goodwin, S. P. (2005). "On the massive stellar population of the super star cluster Westerlund 1". Astronomy and Astrophysics. 434 (3): 949. arXiv:astro-ph/0504342. Bibcode:2005A&A...434..949C. doi:10.1051/0004-6361:20042413. S2CID 119042919.
13. ^ Fukui, Y.; Ohama, A.; Hanaoka, N.; Furukawa, N.; Torii, K.; Dawson, J. R.; Mizuno, N.; Hasegawa, K.; Fukuda, T.; Soga, S.; Moribe, N.; Kuroda, Y.; Hayakawa, T.; Kawamura, A.; Kuwahara, T.; Yamamoto, H.; Okuda, T.; Onishi, T.; Maezawa, H.; Mizuno, A. (2014). "Molecular Clouds Toward the Super Star Cluster Ngc 3603; Possible Evidence for a Cloud-Cloud Collision in Triggering the Cluster Formation". The Astrophysical Journal. 780 (1): 36. arXiv:1306.2090. Bibcode:2014ApJ...780...36F. doi:10.1088/0004-637X/780/1/36. S2CID 53118305.
14. ^ Massey, Philip; Hunter, Deidre A. (1998). "Star Formation in R136: A Cluster of O3 Stars Revealed by Hubble Space Telescope Spectroscopy". The Astrophysical Journal. 493 (1): 180–194. Bibcode:1998ApJ...493..180M. doi:10.1086/305126.
15. ^ Sabbi, E.; Nota, A.; Cignoni, M.; Degl'Innocenti, S.; De Marchi, G.; Gallagher, J. S.; Panagia, N.; Prada Moroni, P. G.; Romaniello, M.; Smith1, L. J.; Sirianni, M.; Tosi, M. (2011). "NGC 346: Tracing the Evolution of a Super Star Cluster". Stellar Clusters & Associations: A Ria Workshop on Gaia: 244–249. Bibcode:2011sca..conf..244S.
16. ^ Hunter, Deidre A.; O'Connell, Robert W.; Gallagher, J. S.; Smecker-Hane, Tammy A. (2000). "The Star Clusters in the Starburst Irregular Galaxy NGC 1569". The Astronomical Journal. 120 (5): 2383. arXiv:astro-ph/0009280. Bibcode:2000AJ....120.2383H. doi:10.1086/316810. S2CID 6445978.
17. ^ Hunter, Deidre A.; O'Connell, Robert W.; Gallagher, J. S.; Smecker-Hane, Tammy A. (2000). "The Star Clusters in the Starburst Irregular Galaxy NGC 1569". The Astronomical Journal. 120 (5): 2383. arXiv:astro-ph/0009280. Bibcode:2000AJ....120.2383H. doi:10.1086/316810. S2CID 6445978.