NGC 121
Oldest globular cluster in the Small Magellanic Cloud.
NGC 121 is a globular star cluster located in the southern constellation Tucana. It holds the distinction of being the oldest globular cluster in the Small Magellanic Cloud (SMC), a dwarf satellite galaxy of the Milky Way. English astronomer John Herschel first spotted it on September 20, 1835. Danish astronomer John Louis Emil Dreyer, who compiled the New General Catalogue, described it as "pretty bright, pretty small, little extended, very gradually brighter middle." The cluster lies roughly 200,000 light-years (60 kpc) from the Sun.
This cluster belongs to the West Halo region of the SMC, which is moving outward relative to the rest of the galaxy. It sits about 2.3° northwest of the SMC’s galactic center. Its mass is about 360,000 times that of the Sun. The angular half-light radius measures 27.1 arcseconds, and the tidal radius is 143 arcseconds. NGC 121 is located roughly 32 arcminutes from the massive globular cluster 47 Tucanae, which has a tidal radius of 42.86 arcminutes.
NGC 121 is the only old globular cluster in the SMC that resembles Milky Way globular clusters. Its age is estimated between 10.5 and 11.8 billion years, making it 2–3 billion years younger than the oldest comparable clusters in the Milky Way. The aging giant stars in NGC 121 reveal two distinct stellar populations. The younger population is chemically enriched by material from the first generation. The second generation makes up a relatively low 32% of the total population, but this fraction is higher in the cluster’s central region, suggesting the later generation is more concentrated there.
RR Lyrae variable stars were detected in this cluster in 1988. In 2008, 20 candidate Dwarf Cepheid variables were reported, including SX Phoenicis variables. In 1998, the Hubble Space Telescope identified 42 potential blue stragglers in images of the cluster.
- Discoverer
- John Herschel
- Discovery date
- September 20, 1835
- Constellation
- Tucana
- Distance from sun
- 200,000 light-years (60 kpc)
- Mass
- 3.6×10^5 solar masses
- Angular half light radius
- 27.1″
- Tidal radius
- 143″
- Age range
- 10.5 to 11.8 billion years
Lore & Background
NGC 121 was first discovered by English astronomer John Herschel on September 20, 1835. The compiler of the New General Catalogue, Danish astronomer John Louis Emil Dreyer, described this object as 'pretty bright, pretty small, little extended, very gradually brighter middle'. The cluster is located at a distance of around 200,000 light-years (60 kpc) from the Sun. It forms part of the West Halo, a region moving outward with respect to the rest of the SMC, and lies about 2.3° northwest of the SMC galactic center. The cluster mass is 3.6×10^5 times the mass of the Sun, with an angular half-light radius of 27.1″ and a tidal radius of 143″. It is positioned about ~32′ from the massive globular cluster 47 Tucanae, which has a tidal radius of 42.86′.
Reader's Guide
NGC 121 is notable as the only old globular cluster in the Small Magellanic Cloud that resembles Galactic clusters in the Milky Way. Age estimates for this cluster range from 10.5 to 11.8 billion years old, which is 2–3 billion years younger than the oldest comparable clusters in the Milky Way. The aging giant stars in this cluster reveal two distinct stellar populations: the younger population is chemically enriched from the output of the first generation. The second generation forms a relatively low ~32% of the total population, but this amount is enhanced in the central portion of the cluster, suggesting the later generation is more centrally concentrated. RR Lyrae variable stars were detected in this cluster in 1988. In 2008, 20 candidate Dwarf Cepheid candidates were reported, including SX Phoenicis variables. In 1998, 42 potential blue stragglers were identified via imaging by the Hubble Space Telescope. These features make NGC 121 a key object for studying stellar evolution and population dynamics in a dwarf galaxy environment.
Did You Know?
- The cluster has two distinct stellar populations, with the younger generation comprising about 32% of the total.
- 42 potential blue stragglers were identified in NGC 121 by the Hubble Space Telescope in 1998.
More in Star Clusters 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced