NGC 1818

NGC 1818 is a young globular cluster in the north-west part of the Large Magellanic Cloud, about 3.2 kpc from the center.[7] It was discovered by Scottish astronomer James Dunlop in 1826,[8] and has since been well studied.[4]

NGC 1818
A Hubble Space Telescope (HST) image of NGC 1818.
Credit: HST/NASA/ESA.
Observation data (J2000 epoch)
ConstellationDorado
Right ascension05h 04m 13.300s[1]
Declination−66° 26′ 05.47″[1]
Distance~164 kly (50 kpc)[2]
Apparent magnitude (V)9.7 (B band)[3]
Physical characteristics
Absolute magnitude−8.8[4]
Mass13,500+5,600
−3,700 M☉
Estimated age30[5] or 40 Myr[6]
Notable featuresRare young globular

The cluster has an estimated core radius of 2.67 pc and a 90% light radius of 13.83 pc,[1] with a combined mass of around 13,500 times the mass of the Sun.[5] Age estimates for the cluster range from 25[7] to 40[6] million years. Given this, most stars with a mass equal to the Sun or less are still on the pre-main-sequence. The average stellar metallicity – what astronomers term the abundance of elements with higher atomic number than helium – is −0.4, or about 10−0.4 ≈ 40% of the abundance in the Sun.[4]

There appear to be two distinct stellar populations in the cluster with the more blue (hotter) stars showing slower rotation rates than the redder (cooler) stars.[6] The frequency of binary star systems in the cluster increases with distance from the core, which is the opposite of the normal trend for globular clusters. This may be explained by interactions with other stars in the denser core disrupting binary systems, before mass segregation of the cluster has begun to take effect.[5] The cluster contains few if any blue stragglers, which are the result of stellar mergers.[7]

References
  1. Werchan, Felicia; Zaritsky, Dennis (August 2011). "The Star Clusters of the Large Magellanic Cloud: Structural Parameters". The Astronomical Journal. 142 (2): 10. arXiv:1105.1769. Bibcode:2011AJ....142...48W. doi:10.1088/0004-6256/142/2/48. 48.
  2. Elson, Rebecca; Sword, Richard; NASA. "Hot white dwarf shines in young star cluster NGC 1818". Hubblesite. Retrieved 2020-08-26.
  3. SIMBAD (January 8, 2007), Results for NGC 1818, SIMBAD, Centre de Données Astronomiques de Strasbourg
  4. Liu, Q.; de Grijs, R.; Deng, L. C.; Hu, Y.; Baraffe, I.; Beaulieu, S. F. (July 2009). "The initial mass function of the rich young cluster NGC 1818 in the Large Magellanic Cloud". Monthly Notices of the Royal Astronomical Society. 396 (3): 1665–1674. arXiv:0903.4787. Bibcode:2009MNRAS.396.1665L. doi:10.1111/j.1365-2966.2009.14838.x.
  5. Geller, Aaron M.; et al. (May 2015). "Different Dynamical Ages for the Two Young and Coeval LMC Star Clusters, NGC 1805 and NGC 1818, Imprinted on Their Binary Populations". The Astrophysical Journal. 805 (1): 11. arXiv:1503.05198. Bibcode:2015ApJ...805...11G. doi:10.1088/0004-637X/805/1/11. 11.
  6. Marino, A. F.; et al. (September 2018). "Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence". The Astronomical Journal. 156 (3): 10. arXiv:1807.04493. Bibcode:2018AJ....156..116M. doi:10.3847/1538-3881/aad3cd. 116.
  7. Johnson, R. A.; et al. (2001). "Young star clusters in the Large Magellanic Cloud: NGC 1805 and 1818". Monthly Notices of the Royal Astronomical Society. 324 (2): 367. arXiv:astro-ph/0012389. Bibcode:2001MNRAS.324..367J. doi:10.1046/j.1365-8711.2001.04291.x.
  8. Seligman, Courtney. "NGC 1818". Celestial Atlas. Retrieved 7 December 2018.


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