NGC 6530

NGC 6530 is a young[8] open cluster of stars in the southern constellation of Sagittarius, located some 4,300 light years from the Sun.[3] It exists within the H II region known as the Lagoon Nebula, or Messier 8,[9] and spans an angular diameter of 14.0′.[5] The nebulosity was first discovered by G. B. Hodierna prior to 1654, then re-discovered by J. Flamsteed circa 1680. It was P. Loys who classified it as a cluster in 1746, as he could only resolve stars. The following year, G. Le Gentil determined it was both a nebula and a cluster.[10]

Portion of NGC 6530 seen by Hubble.
NGC 6530
NGC 6530 (right) and the Lagoon Nebula
Observation data (J2000 epoch)
Right ascension18h 04m 31.0s[1]
Declination−24° 21′ 30″[1]
Distance4,360 ly (1,336+76
−68 pc)[2]
4,320 ly (1,325 pc)[3]
Apparent magnitude (V)4.6[4]
Apparent dimensions (V)14.0′[5]
Physical characteristics
Radius7 ly[6]
Estimated age4–6 Myr[7]
H II region
Other designationsCr 362
Associations
ConstellationSagittarius

The brightest six members of the cluster are visible in 10×50 binoculars at magnitudes 6.9 and fainter, while fifteen evenly distributed stars are visible with a 25×100 pair.[11] More than two dozen stars are visible in an amateur telescope.[6] The average extinction AV due to interstellar dust along the line of sight from the Earth is 1.20±0.24, with a color excess E(B − V) of 0.38±0.07.[8]

In total, 3,675 stars in the field of NGC 6530 have been catalogued as candidate members, with the likely members being 2,728.[3] As of 2019, 652 stars have been confirmed as members: 333 of these are classical T Tauri-type variable stars showing a near infrared emission excess, while the remainder are weak T Tauri stars showing a photospheric excess.[9] Candidate stars appear in two main groups at the cluster core and the Sagittarius "Hourglass nebula", with other smaller concentrations. Two such minor concentrations are associated with the stars 7 Sgr and HD 164536.[3]

Age estimates for the members shows a spread in values that suggests more than one burst of star formation. Initial star formation began up to 15 million years ago, but the bulk formed in the last 1–2 million years near the cluster center.[9] Astrometric data suggests the parent molecular cloud collided with the galactic plane some four million years ago, which may have triggered the star formation.[3] The dispersion of velocities for a sample of stars in the cluster suggests it may be gravitationally unbound and there is evidence the star population is expanding, particularly to the north and south.[12]

References
  1. Wu, Zhen-Yu; et al. (November 2009). "The orbits of open clusters in the Galaxy". Monthly Notices of the Royal Astronomical Society. 399 (4): 2146–2164. arXiv:0909.3737. Bibcode:2009MNRAS.399.2146W. doi:10.1111/j.1365-2966.2009.15416.x. S2CID 6066790.
  2. Kuhn, Michael A.; et al. (2018). "Kinematics in Young Star Clusters and Associations with Gaia DR2". The Astrophysical Journal. 870 (1): 32. arXiv:1807.02115. Bibcode:2019ApJ...870...32K. doi:10.3847/1538-4357/aaef8c. S2CID 119328315.
  3. Damiani, F.; et al. (March 2019). "Wide-area photometric and astrometric (Gaia DR2) study of the young cluster NGC 6530". Astronomy & Astrophysics. 623: 21. arXiv:1812.11402. Bibcode:2019A&A...623A..25D. doi:10.1051/0004-6361/201833877. S2CID 119428764. A25.
  4. "NGC 6530". SEDS NGC Catalog. Retrieved 27 April 2022.
  5. Morales, Esteban F. E.; et al. (2013). "Stellar clusters in the inner Galaxy and their correlation with cold dust emission". Astronomy & Astrophysics. 560: A76. arXiv:1310.2612. Bibcode:2013A&A...560A..76M. doi:10.1051/0004-6361/201321626. S2CID 118422539.
  6. Consolmagno, Guy; Davis, Dan M. (2019). Turn Left at Orion: Hundreds of Night Sky Objects to See in a Home Telescope - and How to Find Them. Cambridge University Press. p. 147. ISBN 9781108457569.
  7. Aidelman, Y.; et al. (February 2018). "Open clusters. III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383, and NGC 6530 B-type stars with circumstellar envelopes". Astronomy & Astrophysics. 610: 18. arXiv:1711.01311. Bibcode:2018A&A...610A..30A. doi:10.1051/0004-6361/201730995. S2CID 59375851. A30.
  8. Topasna, G. A.; et al. (February 2020). "Interstellar Extinction in the Direction of the Young Open Cluster NGC 6530". Publications of the Astronomical Society of the Pacific. 132 (1010): 14. Bibcode:2020PASP..132d4301T. doi:10.1088/1538-3873/ab6aba. 044301.
  9. Prisinzano, L.; et al. (March 2019). "The Gaia-ESO Survey: Age spread in the star forming region NGC 6530 from the HR diagram and gravity indicators". Astronomy & Astrophysics. 623: 22. arXiv:1901.09589. Bibcode:2019A&A...623A.159P. doi:10.1051/0004-6361/201834870. S2CID 119394685. A159.
  10. Chen, James L.; Chen, Adam (2015). A Guide to Hubble Space Telescope Objects, Their Selection, Location, and Significance. Springer International Publishing. p. 67. ISBN 9783319188720.
  11. Crossen, Craig; Rhemann, Gerald (2012). Sky Vistas, Astronomy for Binoculars and Richest-Field Telescopes. Springer Vienna. p. 46. ISBN 9783709106266.
  12. Wright, Nicholas J.; et al. (June 2019). "The Gaia-ESO Survey: asymmetric expansion of the Lagoon Nebula cluster NGC 6530 from GES and Gaia DR2". Monthly Notices of the Royal Astronomical Society. 486 (2): 2477–2493. arXiv:1903.12176. Bibcode:2019MNRAS.486.2477W. doi:10.1093/mnras/stz870.
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