IC 4756

IC 4756 is a large bright open cluster in the constellation Serpens. Known as Graff's Cluster, it is bright enough to be seen with the naked eye and considered a fine cluster for binoculars or small telescopes.[2]

IC 4756
Observation data (J2000 epoch)
Right ascension18h 38m 31.2s[1]
Declination+05° 29′ 24″[1]
Distance1.3 kly (400 pc)
Apparent magnitude (V)4.6[1]
Physical characteristics
Other designationsCr 386, Mel 210
Associations
ConstellationSerpens

IC 4756 is also known as the Tweedledee Cluster (paired with NGC 6633 as Tweedledum), also as the Secret Garden Cluster.[3]

Metallicity of IC 4756 is similar to the Sun, at -0.02±0.01 dex.[4]

Open star clusters IC 4756 (left) and NGC 6633. Original astrophoto.

Stars

There are some noteworthy stars in the cluster. HD 172365[5] is a likely post-blue straggler in the IC 4756 that contains a large excess of lithium.[6] HD 172189, also in IC 4756, is an Algol variable eclipsing binary[7] with a 5.70 day period. The primary star in the system is also a Delta Scuti variable, undergoing multiple pulsation frequencies, which, combined with the eclipses, causes the system to vary by around a tenth of a magnitude.[8]

References
  1. "IC 4756". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 30 November 2016.
  2. Michael Bakich. "The Omega Nebula, Graff's Cluster, and planetary nebula IC 1295". astronomy.com. Retrieved 2020-10-05.
  3. O'Meara, Stephen James (2007). Deep-Sky Companions: Hidden Treasures. Cambridge University Press. p. 457-458. ISBN 978-0521-83704-0.
  4. Bagdonas, Vilius; Drazdauskas, Arnas; Tautvaišienė, Gražina; Smiljanic, Rodolfo; Chorniy, Yuriy (2018), "Chemical composition of giant stars in the open cluster IC 4756", Astronomy & Astrophysics, 615: A165, arXiv:1804.01975, Bibcode:2018A&A...615A.165B, doi:10.1051/0004-6361/201832695, S2CID 118857628
  5. "HR 7008 – Star in Cluster". SIMBAD. Retrieved 14 December 2014.
  6. Andrievsky, S. M.; Gorlova, N. I.; Klochkova, V. G.; Kovtyuch, V. V.; Panchuk, V. E. (1999). "The Lithium-rich supergiant HD172365". Astronomische Nachrichten. 320 (1): 35–41. Bibcode:1999AN....320...35A. doi:10.1002/1521-3994(199903)320:1<35::aid-asna35>3.0.co;2-f.
  7. Ibanoǧlu, C.; Evren, S.; Taş, G.; Çakırlı, Ö.; Bozkurt, Z.; Afşar, M.; Sipahi, E.; Dal, H. A.; Özdarcan, O.; Çamurdan, D. Z.; Çamurdan, M.; Frasca, A. (2009). "Spectroscopic and photometric observations of the selected Algol-type binaries - IV. V799 Cassiopeiae, BX Piscium and HD 172189". Monthly Notices of the Royal Astronomical Society. 392 (2): 757. Bibcode:2009MNRAS.392..757I. doi:10.1111/j.1365-2966.2008.14087.x.
  8. Costa, J. E. S.; Michel, E.; Peña, J.; Creevey, O.; Li, Z. P.; Chevreton, M.; Belmonte, J. A.; Alvarez, M.; Fox Machado, L.; Parrao, L.; Pérez Hernéndez, F.; Fernández, A.; Fremy, J. R.; Pau, S.; Alonso, R. (2007). "Pulsational frequencies of the eclipsing δ Scuti star HD 172189. Results of the STEPHI XIII campaign". Astronomy and Astrophysics. 468 (2): 637–642. arXiv:0706.4083. Bibcode:2007A&A...468..637C. doi:10.1051/0004-6361:20065784.



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