IC 4756

IC 4756
IC 4756
Observation data (J2000 epoch)
Right ascension	18h 38m 31.2s
Declination	+05° 29′ 24″
Distance	1.3 kly (400 pc)
Apparent magnitude (V)	4.6
Physical characteristics
Other designations	Cr 386, Mel 210
Associations
Constellation	Serpens
	See also: Open cluster, List of open clusters

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 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]

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

^ ^a ^b ^c "IC 4756". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 30 November 2016.
^ Michael Bakich. "The Omega Nebula, Graff's Cluster, and planetary nebula IC 1295". astronomy.com. Retrieved 2020-10-05.
^ O'Meara, Stephen James (2007). Deep-Sky Companions: Hidden Treasures. Cambridge University Press. p. 457-458. ISBN 978-0521-83704-0.
^ 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
^ "HR 7008 – Star in Cluster". SIMBAD. Retrieved 14 December 2014.
^ 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.
^ 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.
^ 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.