IC 4756 explained

IC 4756
Epoch:	J2000
Ra:	^[1]
Constellation:	Serpens
Dist Ly:	1.3 kly
Dist Pc:	400 pc
Appmag V:	4.6
Names:	Cr 386, Mel 210

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 dex.

Stars

There are some noteworthy stars in the cluster. HD 172365^[4] is a likely post-blue straggler in the IC 4756 that contains a large excess of lithium.^[5] HD 172189, also in IC 4756, is an Algol variable eclipsing binary^[6] 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.^[7]

Notes and References

1. IC 4756 . 30 November 2016 .
2. Web site: The Omega Nebula, Graff's Cluster, and planetary nebula IC 1295. astronomy.com. Michael Bakich . 2020-10-05.
3. Book: O'Meara, Stephen James . 2007 . Deep-Sky Companions: Hidden Treasures. Cambridge University Press. 457-458. 978-0521-83704-0.
4. Web site: HR 7008 – Star in Cluster. . 14 December 2014.
5. Andrievsky . S. M. . Gorlova . N. I. . Klochkova . V. G. . Kovtyuch . V. V. . Panchuk . V. E. . The Lithium-rich supergiant HD172365 . Astronomische Nachrichten . 320 . 1 . 35–41 . 1999 . 1999AN....320...35A . 10.1002/1521-3994(199903)320:1<35::aid-asna35>3.0.co;2-f.
6. 10.1111/j.1365-2966.2008.14087.x. 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. 2009. 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.. 2009MNRAS.392..757I. free.
7. 10.1051/0004-6361:20065784. Pulsational frequencies of the eclipsing δ Scuti star HD 172189. Results of the STEPHI XIII campaign. Astronomy and Astrophysics. 468. 2. 637–642. 2007. 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.. 2007A&A...468..637C. 0706.4083 .