View source for IC 4756

{{Short description|Open cluster in the constellation Serpens}}
{{Infobox open cluster
| name = IC 4756
| image = [[File:Open star clusters IC 4756 and NGC 6633.jpg|280px]]
| caption = Open star clusters IC 4756 (left) and [[NGC 6633]]
| epoch = [[J2000]]
|ra = {{RA|18|38|31.2}}<ref name="SIMBAD">{{cite simbad
  | title = IC 4756
  | access-date = 30 November 2016 }}</ref>
|dec = {{DEC|+05|29|24}}<ref name="SIMBAD"/>
| constellation = [[Serpens]]
| dist_ly = 1.3 [[light year|kly]]
| dist_pc = 400 [[parsec|pc]]
| appmag_v = 4.6<ref name="SIMBAD"/>
| names = [[Collinder catalogue|Cr]] 386, [[Melotte catalogue|Mel]] 210
}}
'''IC 4756''' is a large bright [[open cluster]] in the constellation [[Serpens]]. Known as [[Kasimir Graff|Graff]]'s Cluster, it is bright enough to be seen with the naked eye and considered a fine cluster for binoculars or small telescopes.<ref>{{Cite web|url=https://astronomy.com/observing/observing-podcasts/2014/07/the-omega-nebula-graffs-cluster-and-ic-1295|title=The Omega Nebula, Graff's Cluster, and planetary nebula IC 1295|website=astronomy.com|author=Michael Bakich |access-date=2020-10-05}}</ref>

IC 4756 is also known as the Tweedledee Cluster (paired with [[NGC 6633]] as Tweedledum), also as the Secret Garden Cluster.<ref>{{cite book |last=O'Meara|first=Stephen James |date=2007 |title=Deep-Sky Companions: Hidden Treasures|publisher=Cambridge University Press|page=457-458|isbn=978-0521-83704-0}}</ref>

[[Metallicity]] of IC 4756 is similar to the Sun, at −0.02{{±|0.01}} dex.<ref>{{citation|arxiv=1804.01975|year=2018|title=Chemical composition of giant stars in the open cluster IC 4756|doi=10.1051/0004-6361/201832695|last1=Bagdonas|first1=Vilius|last2=Drazdauskas|first2=Arnas|last3=Tautvaišienė|first3=Gražina|author-link3=Gražina Tautvaišienė|last4=Smiljanic|first4=Rodolfo|last5=Chorniy|first5=Yuriy|journal=Astronomy & Astrophysics|volume=615|pages=A165|bibcode=2018A&A...615A.165B|s2cid=118857628 }}</ref>

==Stars==

There are some noteworthy stars in the cluster. [[HD 172365]]<ref name="HD172365Simbad">{{cite web |url =http://simbad.u-strasbg.fr/simbad/sim-id?Ident=HD+172365 |title = HR 7008 – Star in Cluster|publisher = [[SIMBAD]] | access-date = 14 December 2014}}</ref> is a likely post-[[blue straggler]] in the IC 4756 that contains a large excess of [[lithium]].<ref name="Andrievsky">{{cite journal |last1=Andrievsky |first1=S. M. |last2=Gorlova |first2=N. I. |last3=Klochkova |first3=V. G. |last4=Kovtyuch |first4=V. V. |last5=Panchuk |first5=V. E. |title=The Lithium-rich supergiant HD172365 |journal=Astronomische Nachrichten |volume=320 |issue=1 |pages=35–41 |year=1999 |bibcode=1999AN....320...35A |doi=10.1002/1521-3994(199903)320:1<35::aid-asna35>3.0.co;2-f}}</ref> [[HD 172189]], also in IC 4756, is an [[Algol variable]] eclipsing binary<ref name="Ibanoglu">{{Cite journal
 | doi = 10.1111/j.1365-2966.2008.14087.x
| title = Spectroscopic and photometric observations of the selected Algol-type binaries - IV. V799 Cassiopeiae, BX Piscium and HD 172189| journal = Monthly Notices of the Royal Astronomical Society| volume = 392| issue = 2| pages = 757| year = 2009| last1 = Ibanoǧlu | first1 = C.| last2 = Evren | first2 = S.| last3 = Taş | first3 = G.| last4 = Çakırlı | first4 = Ö.| last5 = Bozkurt | first5 = Z.| last6 = Afşar | first6 = M.| last7 = Sipahi | first7 = E.| last8 = Dal | first8 = H. A.| last9 = Özdarcan | first9 = O.| last10 = Çamurdan | first10 = D. Z. | last11 = Çamurdan | first11 = M.| last12 = Frasca | first12 = A.| bibcode = 2009MNRAS.392..757I| doi-access = free}}</ref> 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.<ref name="Costa">{{Cite journal | doi = 10.1051/0004-6361:20065784| title = Pulsational frequencies of the eclipsing δ&nbsp;Scuti star HD&nbsp;172189. Results of the STEPHI XIII campaign| journal = Astronomy and Astrophysics| volume = 468| issue = 2| pages = 637–642| year = 2007| last1 = Costa | first1 = J. E. S.| last2 = Michel | first2 = E.| last3 = Peña | first3 = J.| last4 = Creevey | first4 = O.| last5 = Li | first5 = Z. P.| last6 = Chevreton | first6 = M.| last7 = Belmonte | first7 = J. A.| last8 = Alvarez | first8 = M.| last9 = Fox Machado | first9 = L.| last10 = Parrao | first10 = L.| last11 = Pérez Hernéndez | first11 = F.| last12 = Fernández | first12 = A.| last13 = Fremy | first13 = J. R.| last14 = Pau | first14 = S.| last15 = Alonso | first15 = R.| bibcode = 2007A&A...468..637C|arxiv = 0706.4083 }}</ref>

==References==
{{Reflist}}

==External links==
* {{commonscat-inline}}
* {{WikiSky}}


{{Serpens|state=collapsed}}
{{Sky|18|39|04.9|+|05|27|09|13000}}

{{DEFAULTSORT:IC 4756}}
[[Category:IC objects|4756]]
[[Category:Open clusters]]
[[Category:Serpens]]