What is Double Cluster?
[[File:Double Cluster.jpg|thumb|right|The Double Cluster]] The '''Double Cluster,''' is '''NGC 884''' and '''NGC 869''', a group of two open [[star cluster]]s in the [[Perseus constellation]]. It has many blue and red stars. Both are visible with the naked eye. NGC&nbsp;869 and NGC&nbsp;884 lie at a distance of 7,500&nbsp;light years.<ref>{{cite journal |bibcode=2001AJ....122..248K |arxiv=astro-ph/0104179 |title=UBVI and Hα photometry of the h and χ&nbsp;Persei cluster |url=https://archive.org/details/sim_astronomical-journal_2001-07_122_1/page/n255 |journal=The Astronomical Journal |volume=122 |page=248 |last1=Keller |first1=Stefan C. |last2=Grebel |first2=Eva K. |last3=Miller |first3=Grant J. |last4=Yoss |first4=Kenneth M. |year=2001 |issue=1 |doi=10.1086/321139|s2cid=117819633 }}</ref> That is relatively close-by (the [[Milky Way]] galaxy has a diameter of roughly 100,000 light years.) ==Stars== ===RS Persei and T Persei=== These are two [[red supergiant star]]s in the star cluster. They are 770 and 510 times larger than the [[Sun]] respectively.<ref>{{Cite journal|url=http://iopscience.iop.org/article/10.1088/0004-637X/785/1/46/meta|doi = 10.1088/0004-637X/785/1/46|title = Chara/Mirc Observations of Two M Supergiants in Perseus Ob1: Temperature, Bayesian Modeling, and Compressed Sensing Imaging|year = 2014|last1 = Baron|first1 = F.|last2 = Monnier|first2 = J. D.|last3 = Kiss|first3 = L. L.|last4 = Neilson|first4 = H. R.|last5 = Zhao|first5 = M.|last6 = Anderson|first6 = M.|last7 = Aarnio|first7 = A.|last8 = Pedretti|first8 = E.|last9 = Thureau|first9 = N.|last10 = Ten Brummelaar|first10 = T. A.|last11 = Ridgway|first11 = S. T.|last12 = McAlister|first12 = H. A.|last13 = Sturmann|first13 = J.|last14 = Sturmann|first14 = L.|last15 = Turner|first15 = N.|journal = The Astrophysical Journal|volume = 785|page = 46|hdl = 10023/5302|s2cid = 17085548}}</ref> ===SU Persei and AV Persei=== These are also red supergiant stars. They both are 780 and 770 times larger than the Sun respectively.<ref>{{Cite journal|url=htt