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IC 2391 (also known as the Omicron Velorum Cluster or Caldwell 85) is an open cluster in the constellationVela consisting of hot, young, blueish stars, some of which binaries and one of which is a quadruple. Persian astronomer A. a.-R. al-Sufi first described it as "a nebulous star" in c. 964. It was re-found by Abbe Lacaille and cataloged as Lac II 5.
It is centred about 490 light-years away from Earth and can be seen with the naked eye. It contains about 30 stars with a total visual magnitude of 2.5, spread out along 50 arcminutes.
The stars' era of formation is similar to open cluster IC 2602 in neighbouring Carina, and has a lithium depletion boundary
age of about 50 million years. The latter group averages about the same distance, placed at about 485 light years away.
Argus Association
The components formed at about the same time as a nearer group, known as the "Argus Association" which one motion model suggests began in their own nebula cloud. These are in a similar direction, roughly the Vela constellation, within the local galactic arm.
The supposed association may chiefly comprise:
Deep-sky Observer's Handbook: Open and globular clusters, Kenneth Glyn Jones, Webb Society (1979), at p3
The History of Astronomy, 2017, Anne Rooney, at p162
Platais, I; Melo, C; Mermilliod, J. -C; Kozhurina-Platais, V; Fulbright, J. P; Méndez, R. A; Altmann, M; Sperauskas, J (2007). "WIYN open cluster study. XXVI. Improved kinematic membership and spectroscopy of IC 2391". Astronomy and Astrophysics. 461 (2): 509. arXiv:astro-ph/0611584. Bibcode:2007A&A...461..509P. doi:10.1051/0004-6361:20065756. S2CID17173564.
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