The Omicron Velorum Cluster is an open star cluster located 574 light-years away in the southern constellation of Vela (the Sails). With an apparent magnitude of 2.5 and an angular size of 50 arcminutes, it is one of the brightest and largest open clusters in the southern celestial hemisphere. It is catalogued as IC 2391 in the Index Catalogue and Caldwell 85 in the Caldwell catalogue.
The cluster takes its name from its brightest member, the hot blue main sequence star Omicron Velorum (ο Vel, HD 74195). Omicron Velorum shines at magnitude 3.60 from a distance of 490 light-years. It is a slowly pulsating B-type star (SPB) whose brightness varies from magnitude 3.57 to 3.63 over a period of 2.8 days due to pulsations.
With 5.5 times the Sun’s mass, Omicron Velorum is not massive enough to end its life as a supernova. However, it will burn through the supply of hydrogen in its core faster than most other stars due to its high mass. The star has an estimated age of 39.8 million years. It will evolve away from the main sequence in the relatively near future.
Open cluster IC 2391 in Vela. Image created using the Aladin Sky Atlas software from the Strasbourg Astronomical Data Center and DSS (Digitized Sky Survey) data. DSS is one of the programs of STScI (Space Telescope Science Institute) whose files are in the public domain. Credit: Roberto Mura (CC BY-SA 4.0)
Omicron Velorum has a radius of 4.3 solar radii and shines with 1,000 solar luminosities. It has a surface temperature of about 16,200 K.
The Omicron Velorum Cluster hosts around 30 stars with a total apparent visual magnitude of 2.5 within an angular diameter of 50 arcminutes. In comparison, the full Moon has an apparent diameter of 30 arcminutes.
Other relatively bright members include the variable blue B-type subgiants HY Velorum (HD 74560) and NZ Velorum (HD 74146), the Cepheid variable HD 74196, the blue main sequence star HW Velorum (HD 74071), and the Alpha2 Canum Venaticorum variable KT Velorum (HD 74535), a hot, blue, B-type giant. These stars have apparent magnitudes between 4.815 and 5.47 and are visible to the unaided eye from areas without light pollution.
The Omicron Velorum Cluster has a similar lithium depletion boundary age of 50 million years as the Southern Pleiades (Theta Carinae Cluster, IC 2602) in the constellation Carina. The Southern Pleiades cluster lies in the region of the Diamond Cross, formed by the cluster’s brightest member Theta Carinae with Miaplacidus (Beta Carinae), Upsilon and Omega Carinae. The cluster is considerably brighter than IC 2391 and lies at a similar distance, 486 light-years away.
The stars of IC 2391 may be members of the disputed Argus association, a young moving group proposed by Torres et al in 2008. A 2018 study led by B. Zuckerman of the Department of Physics and Astronomy, University of California, supports the existence of the stellar association, confirming that the highest concentration of Argus stars is near the Omicron Velorum Cluster. The bright Denebola (Beta Leonis) in the constellation Leo is among the proposed members of the group.
Omicron Velorum Cluster (IC 2391) captured by by the iTelescope network (T59). Image: Wikimedia Commons/YayLol123 (CC BY-SA 4.0)
Facts
The Omicron Velorum Cluster was recorded by the French astronomer Nicolas-Louis de Lacaille in 1751. Lacaille spent several years studying the far southern sky from the Cape of Good Hope in South Africa. He discovered IC 2391 with his small refractor and catalogued it as Lac II 5.
Lacaille, however, was not the first to discover the cluster. The 10th century Persian astronomer Abd al-Rahman al-Sufi described IC 2391 as a “nebulous star” in 964.
The open cluster NGC 2669 appears near IC 2391 in the sky, but the two clusters are not physically related. NGC 2669 is believed to lie at a much greater distance. Various sources list distances of 2,174.4 light years (667 parsecs) and 3,410 light years (1,046 parsecs). With an apparent magnitude of 6.1 and an apparent size of 8.4 arcminutes, NGC 2669 is relatively bright and can be observed in small telescopes.
NGC 2669 (left) and the Omicron Velorum Cluster, image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)
Location
The Omicron Velorum Cluster is easy to find because it appears next to the False Cross, one of the most familiar features of the southern sky. Formed by the bright stars Avior (Epsilon Carinae) and Aspidiske (Iota Carinae) in Carina with Alsephina (Delta Velorum) and Markeb (Kappa Velorum) in Vela, the asterism is often mistaken for the brighter and smaller Southern Cross in the constellation Crux.
IC 2391 appears just north of Alsephina, in the region between Alsephina and Regor (Gamma Velorum).
The location of the Omicron Velorum Cluster (IC 2391), image: Stellarium
The Southern Beehive Cluster (NGC 2516, Caldwell 96) lies in the same area of the sky, near the brighter Avior.
At declination -53°, the Omicron Velorum Cluster never rises for observers north of the latitude 37° N and stays low above the horizon from northern equatorial latitudes. It is best seen from the southern hemisphere.
The best time of the year to observe IC 2391 and other deep sky objects in Vela is during the month of March, when the constellation appears higher above the horizon in the early evening.
Omicron Velorum Cluster – IC 2391
| Constellation | Vela |
| Object type | Open cluster |
| Right ascension | 08h 41m 10.1s |
| Declination | −52° 59′ 28′′ |
| Apparent magnitude | 2.5 |
| Apparent size | 50’ |
| Distance | 574 light-years (176 parsecs) |
| Names and designations | Omicron Velorum Cluster, ο Velorum Cluster, IC 2391, Caldwell 85 (C85), Collinder 191 (Cr 191), Lac II 5, C 0838-528, Cl VDBH 42, OCl 767.0, Escorial 31, Theia 114, MWSC 1529 |