Omicron¹ Canis Majoris (ο¹ CMa, ο¹ Canis Majoris) is a red supergiant star in the constellation Canis Major. It is also a variable star.

Quick Facts Observation data Epoch J2000 Equinox J2000, Constellation ...

ο¹ Canis Majoris

Location of ο2 CMa (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Canis Major
Right ascension	06h 54m 07.95237s[1]
Declination	−24° 11′ 03.1597″[1]
Apparent magnitude (V)	3.87[2] (3.78 - 3.99[3])
Characteristics
Spectral type	K2.5 Iab[4]
U−B color index	+1.99[2]
B−V color index	+1.73[2]
Variable type	Lc[3]
Astrometry
Radial velocity (Rv)	+36.3[5] km/s
Proper motion (μ)	RA: −4.479[6] mas/yr Dec.: 4.503[6] mas/yr
Parallax (π)	2.54 ± 0.36 mas[6]
Distance	approx. 1,300 ly (approx. 390 pc)
Absolute magnitude (MV)	−4.69[4]
Details
Mass	7.83 ± 2.0[7] M☉
Radius	212[1] R☉
Luminosity (bolometric)	10,280 - 10,570[1] L☉
Surface gravity (log g)	0.6[4] cgs
Temperature	4,015±170[1] K
Metallicity [Fe/H]	−0.11[8] dex
Rotational velocity (v sin i)	12.31[9] km/s
Age	18[10] Myr
Other designations
16 Canis Majoris, HR 2580, HD 50877, CD−24°4567, HIP 33152, SAO 172542, GC 9059
Database references
SIMBAD	data

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Johann Bayer gave two adjacent stars the Bayer designation of ο Canis Majoris in 1603, but without distinguishing between the stars. John Flamsteed gave the two omicron stars his own numbered designations of 16 and 24 Canis Majoris in the early 18th century.^[11] Friedrich Wilhelm Argelander labelled the stars as ο¹ and ο² in his atlas Uranometria Nova.^[12] Nicolas Louis de Lacaille labelled it c Canis Majoris, but this was not upheld by subsequent cartographers.^[13] Its Henry Draper Catalogue designation is HD 50877. The two Omicron stars marked the centre of the Great Dog's body on Bayer's 1603 Uranometria.^[14]

The distance to ο¹ Canis Majoris is uncertain. It is strongly associated with the Collinder 121 stellar association, located around 3,500 light years (1,085 parsecs) distant.^[15] Its original Hipparcos parallax placed it at 610 pc, similar to the distance of EZ Canis Majoris, another member of Cr 121. ο¹ CMa appears to be interacting with the nebula around EZ CMa, implying the two are at the same distance. However, the revised Hipparcos parallax is only 0.22 mas, with a margin of error of 0.43 mas, so the distance is not well-defined but likely to be large. The distance to EZ CMa is now thought to be around 1,500 pc. Conversely, though only separated by 2 degrees from the blue supergiant ο² Canis Majoris, the two appear to be unrelated.^[10]

The star itself is an orange K-type supergiant of spectral type K2.5 Iab and is an irregular variable star, varying between apparent magnitudes 3.78 and 3.99. A cool star, its surface temperature is around 3,900 K.^[4] Around 8 times as massive as the Sun with around 280 times its diameter, it shines with 16,000 times its luminosity. Interstellar dust only obscures it slightly, and its apparent magnitude is 0.16 fainter than it would be if unobscured.^[4] Thought to be around 18 million years old, ο¹ Canis Majoris is undergoing nuclear fusion of helium in its core to generate energy and will one day explode as a type II supernova.^[10]