Epsilon Canis Majoris

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Epsilon Canis Majoris
Observational Data
Designation Adhara
21 Canis Majoris
Right ascension 07h 25m 38.8983s[1][2]
Declination +09° 16′ 33.9268″[1][2]
Constellation Canis Major
Type of object Bright giant star
Magnitude Apparent Mag: +1.50[3]
Absolute Mag: -4.10 / -3.97[3]
Distance from Earth 405.17 ly[3]
Radial velocity -6.90 ± 0.20 km/s[1][2]
Proper motion RA: -3.338 mas/yr[1][2]
Dec: -8.497 mas/yr[1][2]
Parallax 4.2624 ± 0.2288 mas[1][2]

Epsilon Canis Majoris (Adhara, 21 Canis Majoris) is a bright giant star and, unusually for its Bayer designation of epsilon, is the second brightest in the constellation of Canis Major.[4] The 22nd most luminous star in the sky, it can easily be observed with the unaided eye and is most easily seen in February. This means along with other stars such as Sirius, it is often used for celestial navigation. The star is in fact a binary system where the larger primary star is designated "Epsilon Canis Majoris A" and the smaller secondary "Epsilon Canis Majoris B". Epsilon Canis Majoris A is known for being the brightest source of ultraviolet radiation known.[5]

Adhara's name comes from the Arabic "aðāra" or "Al Adhara" where it translates as "maidens".[6] It is not known who first called the star by this name, though the Egyptian astronomer Al Achsasi al Mouakket refered to it as "Aoul al Adzari".[7] Sometimes the star is spelled slightly differently, various spellings include: Udra, Udara, Adard and Adara. The star is also known in Chinese as 弧矢七 (Hú Shǐ qī, Seventh Star of Bow and Arrow), where is is part of a Chinese asterism called the "Bow and Arrow". It is one of the 23 stars on the national flag of Brazil, where it represents the Brazzilian state of Tocantins.

Properties and structure

The Epsilon Canis Majoris system can be found 405.17 light years from Earth.[7] The larger Epsilon Canis Majoris A is thought to have a mass of 12.6 solar masses and radius 13.9 times larger than the Sun's. It possesses an apparent magnitude of +1.50 and a spectral class of B2. The star is though to rotate with an equatorial speed of 38 km/s.[4] With a surface temperature of 21,900 kelvin, the star appears blue-white and is extremely luminous; it emits roughly 20,000 times more energy per second than the Sun.[8][9] The high temperature leads to large amounts of radiation emitted in the ultraviolet portion of the star's spectrum; it is 38.700 times brighter than the Sun here.[10] In fact is is known as an extreme ultraviolet source and was discovered by the Extreme Ultraviolet Explorer (EUVE) all-sky survey to be brighter than any other star in UV light, but as UV can't be seen by humans, it drops down to the 22nd brightest star in the night sky. The UV light is able ionize hydrogen atoms and molecules close to the star.[11] The star is used to determine the ionization state of the Local Interstellar Cloud as it is the strongest UV source near us.[10] The cloud is 30 ly across and our own Solar System is contained within it.

Less is known about the companion Epsilon Canis Majoris B. It is perhaps 250 times fainter than Epsilon Canis Majoris at a magnitude of +7.50, though some sources place it fainter still at +8 or even +9.[8][4] It is thought to have an absolute magnitude of +1.9.[7] Epsilon Canis Majoris B is hard to observe except with a large telescope as it is located only around 900 Astronomical unit or 7.5 arcseconds away from Epsilon Canis Majoris A and Epsilon Canis Majoris A is so much brighter.[12]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Epsilon Canis Minoris. Simbad Astronomical Database. simbad.u-strasbg.fr. Retrieved on June 27, 2020.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Gaia Collaboration (2018). "VizieR Online Data Catalog: Gaia DR2". VizieR Online Data Catalog I/345: I/345. Bibcode2018yCat.1345....0G.  arXiv:0708.1752
  3. 3.0 3.1 3.2 Adhara (Epsilon Canis Majoris, 21 Canis Majoris) Star Facts. universeguide.com. Retrieved on June 27, 2020.
  4. 4.0 4.1 4.2 Jim Kaller (June 6, 2009). Adhara. STARS. stars.asto.illinois.edu. Retrieved on June 27, 2020.
  5. Ken Croswell (February 11, 1995). Electrifying touch of a distant star. newscientist.com. Retrieved on June 27, 2020.
  6. Canis Major Constellation. constellation-guide.com. Retrieved on June 27, 2020.
  7. 7.0 7.1 7.2 ADHARA. star-facts.com (September 25, 2019). Retrieved on June 27, 2020.
  8. 8.0 8.1 Elizabeth Howell (Septermber 27, 2013). Adhara: Brightest Star in Ultraviolet Light. space.com. Retrieved on June 27, 2020.
  9. Adhara. astropixels.comdate=January 31, 2012. Retrieved on June 27, 2020.
  10. 10.0 10.1 Adhara (ε Canis Majoris) Facts. nineplanets.org (January 30, 2020). Retrieved on June 27, 2020.
  11. NASA Technical Reports Server (NTRS) - Epsilon Canis Majoris and the ionization of the local cloud. Epsilon Canis Majoris and the ionization of the local cloud. ntrs.nasa.gov (May 10, 1995). Retrieved on June 27, 2020.
  12. Adhara (Epsilon Canis Majoris). daviddarling.info. Retrieved on June 27, 2020.