Epsilon Canis Majoris

Epsilon Canis Majoris is a binary star system and the second-brightest star in the constellation of Canis Major. Its name is a Bayer designation that is Latinised from ε Canis Majoris, and abbreviated Epsilon CMa or ε CMa. This is the 22nd-brightest star in the night sky with an apparent magnitude of 1.50. About 4.7 million years ago, it was the brightest star in the night sky, with an apparent magnitude of −3.99.<ref name=tomkin98/> Based upon parallax measurements obtained during the Hipparcos mission, it is about 405 light-years distant.<ref name="van Leeuwen 2007"/>

The two components are designated ε Canis Majoris A, officially named Adhara Template:IPAc-en – the traditional name of the system,<ref name=kunitzsch2006/><ref name=IAU-LSN/> and B.<ref name=b/>

Nomenclature

ε Canis Majoris (Latinised to Epsilon Canis Majoris) is the binary system's Bayer designation. The designations of the two components as ε Canis Majoris A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).<ref name=naming/>

ε Canis Majoris bore the traditional name Adhara (sometimes spelled Adara, Adard, Udara or Udra), derived from the Arabic word عذارى 'aðāra', "virgins".<ref name=allen1963/> In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref name=wgsn/> to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire star systems.<ref name=triennial/> It approved the name Adhara for the star ε Canis Majoris A on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.<ref name=IAU-LSN/>

In the 17th-century catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Aoul al Adzari (أول العذاري awwal al-adhara), which was translated into Latin as Prima Virginum, meaning First of the Virgins.<ref name=knobel1895/> Along with δ Canis Majoris (Wezen), η Canis Majoris (Aludra) and ο² Canis Majoris (Thanih al Adzari), these stars were Al ʽAdhārā (العذاري), 'the Virgins'.<ref name=allen1963/><ref>ε CMa as Aoul al Adzari or Prima Virginum (First of the Virgins), Omicron² Canis Majoris as Thanih al Adzari or Secunda Virginum (Second of the Virgins) and Delta Canis Majoris as Thalath al Adzari or Tertia Virginum (Third of the Virgins). Eta Canis Majoris should be Rabah al Adzari or Quarta Virginum (Fourth of the Virgins) consistently, but it was given by the name Aludra, meaning Virgin (same meaning with Adhara or Al ʽAdhārā)</ref>

In Chinese, {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), meaning Bow and Arrow,<ref>弧矢 (Hú Shǐ) is westernized into Koo She. R.H. Allen opined that Koo She refers to the asterism including Delta Velorum and Omega Carinae. AEEA opinion is, Delta Velorum is member of {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), meaning Celestial Earth God's Temple asterism and Omega Carinae is not a member of any asterisms. 天社 (Tiān Shè) is westernized into Tseen She and R. H. Allen used the term Tseen She for Chinese name of η Carinae. See Richard Hinckley Allen: Star Names — Their Lore and Meaning: Argo Navis and Template:In lang AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 17 日 Template:Webarchive.</ref> refers to an asterism consisting of ε Canis Majoris, δ Canis Majoris, η Canis Majoris, κ Canis Majoris, ο Puppis, π Puppis, χ Puppis, c Puppis and k Puppis. Consequently, ε Canis Majoris itself is known as {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx).<ref>Template:In lang AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 17 日 Template:Webarchive</ref>

Physical properties

ε Canis Majoris is a binary star.<ref name=Perryman1997/><ref name=GSM/> The primary, ε Canis Majoris A, has an apparent magnitude of +1.5 and belongs to the spectral classification B2. Its color is blue or blueish-white, due to the surface temperature of Template:Val. It emits a total radiation equal to 38,700 times that of the Sun. This star is the brightest source of extreme ultraviolet in the night sky.<ref name=wilkinson1996/> It is the strongest source of photons capable of ionizing hydrogen atoms in interstellar gas near the Sun, and is very important in determining the ionization state of the Local Interstellar Cloud.<ref name=vallerga1995/> Its rotation period is estimated to be about 5 days.<ref name=Arias2023/>

The exact evolutionary status of Template:Nobr is uncertain. Spectroscopically it has been given the class B2 II,<ref name=Tetzlaff2011/> with the luminosity class of II indicating that is a bright giant, more luminous than a typical giant (luminosity class III). However, it appear less luminous than the expected for this luminosity class, and is more likely of class B2 III-II.<ref name=Shull2025/> Two studies suggest Template:Nobr is still in the late main sequence (TAMS), rather than being a giant.<ref name=Neiner2017/><ref name=Arias2023/> One of these even suggested it could be the final product of a stellar merger.<ref name=Arias2023/>

The +7.5-magnitude (the absolute magnitude amounts to +1.9) companion star, ε Canis Majoris B, is Template:Val away with a position angle of 161° of the main star. Despite the relatively large angular distance the components can only be resolved in large telescopes, since the primary is approximately 250 times brighter than its companion.<ref name=wds/>

A few million years ago, ε Canis Majoris was much closer to the Sun than it is at present, causing it to be a much brighter star in the night sky. About 4.4 million years ago, Adhara was Template:Convert light-years from the Sun, and was the brightest star in the night sky with a magnitude of Template:Val.Template:Efn No other star has attained this brightness since, nor will any other star attain this brightness for at least five million years.<ref name=tomkin98/><ref name=Shull2025/>

In culture

USS Adhara (AK-71) was a U.S. Navy Crater-class cargo ship named after the star.

ε Canis Majoris appears on the national flag of Brazil, symbolising the state of Tocantins.<ref name=flag/>