Adhara: Brightest Star in Ultraviolet Light

Adhara is a bright star in the Northern Hemisphere constellation Canis Major (Great Dog). The constellation is also home to Sirius, the brightest star in the sky, but Adhara is far less known despite its proximity in Earth's sky.

The star — which is about 1.5 apparent magnitude — is actually a binary star, but the light of the primary star is so bright that it overwhelms the companion in small telescopes. Adhara, however, is the brightest source of ultraviolet light when seen from Earth's sky. It is the 19th brightest star in Earth's night sky. [The Brightest Stars in the Sky: A Starry Countdown]

Despite its relative obscurity, astronomer Jim Kaler praised the sight of Adhara in the night sky.

"In fact, Adhara, a class B (B2) bright giant, is quite the magnificent star," he wrote. "Among the hotter of bright stars, Adhara shines with a surface temperature of some 21,900 degrees Kelvin [39,000 Fahrenheit or 22,000 Celsius], which gives it a sparkling bluish cast."

Locating Adhara

Adhara, also known as Epsilon Canis Majoris, is in the constellation Canis Major. It is 570 light-years from Earth. Adhara's location is:

• Right ascension: 6 hours 58 minutes 37.5 seconds
• Declination: -28 degrees 58 minutes 20 seconds

Adhara and its binary

Adhara, sometimes spelled Adara, derives from an Arabic phrase, "Al Adhara," which can be rendered in English as "the Maidens" or "the Virgins," according to Richard Hinckley Allen's "Star Names: Their Lore and Meaning." There does not appear to be much material available on why the star has that name, or who exactly decided to designate it.

Astronomy with a telescope, however, revealed the star as a binary. The brighter part of the pair is so luminous that "if placed at the distance of Sirius (just over 8 light-years), it would shine 15 times brighter than Venus," wrote astronomer David Darling.

The companion is fairly large away from the bright star from the perspective of Earth, but it is about 250 times fainter. This makes it hard to see in anything besides a large telescope, despite its apparent magnitude of 7.5.

The binary stars are about 900 astronomical units or Earth-sun distances apart, and take about 7,500 years to orbit each other, Kaler wrote.

Adhara in ultraviolet

If people could see in ultraviolet light, Adhara would be the brightest star in the sky. When taking that band of light into account, the star is about 22,300 times more luminous than the sun, Kaler added.

Adhara was discovered as the brightest extreme ultraviolet source during the Extreme Ultraviolet Explorer (EUVE) all-sky survey. EUVE was a NASA satellite that launched in 1992 and worked successfully for more than eight years until the agency terminated it for cost and scientific merit reasons.

Adhara was also studied during shuttle mission STS-95 in 1998 using UVSTAR, the UltraViolet Spectrograph Telescope for Astronomical Research.

A 2000 paper in the ASP Conference Series noted that the telescope observed "rapid flux variations" in two ultraviolet bands, with the changes taking place in about a minute — very rapidly given most take place in hours or days. At the time, the astronomers said they had no explanation for the phenomenon, but they noted that late O and early B stars (the class that Adhara belongs to) sometimes exhibit instabilities.

Studying Adhara in ultraviolet allows astronomers to learn more about its stellar wind and atmosphere in general, the astronomers added.