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| Brightest Stars on Jun 29, 2009

Hadar: Southern pointer star

The southern star Hadar, also called Beta Centauri, is the 11th brightest star visible in Earth’s sky. It is more famous than it might be otherwise for its location on our sky’s dome right next to Alpha Centauri, the closest star system to Earth. Alpha Centauri and Beta Centauri (Hadar) are sometimes called the Southern Pointer Stars. They point to the Southern Cross.

How to see it

The southern luminary, Hadar or Beta Centauri, is the 11th brightest star visible in Earth’s skies. It is more famous than it might be otherwise for its location on our sky’s dome right next to Alpha Centauri, the 4th brightest star and closest star system to Earth.

Alas, neither Alpha Centauri nor Hadar (Beta Centauri) can be seen at all in the continental U.S. except very briefly from South Texas and parts of Florida (as well as Hawaii). Still, Beta Centauri is very prominent as seen in the southern hemisphere. It is bluish in color, with a magnitude of 0.61 . Alpha and Beta Centauri are the southernmost bright stars in the fairly large and sprawling constellation of Centaurus, the Centaur. Both are near the famed Southern Cross, and all are roughly 30 degrees from the South Celestial Pole.

Alpha Centauri and Beta Centauri (Hadar) are sometimes called the Southern Pointer Stars. They point to the Southern Cross. The value is questionable though, since the Southern Cross is distinctive in its own right.

Photo of the stars Hadar and Rigel Kent (Alpha Centauri) to the left of the Southern Cross by EarthSky facebook friend Jv Noriega from Cape Santiago, Philippines. Click here for a larger view. Thank you Jv Noriega!

Meanwhile, for northern observers, there really are no good pointer stars to Alpha Centauri and Hadar. If you live far enough south to see them, you will need to look for them low on the southern horizon at exactly the right time. The right time might come, for example, at roughly 1 a.m. (local Daylight Saving time) in early May. By early July, Hadar culminates (reaches its highest point) to the south) by nightfall. It also can be seen briefly in the predawn spring skies. At any of these times, if you are a northern hemisphere observer at a latitude like that of the southernmost U.S., you might glimpse Hadar making a tiny arc across the southern sky.

History and Myth

Beta Centauri’s proper name – Hadar – apparently derives from Arabic for “ground,” possibly referring to its nearness to the horizon as seen from low latitudes. Richard Hinckley Allen reports that other stars in Centaurus, including Alpha, may also have been referred to with this title. Beta Centauri is also sometimes called Agena (“derived from Latin words for the knee”), obviously referring to the anatomy of its classical depiction.

The Centaur itself was supposed to be the son of the god Chronos and a sea nymph. Uncharacteristically wise and just, this Centaur, often known as Chiron, was a favorite of Apollo and Diana and figures in some minor mythology of its own. Alpha and Beta Centauri share little in the classical mythology, although were often considered together. In Africa they were named for two men who once were lions, and in Australian aboriginal myths they represented two brothers who hunted and killed a large emu called Tchingal. At one time in China they were known, according to Allen, as Ma Wei, the “Horse’s Tail.”

The flag of Australia contains the Southern Cross as well as an addition bright star placed in the approximate position of Beta Centauri. However, the star is intended to represent the Australian “Commonwealth Star” rather than either Alpha or Beta Centauri.

Science

Beta Centauri is a B1III star, which means that it is very hot and very large. The “III” classification means that it is a giant star, rather than a main sequence star like our sun. Giants and supergiants have left the main sequence, and have entered into the terminal stages of star-life. These stars may still have tens of millions of years left, but this is short by comparison to the billions of years a star like the sun spends in its adult life – that is, its life classified as a main sequence star.

By Hipparcos mission data, Beta Centauri is at a distance of about 525 light-years, although this may be subject to some debate. If the Hipparcos data is to be taken, of the top 20 brightest stars in our sky, this star ranks as 4th in both intrinsic brightness and distance. A number of other physical characteristics of Beta Centauri are not entirely clear. Dr. James Kaler places it at a distance of 335 light-years. He also gives it a probable surface temperature of about 22,500 K (about 40,000 F or about 4 times as hot as the sun), and a mass of about 9.5 times that of the sun. With that temperature and mass, Beta Centauri must be about 15,500 as luminous as our sun, with a radius 8 times as great. If it is at the greater distance, those values will increase as well.

To make things more complicated, Beta Centauri is not one star, but three. About a second of arc away the companion star, Beta Centauri B, poses a difficult telescopic sight. Analysis of the light from Beta Centauri B reveals that it is a very close binary star. Thus the star seen as a single point of light to the human eye is in fact triple.

Beta Centauri’s position is RA:14h 03m 49s, dec:-60° 22′ 23″.