Algieba
Published on Feb 26, 2008 at 11:06 pm.
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Filed under stars.
During the lunar eclipse last week, Saturn was just below the Moon, and the star Regulus was just above. A number of my students, though, asked “What’s that other star up off to the left?” I told them that the star’s name was Algieba, and that it was part of Leo, the Lion. In particular, Algieba is the second brightest star in the asterism at the front of Leo that is often called the Sickle, because of it’s backwards question mark shape. In most representations of Leo, the Lion, the upper part of the sickle is the head of the lion, and Algieba, Regulus, and Zosma make up the lion’s body. Denebola is at the end of the lion’s tail (in fact, Denebola is a butchering of the words meaning “the tail of the lion”). The name Algieba means “forehead,” but that doesn’t really make sense given the position of the star in typical representations of the lion. John Flamsteed and several other astronomers of the 17th and 18th Centuries called the star Juba, meaning “mane,” which better reflects the star’s position in the constellation. Richard Allen, in his book about star names, suggests that perhaps that may have been its original name in Latin, but that it somehow got mistranslated or misunderstood when transcribed into Arabic. At any rate, the name Algieba is today pretty well acknowledged as being this star’s typical name.
Besides being one of the brighter stars in Leo, I remember Algieba because it is a typical target that I give my students to look at a binary star. Through the telescope, Algieba is a close binary compared with many of the others that I have the introductory students look for. It is only about 4.4 arcseconds separation. That is well within the resolution of all of the telescopes that we use, but it sometimes means using a higher power eyepiece than the ones that the students use to first find the star. Through the telescope, Algieba has always looked like a pair of yellowish stars, with the dimmer one being slightly more yellow. However, the stars are spectral type K0 and G7, which really makes them more of an orange color, or at least yellow-orange. The K0 star is magnitude 2.28, and the G7 star is magnitude 3.51. The combined magnitude of both stars is 1.98. Algieba’s two stars are nearly 170 AU apart. That is over 5.75 times farther apart than Neptune is from the Sun. In fact, these two stars are just under one light day apart! At that distance, they take nearly 620 years to orbit one another.
Both stars are giants, meaning that they have exhausted the hydrogen that they had been fusing in their cores. They have evolved off of the Main Sequence and are dying. We really don’t know how long ago they left the main sequence, so we don’t know at what stage they are in their death throes. Most likely, they have left the main sequence but not yet begun to fuse helium in their core. Both of these stars have more mass than the Sun, and they likely have Main Sequence lifetimes of between 2 and 4 billion years. So, they are at least that old. However, they have only 30% of the metallicity of the Sun. Normally, that would suggest that they would have formed some time before the Sun. However, the Sun is about 5 billion years old. So, that means that the stars of Algieba must have formed from gas that was less enriched with metals than that which formed the Sun. That means that they must have formed in some other part of the galaxy. Algieba had a very large motion in space, moving at about 68 km/s relative to the Sun. That, too, suggests that Algieba is an interloper in this part of the galaxy.
-Astroprof
Star chart created using Starry Night Pro 6
