Antares
Published on Aug 14, 2007 at 9:12 pm.
3 Comments.
Filed under stars.
On Saturday night, I gave a public presentation for a star party. Part of that involved pointing out some of the stars and constellations visible. Currently, the brightest thing in the southern sky is the planet Jupiter. You see it in twilight before any of the stars. But, once it gets dark enough, you see a rather bright star below Jupiter (It would be above Jupiter in the northern sky for people living south of the Tropic of Capricorn.). This star is distinctly reddish, and it is known all over the world as Antares. It has a Bayer designation of Alpha Scorpii and a Flamsteed number of 21 Scorpii.
The name “Antares” comes from the Greek meaning “rival” (ant) of “Mars” (Ares). This presumably comes from the fact that this star is about the color of Mars and is about as bright as Mars normally appears. The Chinese refered to Antares as Who Sing, “The Fire Star,” and it supposedly was tied to the success or failure of silkworm crops. To the Romans, Antares was Cor Scorpii, the “Heart of the Scorpion.” It was also the “Scorpion’s Heart” in Arabic: Kalb al ‘Akrab. In Central Asia, it was the “Gravedigger of Caravans.” Apparently, when Antares was rising in the mornings, Caravans were beset with bandits, robbers, and murderers.
With a magnitude of 1.1, Antares is one of the 20 brightest stars in the sky (being 15th or 16th, depending upon how you count the others), and is one of the brightest stars along the ecliptic. Antares, along with Aldebarran, Regulus, and Fomalhaut, is one of the four Royal stars of navigation.Â
Antares is rather bright in the sky, but when you consider that it is also rather distant, about 600 light years distant, you can appreciate that is it is intrisically very bright, shining with the light of about 10,000 Suns. But, the red color is due to Antares being rather cool, about 3500K. In order to shine as brightly as it does and yet be so cool, Antares must be huge. As it turns out, Antares has a diameter some 700 times that of the Sun. That truly is huge. That makes Antares one of the physically largest stars known. The radius of Antares is some 3.25 Astronomical Units.  If it were placed where the Sun is, then the outer edge of Antares would extend all the way past the orbit of Mars.  Yet, with all of that bulk, Antares has perhaps only about 15 to 18 times the mass of the Sun. That gives a density for Antares of about 4×10-6 kg/m3. That’s about 1/200,000 the density of air! That means that most of the star is only slightly more than a vacuum. A star of this sort is a red supergiant.Â
Stars like Antares are at the end of their lives. After a star uses up the hydrogen in its core that it is fusing into helium as an energy source, the core collapses and heats up.  The hydrogen near the core then gets hot enough to fuse hydrogen, and this release of energy farther out from the center of the star pushes outwards on the outer layers of the star. That causes the star to balloon outwards. As the star swells up, the outer layers of the star expand and cool. That is how you get a red giant or red supergiant. Less massive stars will eventually shed their outer layers into space, leaving a planetary nebula and a white dwarf behind. However, Antares is massive enough to explode in a supernova explosion, leaving a neutron star and an expanding cloud of gas. All of the final activity of a star about to go supernova, though, happens near the core and the surface doesn’t show and evidence of that. So, Antares could go supernova any time (in the next few days, the next few decades, or the next few thousands of years).Â
So, when you go out at night and look at Antares, you can think of some of these things.
-Astroprof
Finder chart created using Starry Night Pro
Size comparison chart from Wikimedia
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Astrogeek on August 15, 2007 at 12:44 am: 1
Antares is one of my favorite summertime objects to point out during public programs. Thanks for all of the additional detail. There were some things I didn’t realize (like the density being 1/200,000 of air!)
Kazarelth on August 17, 2007 at 7:31 am: 2
I like that star. The heart of Scorpio.
I practically loved it because of its ‘underdog’ status. Heh… Anti-Ares. the rival of Mars.
jason m on December 4, 2008 at 7:22 am: 3
thanks im doing a project on it and it helped