The Heart of the Scorpion

At star parties this time of year, I’m often asked about the bright, flickering, red star rising over the southeastern horizon after sunset: “Is that Mars”?

No, not Mars.  It’s the dazzling red supergiant Antares, the brightest star in the constellation Scorpius and one of the brightest stars embedded in the sweeping arc of the Milky Way.  It’s beautiful from the northern hemisphere, and even more dazzling from the south, where it lies almost directly overhead for the next month or two.

The ancient Greeks, too, thought this star resembled Mars, and named it ant-Ares, Greek for “compared to Ares” (Ares was the Greek name for Mars, the god of war).

Like all red supergiants, Antares will soon run out of fuel in its core.  It’s burned hydrogen, then helium, then carbon, and neon, and magnesium, into heavier elements like iron.  But it can go no further: fusing iron into heavier elements creates no energy.  So when the fuel finally runs out, Antares will collapse and detonate as a supernova, the biggest explosion known in the universe.  From our sky, it will shine bright enough to cast shadows in the dead of night for many weeks.

And this could happen anytime, though forecasting supernovae is less exact than forecasting earthquakes.  Astronomers believe Antares could explode in a week from now, perhaps, or in a century, or in a million years.  But whenever it goes, it will be quite a show.  And no, it will pose no danger to Earth.

The comparative size of the star Antares, which the Romans called Cor Scorpii (the Heart of the Scorpion).

Since its days as a hot, blue main sequence star, Antares has swelled to immense size.  Though it’s nearly 600 light years away, the star subtends an angle of 0.04 arc-seconds… large enough for astronomers to measure directly.   Simple trigonometry suggests Antares spans some 820x the radius of our sun.  Were it at the center of our solar system, Antares would engulf the inner planets, including Earth and Mars, and stretch almost to the orbit of Jupiter.  This image above gives some idea of Antares’ size relative to other well-known giant stars, and to our own modest home star.

Antares, which is slightly variable like many end-stage stars, is listed as the 15th or 16th brightest star in the sky at visual wavelengths.  It’s visual magnitude is +0.96. Intrinsically, it’s about 10,000x as bright as our Sun, and some 60,000x brighter if infrared wavelengths are included.

Antares also presents a good challenge for backyard stargazers with a telescope.  The star has a companion, a blue-giant star called Antares B, that’s nearly 3 arc-seconds away from the reddish Antares A, but some 370x fainter.  Seeing the fainter star is not easy… it’s like trying to look for a firefly in the glow of a bright streetlight.

Still, it’s worth a try.  You’ll need a good 8-10″ scope to separate the two stars, and a magnification of perhaps 200x.

Occasionally, when the Moon passes in front of Antares, bright Antares A is blocked for a brief time, making it easy to spot Antares B, even with a good 3″ scope.  Many observers say the fainter star looks greenish compared to its brighter red companion.

Have you seen the pair?  Let us know what it looked like to you…