Antares, alpha Scorpii, is the brightest star in the constellation Scorpius and the 16th brightest star in the sky. Antares is a class M supergiant star. What does that mean? Well, with a radius of about 800 times the Sun; if it were placed in the center of our solar system, its outer surface would lie between the orbits of Mars and Jupiter.

Antares is big...really big!

Antares is approximately 600 light-years distant. It is a type LC "slow irregular variable" star, whose apparent magnitude slowly varies from +0.88 to +1.16. Its absolute magnitude is -5.28 Mv. Its visual luminosity is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the bolometric luminosity equals roughly 65,000 times that of the Sun.

The mass of the star is calculated to be 15 to 18 solar masses, which isn't very massive considering its bloated dimensions, so Antares has a very low average density.

Antares has a hot blue companion star, Antares B, of spectral type B2.5. It is normally difficult to see in small telescopes due to Antares' glare, but it can be picked out from the glare in 6 inch or larger telescopes on good nights. I've seen it in telescopes on still nights and to me it looks green, but this is probably a contrast effect. The orbit is poorly known, with an estimated period of ~878 years.

What Antares and its companion star might look like from a planet orbiting at a safe distance. 
Copyright Don Dixon 1997


Levinson Axelrod said...

Enjoyed reading this article... Very interesting finds.

Eric M. Bram said...

Regarding your comment that Antares' companion looks green but that "is probably a contrast effect," I have some personal experience on that topic, because I am one of the few human beings who have seen Antares' companion star by itself, without the light of Antares.

The occasion was an expedition to the hills of West Virginia one bitter winter in the late 1960s, to view the grazing occultation of Antares. As you know, a lunar occultation of a star occurs when the moon passes in front of the star, and a grazing occultation occurs when just the very edge of the moon grazes the star as seen from the ground. In other words, when the observer happens to be at the northernmost or southernmost limit of the moon's "shadow" (as cast by the light of the star) as that shadow passes across the Earth.

In this case it was the southern limit, which is favored by amateur astronomers because the Moon's southern polar region is rich with mountains, so the star being occulted often appears to blink on and off as the various mountaintops pass in front of it. Amateur astronomers who want to observe this event calculate where the limit will be, and travel large distances to observe the grazing application from that place, with the observers usually stretched along a line laid perpendicular to the limit. The resulting observations can be used to map the "profile" of the Moon's mountains with much greater resolution than can be done with a telescope.

I was on a small hillside outside of a farmhouse in the small West Virginia town we chose as our observing site. It was bitterly cold. As I watched Antares waiting for the "graze" to begin, Antares looked as usual in my six-inch Celestron, bright orange with a brilliant emerald green tinge at one side from the unresolvable companion.

What I did not know, but would soon find out, is that on this particular day (or rather, night) Antares' companion was to the north of its primary, so there would be times when the mountains of the Moon would block the light of Antares, but not of its companion. It is difficult for a terrestrial telescope to split the pair, and even when it can the primary is so close to the companion, and so bright, that light from its diffraction rings can appear to come from the direction of the companion. But at a distance of some 250,000 miles the Moon made an excellent light baffle and ensured that when it was blocking Antares but not the companion, only the light of the companion will get through to my telescope. At the first occultation event I was amazed and delighted to see the companion star sitting all by itself. The color was an indescribable vivid blue, a pale (yes, vivid but pale) turquoise color that I've never seen before or since.

(Continued below)

Eric M. Bram said...


So why does the companion look green when viewed along with Antares? I do not believe it was a "contrast effect" or some low-light optical effect like the Purkinje shift, since the color did not change over time as my eyes "adapted" to lower light levels when Antares disappeared: there was emerald green at one side of Antares, and then all of a sudden there was that incredible blue. And anyway, the overall light levels did not change since there was always a sizable lit portion of the Moon in my field of view, so my eyes were not particularly dark adapted at that time; I was looking with fairly normal vision at a 5.5 magnitude star through a 6 inch telescope--plenty bright enough to see without dark adaptation.

So here's the actual reason, in my opinion, that the companion appears green when observed by us amateur astronomers while viewing Antares: even when a close double star can be resolved, light from the diffraction pattern of each star (multiple diffraction rings, ever decreasing in brightness with distance from the star) extends beyond the Airy disk and onto the other star. The brightness of even the first diffraction disc is much smaller than that of the Airy disk, so this usually makes no difference. But in this case it does, because Antares is more than 60 times brighter than its companion. I believe that the green color of the companion when viewed along with Antares through a telescope is caused by yellow light from the diffraction pattern of the much brighter orange primary mixing with the blue light of the much fainter companion. Remember when you were a kid and mixed yellow and blue paint or crayon? The result is green.

Yes, yes, I know Antares is orange, and orange and blue are complementary colors so the result should be brown. Maybe Antares isn't truly orange, maybe the shorter wavelength yellow light from it is diffracted farther, or maybe there is some other reason having to do with bright light from stars mixing together--I don't know. But I'm pretty sure from my experience that the green color we see is caused by mixing of the light from Antares with that of its companion, rather than an optical contrast or low-light level effect.

Astrofabio said...

there may be planets orbiting Antares?