Tonight – August 28, 2017 – let the moon introduce you to Antares. It’s a red star and the brightest star in the constellation Scorpius the Scorpion. Look first for the moon, and the nearby bright star will be Antares. The other bright starlike object making a triangle with the moon and Antares tonight will be Saturn, 6th planet outward from the sun in our own solar system.
You can distinguish Antares from Saturn by color. Antares, the heart of the Scorpion, glows red while Saturn displays a golden color. If you have difficulty discerning color with the eye alone, try your luck with binoculars.
Any red-looking star that you can see with the unaided eye is either a red giant or red supergiant star. Antares is a red supergiant. This star, which is in the autumn of its years, is expected to explode as a supernova one of these days. No telling when that will be, however. It could happen tomorrow or a million years from now.
Although Antares lies way out there, at some 600 light-years distant, this star easily shines at 1st-magnitude brightness. In order to beam so brightly in our sky, this star must be extremely luminous, that is, intrinsically very brilliant as opposed to merely appearing bright because of a nearer distance.
Antares’ red color indicates a relatively cool surface temperature, and cool stars shine less brilliantly than hot stars of the same size. But Antares is just so big! Its sheer size makes this star more luminous than many stars with higher surface temperatures.
Just how large is this incredible star? It’s not known with absolute certainty, but its radius is thought to be about 3 times the Earth’s distance from the sun (3 astronomical units). That’s about 3/5th the way from the sun to the orbit of Jupiter, the 5th planet outward from the sun. The radius of Antares is the equivalent of approximately 650 solar radii.
Presuming a radius of 650 solar radii and therefore a diameter of 650 solar diameters, that means the surface area of Antares exceeds that of our sun by some 122,500 times (Antares’ surface area = 650 x 650 = 122,500 solar). But Antares’ volume is actually a few hundred million times greater than the sun’s (Antares’ volume = 650 x 650 x 650 = 271,630,000 solar). And just to think that the sun has the volume of 1,300,000 Earths!
Bottom line: Tonight – August 28 – let the moon be your guide to Antares, a red supergiant star whose humongous size is truly difficult to fathom!
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.