Have you ever heard of a star named after one of the planets? Well there actually is one! And it, along with its namesake, is currently visible in the evening sky after sunset. Both objects are unmistakably bright and will jump out at you facing south as the twilight fades.
The star we’re referring to marks the heart of the sprawling summer constellation Scorpius. It is radiant first-magnitude Antares, whose name comes from the Greek for “Rival of Mars” because its reddish-orange color closely mimics that of the planet Mars itself. Compare the two for yourself and see if you agree. But these hues come from two radically different causes. Mars is ruddy orange because its surface is largely oxidized or “rusted.” Antares on the other hand is a huge red super-giant sun and relatively cool compared to stars like our own Sun and blue-white ones. (Star color is primarily an indication of temperature—not chemical composition as many believe.) Antares is, in fact, one of only two red super-giants among the first-magnitude stars, the other being Betelgeuse in the winter constellation Orion. And it is truly a giant—if placed where the Sun is, it would extend out to the orbit of Mars, engulfing all of the terrestrial planets including Mars itself.
While the ancients didn’t know anything about the physical nature of these ruddy lights in their night sky, they did notice two differences between them. Most obvious is that Antares flickers (which goes well with it fiery appearance!) while Mars shines with a steady light. Also, Antares remains fixed in relation to all the other stars while Mars slowly moves around the sky. As we know today, the reason for the latter motion is that Mars is a planet orbiting our home star while Antares is simply one of the hosts of other stars. As to the flickering of Antares, like all stars it’s so distant that it appears essentially as a point source to the unaided eye. Its single ray of incoming light is, therefore, easily bounced about by atmospheric turbulence causing it to scintillate or twinkle. Mars on the other hand like the other planets displays a minute disk composed of a bundle of rays, making it largely immune to turbulence. Thus the old adage: “stars twinkle, planets don’t.”
We should point out that when looking at these two vastly different orbs, notice a bright yellowish “star” currently positioned about midway between them. This is the magnificent ringed planet Saturn. If you have a telescope—any telescope (magnifying about 25x or more) — be sure to gaze upon this supernal wonder of the heavens!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.