From the Southworth Planetarium
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THE DAILY ASTRONOMER
July 14, 2010
The Martian Rival
Some people study the stars intensely, but care nothing for astronomy. These inquisitive folks are more interested in a star’s name than in its physical characteristics. One can well understand the fixation, for star names are as fascinating as they are complicated. Various cultures have assigned names to the night sky’s brightest stars. (Many of these names are, lamentably, lost to history.) Of the more than five naked-eye stars fewer than four hundred still have recorded proper names. The International Astronomical Union is the organization that assigned these star names, most of which derive from Arabic, Greek or Roman sources.
Almost all star names have stories behind them: either in their mythological association, or in how they relate to a certain aspect of the star, itself.
A latter case is in Antares, a red supergiant star that marks Scorpius the Scorpion’s heart. (One can find Scorpius in the southern evening sky, tonight.)
The name “Antares,” means “Rival to Mars.” “Ares” is the Greek name for Mars, the Roman War God. Antares is considered the Martian rival because their colors are both ruddy red. Even though the hues are similar, the reasons why the star and planet have these shades is not. The Martian surface is covered by a thin layer of oxidized iron (rust). This material appears reddish and lends the planet its color. Antares is red because it is a relatively cool star with an effective temperature of 2,300 degrees. While furiously hot by planetary standards, a 2,300 “surface” temperature is chilly for a star. Red stars are the coolest stars, and blue-white stars the hottest.
What is more remarkable about Antares is that we can observe this color at all! Even when viewed telescopically, most stars don’t reveal distinct shades. As they are so far away, they seem to be merely whitish, for our eyes are not developed to detect color at low light levels. For a star to appear colored, it would have to be quite close or large. At a distance of 440 light years, Antares is not particularly close, but it is huge, with a volume more than 300 million times larger than the Sun’s! (Remember: 1.2 million Earth could easily fit inside the Sun, so one can well imagine -or, actually, can’t imagine-the size and scope of the Scorpion’s heart star.)
Like Orion’s star Betelgeuse, which is 160 million times larger than the Sun in volume, Antares is so big that we can actually detect its reddish coloring.
Some consider Antares the Summer Red Supergiant and Betelgeuse the Winter Red Supergiant. Indeed, these stars do look similar. Why, then, should Antares be the Martian rival instead of Betelgeuse? The answer to this question could perhaps be in the position. Antares is closer to the ecliptic than Betelgeuse. After all, Scorpius is an ecliptic constellation. Orion isn’t. The ecliptic is the Sun’s annual path through the sky. All of the planets travel within a band centered on it. Therefore, Mars will venture closer to Antares than it ever will to Betelgeuse. When in close proximity, Mars and Antares seem almost like twin stars. They are indeed vastly different in terms of size, structure and distance from us. Yet, we discovered those details many years after the ancients named Antares and made it Mars’ rival star.
