Jimmy Westlake: My Favorite Stars: La Superba
March 12, 2012
Note: Throughout the next year, I will share with you in this column some of my favorite stars — stars that you, too, can observe and perhaps add to your list of favorites. They are presented in no particular order other than when they are best observed.
Have you ever seen a red star? No, I mean a really red star. Tucked in under the handle of the Big Dipper is one of the reddest stars in the sky, named La Superba.
With a name like that, it had better be pretty impressive, and it is. La Superba is the brightest carbon star in the sky. With a surface temperature of only 3,500 degrees F, it also is one of the "coolest" naked eye stars and, for a star, cool means red.
La Superba is classified as a carbon-rich giant that is nearing the end of its life. Most stars do not contain large amounts of carbon in their outer layers, but stars like La Superba are well advanced in age and have fused much of their original hydrogen into helium and then into carbon in their superhot cores. When convection dredges this carbon up to the star's surface, it cools and forms simple carbon molecules like cyanogen and carbon monoxide. These molecules strongly absorb the blue and violet light from the rainbow of colors emitted by the star, making it appear redder than its temperature would indicate. When the 19th century astronomer Father Angelo Secchi observed the vivid red color of this star, it so impressed him that he dubbed it La Superba.
La Superba also is known by its variable star designation Y Canum Venaticorum, or Y CVn for short, and is classified as a semi-regular variable star. Throughout the course of about 160 days, Y CVn varies in brightness from just above naked eye visibility to just below naked eye visibility. Even when the star is at its brightest, your eye will not perceive much color. You'll need to use a regular pair of binoculars or a small telescope to gather enough light to make the vivid red color jump out at you.
La Superba is located about 700 light-years from our solar system. It is so large that if you plucked the sun from the center of the solar system and replaced it with La Superba, the giant red star would swallow the orbits of Mercury, Venus, Earth and maybe Mars. It pumps out more total energy than 4,400 stars like the sun.
Giant red stars like La Superba are in the final stages of their lives and might provide us with a glimpse of what will happen to the sun when it runs low on fuel. La Superba already is spewing large amounts of its mass into space, but in the not-to-distant future, the outer layers of La Superba will be blown away from its hot carbon core, producing a spectacular planetary nebula. The colorful glowing gases will expand away from the central star, which is on its way to becoming a white dwarf — the super-compressed remains of the dead star.
To locate La Superba in the sky, go outside at about 9 p.m. in mid-March and find the Big Dipper standing on its handle in the northeastern sky. If you imagine the curve of the Big Dipper's handle to be a portion of a circle in the sky, La Superba will be near the very center of this circle, not far from the medium bright star Cor Caroli in the constellation of Canes Venatici, the Hunting Dogs. Aim your binoculars at that spot, and if La Superba is in your field of view, its striking red color probably will make you say, "Wow!"
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out Westlake's astrophotography website at http://www.jwestlake.com.