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Demon Star Winks from Afar
By Wil Milan
Special to SPACE.com
posted: 09:11 am ET
07 February 2000
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winking_star_000207 If you go outside this evening and look to the north you'll see the bright constellation Perseus, and within it you'll find the bright star Algol. If you observe that star long enough -- it may take a few hours, and you may have to wait a night or two -- you'll see that it does a curious thing. Over the space of a couple of hours, it will become much dimmer, its brightness rapidly reduced by more than half. If you keep watching, within another couple of hours its brightness will return. It's as if the star gave us a quick wink, a wink that is repeated like clockwork every 69 hours. The strange behavior of Algol can be observed with the naked eye, and apparently was noted even by ancient skywatchers. Many cultures have considered Algol's wink to be an evil or ominous trait, and it's that evil connotation that has given the star its name: "Algol" is Arabic for "demon star." A whirling dance What causes Algol to wink was once one of the great mysteries of astronomy, but in the end it turned out to be quite simple: Algol is not one star, but two: one compact and very bright, the other larger and dimmer. They revolve about each other every 69 hours and, as it happens, their orbit is almost exactly on edge to us. 
The inner binary of Algol as viewed from Earth. Shapes of stars, distorted by gravity, are true to observations. Credit: Lawrence A. Molnar, Calvin CollegeSo every 69 hours the larger, dimmer star moves in front of the brighter star, blocking its light, causing Algol to "wink." As soon as the dim companion moves out of the way, both stars are again fully visible and Algol's brightness is restored. (There's a third member of the Algol system, another star that orbits the inner two, but it is not involved in the eclipsing dance.) The company of stars Algol's winking behavior is unusual, and it is due to the coincidental alignment of the two companions' mutual orbit and our line of sight. But what is not unusual about Algol is that it is a multiple star: It turns out that most stars have one or more companions. Everywhere we look in the sky, lone stars appear to be the minority: - Polaris, the North Star, is not one star but two, a large yellow-white giant orbited by a much smaller blue companion. If you have a home telescope of at least 3 or 4 inches aperture, point it at Polaris and you'll be able to see its companion as a tiny blue dot just outside the glare of the main star.
- Rigel, the brilliant blue-white giant that marks one corner of the constellation Orion, also has a small blue companion. It too can be seen with some home telescopes, but is much harder to pick out because it is so close to the main star that it is often lost in the glare.
- Dazzling blue-white Sirius, traditionally known as the "Dog Star" for its presence in the constellation Canis Major (which means "the large dog"), has a white dwarf companion appropriately nicknamed "the Pup."
- Alpha Centauri, the closest star to Earth (4.3 light-years away) is really a triple star, made up of a close-orbiting pair of stars, which in turn are orbited by a lonely dwarf star much farther out. At its present point in its orbit Alpha Centauri's dwarf companion is the single star in closest proximity to our own, and therefore the dwarf star has been given the name Proxima Centauri.
Artist's vision of the double star Phi Persei shows a small star along with another that is 9 times as massive, surrounded by a disk of gas. Credit: NASA/D. Gies (CHARA, GSU)/W. Pounds The complexity multiplies Some multiple-star systems are quite complex. Epsilon Lyrae, in the constellation Lyra, can be easily seen to be a double star, even with a small telescope. But it's more than that: A larger telescope will show that each of the two stars is in itself a double star, giving it its common nickname as "the double-double star." But for sheer wonderful complexity it's hard to top the multiple-star system that forms the heart of the Orion nebula. Even with a small telescope it's easy to see that the bright "star" that powers the core of the nebula is really four stars. This multiple star is known as the Trapezium for the shape the four companions make. A larger telescope will show that there are two more stars involved and spectroscopic measurements have shown that each of the Trapezium stars is in turn a double star, making at least 10 members altogether. What about our sun? As far as we know, our sun does not seem to have a companion -- or does it? One current theory is that our sun may have a companion that orbits at a great distance, once every 32 million years.As the theory goes, the companion has remained undetected because it is either a very dim dwarf or perhaps a dark star, still traveling its orbit but with its light long ago extinguished. But it still makes its presence known by the damage it does when it approaches the sun every 32 million years. According to the theory, during those times of closer approach the gravitational pull of the dark star (which has been ominously named "Nemesis") jars many comets from their far-flung orbits, causing them to rain down on the inner planets. Stars from dinosaurs By a curious twist, the impetus for believing that our sun may have a dark companion does not arise from any astronomical evidence, but from the study of dinosaurs. In the fossil record there seems to be evidence of mass extinctions of species at intervals that are multiples of 32 million years, the last one being the catastrophic event that wiped out the dinosaurs 65 million years ago. Based on this observation, some have theorized that the sun has an invisible companion that can trigger such catastrophes each time it makes a close approach. This is obviously very flimsy evidence, and some would say it's not evidence at all but pure conjecture. It could be that extinctions are random events and our sun is just another of the many stars that have no companions, but instead travel through the cosmos alone. Though most stars appear to have companions, lone stars are certainly not uncommon. But the search continues for the mysterious Nemesis, and new instruments that can better detect the heat and dim glow of such a dwarf star may speed the search. However, considering the trail of destruction that might follow the presence of Nemesis, we should be happy if our sun is a lone star, traveling through the galaxy with only curious humans for companions. Wil Milan is an astrophotographer based in Arizona. Some of his work can be seen at http://www.astrophotographer.com/
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