Observing Project 12F Irregular and Eruptive Variables

Not every variable star behaves on a precise or even semiregular schedule. Sometimes certain stars, giant stars most likely, may suddenly erupt and gain considerable brightness for a time before fading back. Sometimes after such an eruption, the star may actually fade considerably below its normal brightness before slowly recovering.

Gamma Cassiopeiae is an example of such a star. Throughout the recorded history of modern astronomy, Gamma Cassiopeiae maintained a constant brightness of magnitude +2.25. But suddenly in 1937, the star brightened to magnitude +1.6 as it ejected a shell of gas. By 1940, Gamma Cassiopeiae had faded all the way to magnitude +3.0 and did not recover its original luminosity until 1966. What causes a seemingly stable star to suddenly erupt in this way is not known but any large star in the sky can behave in this way.

In July 2000, Delta Scorpii (the middle star in the head of the scorpion) flared from its normal magnitude +2.3 to +1.8 and some estimates of its brightness ranged as high as +1.6 in 2002. Delta Scorpii normally shines with 3,000 times the luminosity of the Sun, but the eruption swelled that value to 5,000 Suns, a 67% increase in luminosity. Delta Scorpii has since been slowly fading back towards normal. How much it will fade is not known and the star will have to be monitored carefully for many years. Both Delta Scorpii and Gamma Cassiopeiae are spectral type B0 stars that have begun to evolve off the main sequence into the helium-fusing stage of their lives.

Just because stars are cool and faint does not mean they cannot be eruptive in nature. The tiny red dwarf Wolf 359, located in Leo the Lion, is an example of a UV Ceti type variable. Such stars are prone to sudden and violent outbursts of activity. Unlike with eruptive giants, which display activity on a long-lasting and global scale, UV Ceti types display a sudden rapid brightening that occurs over seconds and then return to normal brightness over less than thirty minutes. Wolf 359 is not particularly violent like UV Ceti or Proxima Centauri can be, but it is much more easily observed than the other two. Wolf 359 can suddenly brighten from its normal magnitude +11.0 to magnitude +9 and fade back within a few minutes. Another interesting, though unobservable aspect of flare star behavior is that while a star may brighten one or two magnitudes visually, flare stars are at their brightest in the X-ray region of the spectrum. A flare on Wolf 359 may produce X-rays 10,000 times more intense than the most powerful solar flare on our Sun. Most of the red dwarves in the Sun's vicinity are known to be UV Ceti type variables including Proxima Centauri, DX Cancri, Lalande 21185, Ross 154, Ross 248 and Barnard's Star. Despite the fact that all are less than twelve light years from Earth, all require telescopes to see, but they can be worth monitoring.

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