A few centuries ago, the current astronomical theories could not account for the strange stars in the sky that brightened, dimmed, and then brightened again. Today, astronomers know what's behind these variable stars, and they're well worth looking for. Variable stars fall into two basic categories: eclipsing stars and intrinsic variables.
Eclipsing stars occur in a binary system where one star crosses partially or completely in front of another, causing an apparent dimming of the starlight as seen from Earth. A fine example of an eclipsing binary is the star Algol, in the constellation of Perseus.
Intrinsic variables are stars whose light fluctuations are inherent in the structure of the star. These stars can change physically in terms of color, spectrum, effective temperature, and heat output. In addition, the radial velocity of such stars as they travel through space can also change as a result of changes in the star's rate of convection, expansion, and contraction.
The vast majority of variables stars are periodic or almost periodic in their variability, but many can change without warning, often for reasons we do not fully understand.
A Wonderful Star
The first intrinsic variable star to be discovered was the long-period variable named Mira.
In 1596, David Fabricus, an amateur astronomer and native of Friesland in the Netherlands, saw a new third magnitude star in the constellation of Cetus. As the intruder faded in the days and weeks that followed, it was logical to suppose that it was a nova.
Then, in 1638, Johann Fokkens Holwarda, also of Friesland, watched this ruddy star brighten and grow dim again over an eleven-month period. While a nova would not be expected to reappear, this one was apparently flashing off and on again, which contradicted the still current Aristotelian dogma that asserted the heavens were both perfect and constant.
As astronomers became aware of these unusual fluctuations, they honored the star with the name Stella Mira, Latin for wonderful star.
Mira grows brighter, then fainter, then brighter again in regular, predictable cycles of approximately 332 days. It rises to its greatest splendor twice as fast as it fades again to obscurity. At its dimmest, Mira is about fifteen times dimmer than the faintest star you can see without a telescope; while at its maximum brightness, it usually reaches third magnitude—250 times brighter that its dim alter ego.
Red giants like Mira are old stars, nearly out of hydrogen and about to enter the new, helium-burning phase of their lives. Mira's pulsing may be directly related to the onset of the helium reaction and is thought to involve at least the outer layers of the star; but the specifics of the mechanism are still in doubt.
Mira has a companion, first discovered in 1923, that also varies in brightness, but it has nothing to do with Mira's variations. The companion is never bright enough to see without very powerful telescopes.
Strange Eruptions
Sometimes a star can suddenly brighten unexpectedly and remain unusually bright for an indeterminate length of time.
In July of 2000, the familiar head of Scorpius took on a new look as its middle star, Dschubba, suddenly and unexpectedly brightened to a magnitude of 1.9 from its usual magnitude of 2.3. The change was enough to make Dschubba quite plainly the brightest star in the row of Graffias, Dschubba, and Pi Scorpii, sometimes referred to as the Crown of the Scorpion.
Now, some years after its initial outburst, Dschubba has become even brighter, and currently hovers near a magnitude of 1.6.
If Dschubba is following the pattern of Gamma Cassiopeiae—another variable star with a similar record—it may have an interesting future ahead. In April 1937, Gamma Cassiopeiae brightened to a magnitude of 1.6 from magnitude of 2.3, and remained at that peak for many months. It then dwindled to a magnitude of 3.0 over the next three years, fainter than had ever been noted, and then after three decades, finally returned to its normal magnitude of 2.3.
What's behind these strange eruptions? It turns out that Dschubba and Gamma Cassiopeiae are both rapidly rotating stars that occasionally fling matter into space from their equators.