A few centuries ago, theory could not account for strange stars that brightened, dimmed, then brightened again. Today, astronomers know what's behind these variable stars, and two of them are worth looking for right now.

Variable stars can appear to brighten and fade at both regular and irregular intervals. They fall into two basic categories.

There are eclipsing stars, in which one star crosses partially or completely in front of another causing an apparent dimming of starlight as seen from Earth. A fine example of an eclipsing binary is the star Algol, in the constellation of Perseus, which we'll address in detail in a separate Spacewatch column later this fall.

The vast majority of these stars are periodic or almost periodic in their variability, but many can change unpredictably, often for reasons we do not fully understand.

People have been monitoring Mira's changes for centuries. In 1596, David Fabricus (1564-1617), an amateur astronomer and native of Friesland, The Netherlands, saw a third magnitude star in the constellation of Cetus, the Whale. As the intruder faded in the following days and weeks, it was logical to suppose that it was a nova.

Then, Johann Fokkens Holwarda (1618-1651), also of Friesland, watched this ruddy star brighten and grow dim again over an 11-month interval during 1638. While a nova would not be expected to reappear, this one apparently was flashing off and on again. Its variable brightness contradicted the Aristotelian dogma of the times, that the heavens were both perfect and constant.

As astronomers became aware of the unusual fluctuations, they honored the star with the name Mira, the "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 to obscurity again. At its faintest, Mira is about fifteen times dimmer than the faintest star that you can see without a telescope. At maximum it usually reaches third magnitude, about 250 times brighter than at its lowest emission level.

Mira's pulsations are thought to involve at least its outer layers. Red giants like Mira are old stars, nearly out of hydrogen and about to enter a new helium-burning phase of their lives. Mira's pulsing may be directly related to the onset of the helium reaction, but this is a highly controversial question.

Mira also has a companion, detected visually 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 pretty powerful telescopes.

Currently, Mira comes above the eastern horizon shortly before midnight local daylight time. Although now nearly a month past maximum brightness, it still should be luminous enough to be visible to the unaided eye. Try looking for it the first clear night this week.

Sometimes a star can suddenly brighten unexpectedly and remain unusually bright for an undetermined length of time.

Now, some two years after its initial outburst, Dschubba has become slightly brighter, currently hovering near magnitude 1.6.

If Dschubba is following the pattern of another similarly performing variable star, Gamma Cassiopeiae, it may have an interesting future ahead of it. Gamma Cass also brightened to magnitude 1.6 from magnitude 2.3 in April 1937 and remained at that peak for many months. It then dwindled to magnitude 3.0 over the following three years, fainter than had ever been noted. Then, after three decades, it finally got back to its normal brightness of magnitude 2.3.

What's behind these strange flarings? Dschubba and Gamma Cass are rapidly rotating stars, occasionally flinging hot masses of gas out into space from their equators.