Most stars burn with a steady and unvarying flame for the 10 or so billion years of their existence. But a few live out part of their lives by pulsating, changing in their apparent brightness in periods that can vary from minutes to years.
Such stars are called variables, and the autumn skies possess some of the most important and brightest of each class of such stars. About 30,000 variable stars have been recognized out of the 100 billion suns of our galaxy.
Variables can be divided into three classes: long-period, short-period, and eclipsing variables.
The group of pulsating stars with the most members is the long-period variety.
They are often red stars, suggesting that they are old and have burned up a large percentage of the hydrogen fuel that stokes their thermonuclear reaction. Their periods of pulsation often last for months or even years.
The best example of these beating red giants is Mira in the constellation Cetus. Cetus stretches low across the southeastern sky in the early evening right now. The stars of Pisces, right above Cetus in the southeastern sky, point downward right at Mira.
“Mira” means “the Wonderful” in Arabic, and it is a strange star indeed. Red giants like Mira often expand to huge sizes when their lives are coming to an end. In fact, Mira is one of the largest stars that we know of. It is over 300 million miles in diameter. Compare that to the mere 900,000-mile diameter of the sun. If the sun suddenly expanded to Mira’s size, it would swallow up all the inner planets, including Earth and Mars.
Mira pulsates with a period of about 331 days. At its maximum it is almost as bright as Polaris, the North Star, and is easily visible to the naked eye. At its minimum brightness, it takes a telescope or binoculars to see it, and it seems to have completely disappeared from the constellation!
Short-period, or Cepheid, variables get their name from Delta Cephei in the constellation Cepheus. Look for Cepheus high in the northern sky, right above the Little Dipper. It is shaped a bit like a child’s drawing of a house—a five-sided figure with a tall “peak” facing northeast. At the base of the “house” on the southeastern corner is a small triangle of stars. Delta Cephei is the eastern-most star in the triangle.
Delta doesn’t pulsate as dramatically as Mira. Still, Delta is twice as bright at its maximum as at its minimum and never shrinks below naked-eye level. It takes only about 5 1/2 days for it to complete its cycle. Its variability wasn’t discovered until 1784, so it’s pulsations couldn’t have been as obvious as Mira’s, which have been known since ancient times.
Variables like Mira and Delta Cephei pulsate because, as they contract, internal pressures build up inside them. Those pressures push the outer layers of the star outward. At their largest size, they are at their brightest. As they get bigger, they radiate heat energy into space, causing the internal pressures to lessen. They then contract, reaching their faintest point about halfway between their greatest and least size. When they reach their smallest size, the internal pressures build up again, and the cycle begins anew.
Mira and Delta are thus called intrinsic variables because their brightness is a direct result of their internal dynamics.
Another class of variable stars seems to increase and decrease in brightness because another, fainter, star gets in the way. The best example of such eclipsing variables is Algol, said by the ancients to be the head of the snake-haired monster Medusa.
It is found in the constellation Perseus, which is in the northeastern sky to the east of the “W” shape of the constellation Cassiopeia. Perseus looks like an upside down “Y.” And Algol is the white, naked-eye star to be found on the right side of the “Y” shape.
Algol is a binary star—really two stars circling around each other. The main star of Algol is a hot, bright, middle-aged star about three times our sun’s diameter. It’s companion is an older, fainter star, which is about 20 percent bigger than the main star but gives off too little light to be seen from Earth.
About every three days, the fainter star passes in front of the brighter one, and for several hours, Algol seems to get much dimmer in the sky. As the fainter star gets past the brighter one, Algol seems to return to its normal brilliance.
It’s fun – and sometimes quite startling – to watch these stars dim and get brighter over the course of time. Many is the time that I’ve looked over at the constellation Cetus and become momentarily confused. It looks different. There is a star missing. What happened to it?
But then I remember that it is only Mira that is gone, its light reduced to minimum, a beautiful, red star beating slowly like the heart of the universe.
Tom Burns is director of the Perkins Observatory in Delaware.