What Is A Variable Star?

On August 3, 1596, German astronomer David Fabricius need a reference star while observing the planet Mercury and looked to Omicron Ceti, an unremarkable third-magnitude star nearby. Several weeks later, Fabricius noticed that the star had increased in brightness by one magnitude, and then as he continued his observations into October, the star disappeared entirely from his view. But when Fabricius looked in the sky again on February 16, 1609, the star was back in its place. Omicron Ceti soon received a new name, “Mira,” meaning “astonishing” in Latin.

Fabricius had discovered one of the first-known variable stars, and today there are more than 200,000 known variables, including tens of thousands in the Milky Way alone. Variable stars are stars that change in brightness or in some other characteristic, such as the spectrum in which they shine, over time.

There are many reasons why a star may change over time: Internal forces cause some stars to swell and shrink. Other stars will appear to dim when they are eclipsed by a fainter companion and brighten as that companion moves past. Mira variables, named after Fabricius’s star, are red giants in the late stages of their evolution, now expanding and contracting, but in a few million years, they’ll have become white dwarfs.

Cepheid variable star V1 (Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA))

Cepheid variables, another type of pulsating star, have proven to be more than just stellar oddities–astronomers have been able to use these stars to make monumental discoveries about our universe. Henrietta Swan Leavitt, one of the “Harvard computers” at the turn of the last century, showed that there was a relationship between a Cepheid variable star’s brightness and the time it took for that star to cycle through its brightening-dimming pattern. This discovery led astronomers to develop a method for determining the distance of a Cepheid variable based on its brightness. With that yardstick, Edwin Hubble was able to show that two stars (including the one in the photo above) were too far away to be in the Milky Way and thus proved that our galaxy was not the only one in the universe. Hubble also used Cepheid variables to show that the universe is expanding.

Backyard astronomers can observe and even discover variable stars–it just takes a bit of knowledge and a lot of patience. If someone thinks they’ve found one, the American Association of Variable Star Observers recommends first checking the International Variable Star Index (VSX) as well as other star catalogs to see if it is already known. If it’s not in one of the indices, the discoverer should double check their data and then they can submit it to the VSX. There are likely millions of variable stars in the sky, so there’s plenty of opportunity for future discovery.