Variable Stars: What Are They?

variable star, v838 monocerotis
Variable star V838 Monocerotis lies near the edge of the Milky Way Galaxy, about 20,000 light-years from our sun. Still, ever since a sudden outburst was detected in January 2002, this enigmatic star has taken the center of an astronomical stage. As astronomers watch, light from the outburst echoes across pre-existing dust shells around V838 Mon, progressively illuminating ever more distant regions. NASA

There are many types of stars in the universe, ranging from those like our Sun to the white dwarfs and the red supergiants and blue supergiants. Many "classifications" of stars exist beyond size and temperature, however.

You've probably heard the term "variable star" before — it's used to describe a star that has pulsations in its brightness or in its spectrum. Sometimes the changes are fairly fast and can be noted by observers over the course of a few nights.

Other times, the variations are much slower. To measure spectral variations, astronomers need to look at the stars with special instruments called spectroscopes. These instruments detect minute changes that the human eye would never see. There are more than 46,000 known variable stars in our own Milky Way Galaxy, and astronomers have observed thousands in other nearby galaxies.

Most stars are variable, even our Sun. Its luminosity is fairly small and takes place over an 11-year period. Other stars, such as the reddish Algol (in the constellation Perseus) vary more quickly. Algol's brightness changes every few nights. That and its color earned it the nickname "Demon Star" from stargazers in ancient times.

What Happens in a Variable Star?

Many stars vary because their sizes change. These are called "intrinsic variables" because their changes in brightness are caused by changes in the physical properties of the stars themselves.

They can swell up over a period of time and then shrink. This affects the amount of light they emit.

What causes a star to swell and shrink? It begins in the core, where nuclear fusion takes place. As the energy from the core travels out through the star, it encounters differences in density or temperature in the outer layers of the star.

Sometimes the energy is blocked, which causes the star to grow hotter. That usually expands the star until the heat is released. Then, the material in a layer cools and the star subsides a bit. As it collects again, the star heats up again, and the cycle repeats itself.

Other changes in stars include eruptions, which are usually flares or mass ejections. These are often referred to as flare stars. These activities cause sudden, rapid changes in brightness. The most radical changes in brightness happen when a star explodes outwards, such as in a supernova. A nova can also be a cataclysmic variable when it periodically flares up due to an accumulation of material from a nearby companion.

Other stars are sometimes blocked by something. These are called extrinsic variables. Eclipsing binaries cause changes in a star's brightness as they rotate around each other. From our point of view, it seems like one star gets dimmer for a short period of time. Sometimes an orbiting planet will do the same thing, but the shift in brightness is very tiny. The period (the timing of each dimming and brightening) matches the orbital period of whatever is blocking the light. Another type of extrinsic variable happens when a star with large spots rotates and the region with the spot is facing us.

The star then appears a tiny bit less bright until the spot rotates away.

Types of Variable Stars

Astronomers have classified many different types of variables, usually named after the stars or regions where the first ones of the types were discovered. So, for example, the Cepheid variables are named after the pulsating star Delta Cephei. There are several sub-types of Cepheids, too. Cepheids were used by Henrietta Leavitt when she discovered the relationship between the pulsations of brightness in these stars and their distances. It was also a fundamental discovery in astronomy. Edwin Hubble used her work when he first discovered a variable star in the Andromeda Galaxy. From her calculations, he was able to determine it lay outside our own Milky Way.

Other types of variables include RR Lyrae variables, which are older, lower-mass stars often found in glbular clusters.

They are also used in the period-luminosity distance determinations. The Mira variables are long-period red giant stars that are very evolved. Orion variables are hot young stellar objects that haven't yet "turned on" their nuclear furnaces. They are almost like fussy babies, acting out at irregular times. Other protostar types can also be variables going through the period of contraction that all stars do as their born. These are outbursty variables.

The most massive and active variables (outside of cataclysmic ones) are the luminous blue variables (LBV) and the Wolf-Rayet (W-R) variables. The LBVs are the brightest variable stars known and are losing incredible amounts of mass sometimes in clumps years or centuries apart. The best-known example is the star Eta Carinae in the southern hemisphere sky. W-Rs are also massive stars that are very hot. They may be binaries interacting, or have heated material rotating around them.

In all, there are nearly 60 different types of variable stars, and each one is being heavily studied so that astronomers can understand more about what makes them "tick".

Who Observes Variables

There is a whole subdiscipline in astronomy that focuses on variable stars, and both professional and amateur observers are involved in charting these stars. The American Association of Variable Star Observers (AAVSO.org) has thousands of members who carefully track these objects. Their work is used heavily by professionals who then "zero in" on specific aspects of a star's structure and activity. All of these studies are helping to explain why stars flicker and brighten throughout their lives.

Variable Stars Cultural References

Variable stars have long been known to observers, even since ancient times. It wasn't hard for stargazers to see that some stars varied over short (or long) periods of time. The big problem for ancient astronomers (who often were also astrologers) was how to interpret them. These stars were sometimes feared or given an ominous meaning.

All that changed as astronomers began to understand these objects. Today, the focus is on the events and processes inside of stars.

In popular culture, the most obvious use of the term outside of astronomy lately is in science fiction. While all kinds of stars show up in science fiction, variable stars make their appearances This particularly true of flare stars or supergiants about to explode. For example, at least one Star Trek episode, the crew of the Enterprise had to deal with the consequences of a flare star and the danger it posed to people living on a nearby planet. In another, a flare star threatens the existence of the ship itself.

The best-known use of variable star in recent times was the book Variable Star by Spider Robinson and the late Robert A. Heinlein. In it, a character goes through changes in his life as he decides to head to space to escape a romance that wasn't quite working out. Another book that focused more directly on actual variable stars was Mike Brotherton's Star Dragon, which described the variable SS Cygni (in the constellation Cygnus) as part of the tale.