A variable star is a star whose brightness changes over time as seen from Earth. Sometimes the star itself is changing, while in other cases its apparent brightness varies because of an external effect, such as another star passing in front of it.
These changes can happen over minutes, hours, days or even years, and they can reveal a surprising amount about a star’s physical properties and evolution. Variable stars include pulsating giants, eclipsing binary systems, violently flaring stars and some of the most useful distance markers in astronomy.
What Is a Variable Star?
Astronomers classify a star as variable when repeated observations show that the amount of light reaching us changes over time. The changes may be extremely small or dramatic enough to be noticeable with the naked eye.
Variable stars do not form a single physical class of star. Instead, variability occurs among many different types of stars and for many different reasons. By measuring a star’s brightness repeatedly, astronomers can plot the results as a light curve, revealing the pattern and timescale of its variations.
Why Do Variable Stars Change in Brightness?
Variable stars change in brightness for two fundamentally different reasons. In some cases, the star itself is changing. These are known as intrinsic variables, and their brightness may rise or fall as the star pulsates, erupts or undergoes other physical changes.
In other cases, the star may remain relatively stable but its brightness changes from our point of view. These are extrinsic variables. An orbiting companion might periodically pass in front of the star and block some of its light, for example, while rotation can bring brighter or darker regions of a star’s surface into and out of view.
This distinction provides the simplest way to understand the enormous variety of variable stars. The next few sections look at the major mechanisms that can make a star appear to brighten and fade.
Pulsating Stars Physically Expand and Contract
Some variable stars change brightness because the star itself repeatedly expands and contracts. As its outer layers move outward and inward, the star’s radius and surface temperature change, causing its luminosity to rise and fall in a recognizable pattern.
These pulsating variables include several important groups. Cepheids and RR Lyrae stars pulsate with particularly regular cycles, while long-period variables such as Mira can undergo much slower and more dramatic changes. The different patterns are closely connected to a star’s mass, temperature and stage of evolution.
Pulsation therefore isn’t simply a star getting brighter and dimmer at random. It reflects real physical changes taking place within the star.
Eclipsing Binary Stars Dim as One Star Passes in Front of Another
Not every variable star is actually changing its own brightness. In an eclipsing binary system, two stars orbit one another in such a way that, from Earth, one periodically passes in front of the other and blocks some of its light.
The result is a repeating drop in the system’s observed brightness. Often there are two different dips during each orbit: a deeper one when the brighter star is eclipsed and a shallower one when the fainter star passes behind its companion. The timing and depth of these eclipses can reveal information about the stars’ sizes, temperatures and orbit.
One of the best-known examples is Algol, which noticeably fades about once every 2.9 days before returning to its usual brightness. Its regular changes make Algol one of the easiest examples of an eclipsing variable to understand — and one that can be followed with the naked eye.
Some Variable Stars Erupt or Flare
Some stars vary because they undergo sudden outbursts that temporarily increase their brightness. These eruptive variables can brighten when powerful flares erupt from their surfaces or when unstable stars eject large amounts of material into space.
The changes can be far less regular than the pulsations or eclipses we’ve already seen. Some outbursts last only minutes or hours, while others can persist much longer, depending on the star and the process involved. Red dwarf flare stars, for example, can suddenly release enormous bursts of energy before fading back toward their normal brightness.
More extreme changes can occur in cataclysmic variable systems, where a white dwarf pulls material from a companion star. As this material accumulates, it can trigger a nova, causing the system to brighten dramatically before gradually fading again.
Different Variable Stars Change Over Very Different Timescales
There is no single timescale on which variable stars brighten and fade. Some complete an entire cycle in only a few hours, while others change over days, months or even years.
RR Lyrae stars, for example, typically pulsate over periods of less than a day, while Cepheid cycles can last from days to weeks. Long-period variables such as Mira, an evolved red giant, change much more slowly, with cycles lasting many months. Sudden events such as stellar flares can occur on far shorter timescales.
These differences are not arbitrary. The rate at which a star varies is closely connected to the physical process causing the change, which is why different classes of variable stars can have such distinctive patterns.
Cepheid Variables Help Astronomers Measure Cosmic Distances
Cepheid variables are pulsating stars with an especially useful property: the time they take to complete each cycle is closely related to their true luminosity. In general, the longer a Cepheid’s pulsation period, the more luminous the star really is.
Astronomers can therefore measure a Cepheid’s period, determine how luminous it should be, and compare that with how bright it appears from Earth. The difference allows them to calculate the star’s distance. Because Cepheids are extremely luminous, they can be detected in other galaxies as well as our own.
This period-luminosity relationship made Cepheids one of astronomy’s most important distance indicators. Observations of Cepheid variables helped establish that other galaxies lie far beyond the Milky Way and continue to provide an important step in measuring distances across the universe.
Variable Stars Reveal How Stars Evolve
Changes in brightness can reveal much more about a star than simply whether it is getting brighter or fainter. By studying the pattern, size and timing of these variations, astronomers can learn about a star’s temperature, size, mass and internal structure.
Variable stars are also found at many different stages of stellar evolution. Pulsations can begin as stars move into particular regions of the H-R diagram, while eruptions and other forms of variability can reveal changes occurring as stars age or interact with companions.
This makes variable stars valuable laboratories for studying stellar evolution. Instead of seeing a star as an unchanging point of light, astronomers can use its changing brightness to investigate physical processes taking place both inside and around it.
You Can Observe Variable Stars Yourself
Variable stars are not only objects for professional observatories. Many are bright enough to follow with binoculars or even the naked eye. By comparing a variable with nearby stars and repeating the observation over time, amateur astronomers can watch its brightness change and build their own light curves.
Good targets include Algol in Perseus, an eclipsing binary that noticeably fades every few days, and Mira in Cetus, whose much slower changes take place over many months. Delta Cephei in Cepheus is another famous example, completing its regular pulsation cycle in just over five days.
Amateur observations can also have genuine scientific value. Organizations such as the American Association of Variable Star Observers collect brightness measurements from observers around the world, helping astronomers build long-term records of how variable stars behave.