Variable Stars: Stars That Change Brightness

Most stars appear constant in brightness from night to night, but thousands of stars vary noticeably over periods ranging from hours to years. These variable stars change brightness for different reasons: some physically pulsate, expanding and contracting like cosmic heartbeats; others are eclipsed by companion stars; and some undergo dramatic outbursts. Observing variable stars is one of the few areas where amateur astronomers make genuine scientific contributions, and many can be monitored with nothing more than your eyes. Locate variable stars in the sky using StarGlobe.

Types of Variable Stars

Pulsating Variables

Pulsating variables are stars that physically expand and contract, changing their surface area and temperature, which causes their brightness to change. The most famous type is the Cepheid variable, named after Delta Cephei. Cepheids are supergiant stars that pulsate with extremely regular periods ranging from 1 to 100 days. Their importance to astronomy is enormous: the period of a Cepheid's pulsation is directly related to its true luminosity. By measuring a Cepheid's period and comparing its known luminosity to its apparent brightness, astronomers can calculate its distance. This period-luminosity relationship, discovered by Henrietta Swan Leavitt in 1912, became one of the most important tools for measuring cosmic distances.

Mira variables are red giant and supergiant stars that pulsate over periods of roughly 80 to 1,000 days. Mira itself (Omicron Ceti) varies between about magnitude 2 and magnitude 10 over approximately 332 days, meaning it goes from easily visible to requiring binoculars. RR Lyrae variables pulsate with periods of less than a day and are found in globular clusters and the galactic halo, serving as distance indicators for old stellar populations.

Eclipsing Binaries

Eclipsing binaries are not truly "variable" in the sense of intrinsic change. They are binary star systems oriented so that one star periodically passes in front of the other as seen from Earth, blocking some of its light. The most famous example is Algol (Beta Persei), which drops from magnitude 2.1 to 3.4 every 2.87 days when its fainter companion crosses in front of the brighter star. The eclipse lasts about 10 hours and is easily noticeable to the naked eye.

Beta Lyrae in Lyra is another prominent eclipsing binary that varies between magnitudes 3.3 and 4.4 over a 12.9-day period. Because the two stars in this system are so close that they distort each other's shapes, the light curve is smooth and continuous rather than showing sharp dips.

Eruptive and Cataclysmic Variables

Some variables undergo dramatic outbursts. Novae occur in binary systems where a white dwarf accumulates hydrogen from a companion star until a thermonuclear explosion on the white dwarf's surface causes a sudden brightening of 7 to 16 magnitudes. Recurrent novae, like T Coronae Borealis in Corona Borealis (the Blaze Star), repeat this process every few decades. R Coronae Borealis stars do the opposite: they fade unpredictably when clouds of carbon dust form in their atmospheres.

How to Observe Variable Stars

Monitoring a variable star is straightforward. You estimate its brightness by comparing it to nearby stars of known, constant magnitude. This technique, called visual magnitude estimation, requires only your eyes (for bright variables) or binoculars (for fainter ones). By recording your brightness estimates over weeks and months, you build up a light curve showing how the star's brightness changes over time.

Start with a bright, easy variable. Algol is ideal for beginners because its eclipses are frequent, dramatic, and predictable. You can calculate the predicted minimum times and then watch the star fade and recover over a few hours. Delta Cephei is another good choice, pulsating between magnitudes 3.5 and 4.4 over 5.37 days with clockwork regularity. Understanding star magnitude is helpful for making accurate estimates.

Citizen Science with Variable Stars

The American Association of Variable Star Observers (AAVSO) coordinates a global network of amateur observers who submit brightness estimates to a centralized database. This data is used by professional astronomers for research. Some variable stars require continuous monitoring that professional observatories cannot provide, making amateur contributions scientifically valuable.

Submitting observations is free and the AAVSO provides finder charts, comparison star magnitudes, and instructions for every variable star in its program. Contributing to this effort connects your backyard observations to real astronomical research.

Notable Variable Stars Visible to the Naked Eye

Betelgeuse in Orion is a semi-regular variable that fluctuates between about magnitude 0.0 and 1.6 over periods of roughly 400 days. Its dramatic dimming event in late 2019 and early 2020 attracted worldwide attention and speculation about an imminent supernova, though the cause was likely a combination of pulsation and dust ejection.

Eta Aquilae in Aquila is a Cepheid variable that oscillates between magnitudes 3.5 and 4.4 over 7.2 days. Mira in Cetus varies over about 11 months and can become easily visible to the naked eye at maximum brightness. Chi Cygni is a Mira variable with an extraordinarily large amplitude, varying between about magnitude 3.3 and magnitude 14, one of the largest brightness ranges of any variable star.

Variable Stars and Cosmology

Variable stars have had an outsized impact on our understanding of the universe. Cepheid variables were the key to proving that other galaxies exist beyond the Milky Way. In 1924, Edwin Hubble identified Cepheid variables in the Andromeda Nebula and used the period-luminosity relationship to show that it was far too distant to be part of our galaxy. This established that the Milky Way is just one of countless galaxies, transforming our understanding of the cosmos.

Type Ia supernovae, the thermonuclear explosions of white dwarfs in binary systems, serve as standardizable candles for measuring distances to remote galaxies. Observations of these events in the late 1990s revealed that the expansion of the universe is accelerating, leading to the discovery of dark energy.

Getting Started

Variable star observing requires no special equipment for many targets. Start with Algol and Delta Cephei, both visible on StarGlobe. Practice estimating magnitudes by comparing your target to nearby constant stars. Record your observations with dates and times. As you gain experience, expand to fainter variables using binoculars or a telescope. The sky is full of stars with stories to tell through their changing light, and your observations can contribute to the scientific record.