Variable star

Lore & Background

Variable stars have been observed since the dawn of human history. The first documented periodic variable was the eclipsing binary Algol. The periodic variable Omicron Ceti, later named Mira, was discovered in the 17th century, followed by Chi Cygni then R Hydrae. By 1786, ten had been documented. An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol, but the validity of this claim has been questioned. Aboriginal Australians are also known to have observed the variability of Betelgeuse and Antares, incorporating these brightness changes into narratives passed down through oral tradition. Pre-telescope observations of novae and supernovae events were recorded by Babylonian, Chinese, and Arab astronomers, among others. Of the astronomers in the telescope era, the first periodic variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by David Fabricius in 1596. This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught. The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669; John Goodricke gave the correct explanation of its variability in 1784. Chi Cygni was identified in 1686 by G. Kirch, then R Hydrae in 1704 by G. D. Maraldi. Eta Aquilae, the first Cepheid variable to be discovered, was spotted by Edward Pigott in 1784. John Goodricke himself discovered Delta Cephei and Beta Lyrae. Since 1850, the number of known variable stars has increased rapidly, especially with photography. In 1885, Harvard College Observatory began a program of repeatedly photographing the entire sky for the purpose of discovering variable stars. Variable stars are generally analysed using photometry, spectrophotometry, spectroscopy, and polarimetry. Measurements of their changes in brightness can be plotted to produce light curves. For regular variables, the period of variation and its amplitude can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to the next.

Reader's Guide

Variable stars have played a crucial role in astronomy, particularly through the period-luminosity relationship of Cepheid variables discovered in 1912. Edwin Hubble used this result in 1924 when he discovered a Cepheid variable in what was then termed the Andromeda Nebula, demonstrating that this nebula was an 'island universe' located well outside the Milky Way galaxy, ending the Great Debate about the nature of spiral nebulae. Variable stars now form several standard methods (or rungs) on the cosmic distance ladder used to determine the scale of the visible universe. The periods of eclipsing binaries allow for a more precise determination of the mass and radii of their component stars, which is especially useful for modelling stellar evolution. In 1930, astrophysicist Cecilia Payne published the book The Stars of High Luminosity, in which she made numerous observations of variable stars, paying particular attention to Cepheid variables; her analyses and observations, carried out with her husband Sergei Gaposchkin, laid the basis for all subsequent work on the subject. The 2008 edition of the General Catalogue of Variable Stars lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 'suspected' variables. Amateur astronomers have long played a significant role in variable star observation, with perhaps the oldest such organization being the Variable Star Section of the British Astronomical Association, founded in 1890.

Did You Know?

• The first documented periodic variable star was the eclipsing binary Algol.
• An ancient Egyptian calendar from about 3,200 years ago may record the discovery of Algol, but the claim has been questioned.
• The discovery of Cepheid variables' period-luminosity relationship in 1912 allowed Edwin Hubble to show that the Andromeda Nebula is a separate galaxy.
• About two-thirds of all variable stars appear to be pulsating.

Classification & the Physics of Variability

Variable stars are defined by their changing apparent brightness as observed from Earth. This variation falls into two broad categories. Intrinsic variables experience genuine shifts in their own luminosity—imagine a star physically swelling and contracting, altering how much energy it radiates. Extrinsic variables, by contrast, remain essentially constant in output; instead, something in their system intermittently blocks the light reaching us, such as an orbiting companion passing in front of the star. The patterns these stars display range from smooth cyclical pulsations to erratic, irregular fluctuations, and even brief transient events. The timescales are staggering: some variations complete in under an hour, while others stretch across multiple years. It is worth noting that variability is not exotic—the Sun itself waxes and wanes by roughly 0.1 percent over its eleven-year magnetic cycle. At the far extreme, a supernova can momentarily outshine every star in its host galaxy combined. As of 2023, astronomers have catalogued 58,200 variable stars, with pulsating types (just under 30,000) and eclipsing binaries (over 10,000) dominating the list.

From Ancient Calendars to the Astronomical Revolution

The story of variable stars stretches back to the earliest human observations of the night sky. An Egyptian calendar dating roughly 3,200 years ago may contain the oldest recorded reference to Algol's eclipsing behavior, though scholars debate the claim's validity. Aboriginal Australian oral traditions independently tracked the brightness shifts of Betelgeuse and Antares, weaving those changes into narratives passed down through generations. Babylonian, Chinese, and Arab astronomers also left records of novae and supernovae long before telescopes existed. The telescope era transformed the field. In 1638, Johannes Holwarda recognized that Omicron Ceti—later christened Mira—pulsed on an eleven-month cycle, a finding that, alongside the supernovae of 1572 and 1604, shattered the Aristotelian belief in an eternally unchanging heavens. Geminiano Montanari described Algol's variability in 1669, and John Goodricke correctly explained its eclipsing mechanism in 1784. By 1786, ten variable stars had been formally documented. The advent of photographic plates after 1850 accelerated discovery dramatically; Harvard College Observatory launched a systematic sky-photography program in 1885 specifically to hunt for new variables.

Cepheids, the Distance Ladder, and Measuring the Cosmos

In 1912, Henrietta Swan Leavitt uncovered a profound relationship: the brighter a Cepheid variable is at maximum, the longer its pulsation period. This period-luminosity link turned Cepheids into natural cosmic rulers. In 1924, Edwin Hubble located a Cepheid within what was then called the Andromeda Nebula. Applying Leavitt's calibration, he calculated a distance placing that object far beyond the Milky Way's boundaries, thereby resolving the so-called Great Debate over whether spiral nebulae were mere gas clouds or entire island universes. Cepheids and other variable types now serve as critical rungs on the cosmic distance ladder, the step-by-step method astronomers use to gauge the scale of the observable universe. Beyond distance work, eclipsing binaries offer a different gift: their orbital periods let researchers pin down the masses and radii of component stars with high precision, data indispensable for building and testing models of stellar evolution. In 1930, Cecilia Payne published The Stars of High Luminosity, devoting substantial attention to Cepheid behavior. Her analyses, conducted alongside her husband Sergei Gaposchkin, established the observational foundation upon which all subsequent variable-star research has been built.

Modern Techniques and the Amateur Community

Today, variable stars are studied through a toolkit of photometry, spectrophotometry, spectroscopy, and polarimetry. Brightness measurements plotted over time yield light curves, where peaks are called maxima and troughs are minima. For regular variables, period and amplitude can be measured with high confidence, though many stars show slow drifts or even period-to-period changes. Spectral and polarization data, combined with the light curve, often reveal the physical mechanism behind a given star's variability. The 2008 edition of the General Catalogue of Variable Stars listed more than 46,000 variables within the Milky Way, roughly 10,000 in other galaxies, and over 10,000 additional suspected cases. Amateur astronomers have long been vital contributors. One of the oldest dedicated groups is the Variable Star Section of the British Astronomical Association, established in 1890. Enthusiasts perform genuine scientific work by visually comparing a variable's brightness against nearby reference stars of known magnitude within the same telescopic field, recording the estimated value and the time of observation to build a visual light curve. This citizen-science tradition ensures that variable-star research remains a collaborative, global endeavor.