Variable Stars

Variable stars are stars whose brightness changes over time. The variations can be regular and predictable, or irregular and chaotic. Some change by barely perceptible amounts, while others vary by factors of thousands. Understanding why stars vary has been crucial to modern astronomy, revealing stellar structure, measuring cosmic distances, and discovering exotic objects.

Variable stars fall into two main categories: intrinsic variables and extrinsic variables. Intrinsic variables change brightness because something physical is happening to the star itself - it’s pulsating, erupting, or undergoing internal changes. Extrinsic variables change brightness because of external factors like eclipses or rotation.

Pulsating variables are stars that physically expand and contract. Cepheid variables are the most famous type. These are supergiant stars that pulsate with periods ranging from 1 to 100 days. Delta Cephei, the prototype, varies between magnitudes 3.5 and 4.4 with a period of 5.37 days. The pulsations are driven by a valve mechanism in the star’s outer layers where helium alternately ionizes and recombines, causing the star to expand and contract rhythmically.

Cepheids are crucial to astronomy because they’re standard candles. Henrietta Swan Leavitt discovered in 1912 that a Cepheid’s pulsation period is directly related to its intrinsic brightness - longer period means brighter star. By measuring the period and apparent brightness, astronomers can calculate the distance. This period-luminosity relationship allowed Edwin Hubble to prove that spiral nebulae were actually distant galaxies outside the Milky Way.

RR Lyrae variables are another type of pulsating variable, similar to Cepheids but smaller and less luminous. They have shorter periods, typically less than a day, and are commonly found in globular clusters. They’re also standard candles, useful for measuring distances within our galaxy.

Mira variables are long-period pulsating red giants. Mira itself, Omicron Ceti, varies from magnitude 2 to magnitude 10 with a period of about 332 days. At maximum, it’s easily visible to the naked eye. At minimum, it requires a telescope. These stars are cool red giants losing mass through stellar winds, shedding their outer layers into space.

Eclipsing binaries are extrinsic variables. The brightness changes because one star periodically passes in front of the other from our viewpoint. Algol in Perseus is the prototype - the “Demon Star” that dims noticeably every 2.87 days when a fainter companion eclipses the brighter primary. Ancient astronomers noticed Algol’s variations thousands of years ago.

Cataclysmic variables are binary systems where a white dwarf accretes material from a companion star. The material spirals onto the white dwarf through an accretion disk, occasionally producing outbursts. Novae are dramatic examples - the accumulated material on the white dwarf’s surface suddenly ignites in a thermonuclear explosion, brightening the star by 10 magnitudes or more in days or hours.

BTW supernovae are the ultimate variable stars. Type Ia supernovae occur when a white dwarf in a binary system accretes enough mass to exceed the Chandrasekhar limit of 1.4 solar masses and explodes completely. These explosions are remarkably uniform in brightness, making them standard candles for measuring cosmic distances. They’re how astronomers discovered that the universe’s expansion is accelerating.

One thing is certain: variable star observation is one area where amateur astronomers make real scientific contributions. Professional telescopes can’t monitor thousands of variable stars continuously. Amateur observers submit millions of observations annually to organizations like the American Association of Variable Star Observers. This data helps track long-term behavior and catch unexpected outbursts.