Stellar Variability
Stellar variability refers to any change in a star’s luminosity over time. These variations can be periodic or irregular, slow or rapid, and reveal fundamental information about the star’s internal physics,star, its interactions with a companion, or the conditions in its atmosphere. Variable stars are at the heart of many advances in astrophysics.
Pulsating variable stars
Cepheids are the most famous variable stars. They pulsate regularly—expanding and contracting—with periods ranging from a few days to a few weeks. Their intrinsic luminosity is directly related to their pulsation period, making them remarkable standard candles for measuring cosmic distances. It was thanks to Cepheids that Hubble measured the distance to Andromeda in 1924.
Eclipsing variables
In eclipsing binary systems, two stars orbit each other in a plane nearly aligned with our line of sight. When one star passes in front of the other, the total brightness of the system decreases periodically. These eclipses make it possible to measure the radii, masses, and temperatures of the two stars with great precision. The method of detecting exoplanets by transit is directly inspired by this principle.
Cataclysmic and Explosive Variables
Some variations are due to explosive phenomena. Novae result from the accretion of matter onto a white dwarf in a binary system, causing a surface thermonuclear explosion. T Tauri stars, which are young and still forming, are very active and exhibit variations due to their intense stellar winds and magnetic activity.
Importance in Astronomy
The study of variable stars has made it possible to measure the distances to galaxies, to probe theinteriors of stars through asteroseismology, and to detect thousands of exoplanets. The Kepler and TESS space missions have continuously observed millions of stars, detecting the slightest variations and revealing a variety of unexpected stellar behaviors.
Concrete example
Betelgeuse experienced a dramatic drop in brightness between late 2019 and early 2020 — the Great Betelgeuse Dimming — which fueled speculation about an imminent supernova. The explanation turned out to be the ejection of a dust cloud by thestar, which obscured part of its light. A perfect example of unexpected stellar variability.
Frequently Asked Questions
Is the Sun a variable star?
Yes, but only slightly. The Sun’s luminosity varies by about 0.1% over an 11-year cycle, linked to sunspot activity. These variations have measurable effects on Earth’s climate, but remain very small compared to true variable stars.
How do we detect a variable star?
Through photometry: by very precisely measuring a star’s brightness at regular intervals and detecting variations. Space telescopes such as Kepler and TESS measure the brightness of millions of stars simultaneously with an accuracy of a few parts per million.
Do all variable stars pulsate?
No. Stellar variability can have many causes: internal pulsations, eclipses by a companion, surface eruptions, transits of planets in front of thestar, star spots, or ejections of matter. Each type of variability reveals different information about the star.