What Is a Variable Star? Stars That Change and What the Changes Reveal

Variation has two broad causes and distinguishing them is the first step in any analysis. Intrinsic variation means the star itself changes output, which happens when it pulsates, expanding and contracting in a cycle driven by a layer in its interior that alternately traps and releases radiation, or when it erupts, or when its surface is unevenly covered with spots and it rotates. Extrinsic variation means the output is constant and something blocks or adds to the light reaching the observer, which happens when a companion star passes in front in an eclipsing system, or when a planet transits, or when an intervening mass bends light. The shape of the brightness curve generally distinguishes these, since pulsation, eclipse and eruption produce characteristically different profiles.

Several classes are important enough to have shaped astronomy:

• •Cepheid variables, pulsating with a period tightly related to their intrinsic brightness, which makes them distance indicators
• •RR Lyrae stars, pulsating faster and fainter, used for distances within and near our galaxy
• •Mira variables, red giants pulsating over many months with enormous changes in brightness
• •Eclipsing binaries, where two stars pass in front of each other, giving masses and sizes directly
• •Cataclysmic variables, where material transferred between a close pair produces outbursts
• •Supernovae, which are the extreme case and which a particular type of serves as a distance indicator

This is the area of astronomy where observers without professional equipment contribute most substantially. Monitoring a star's brightness over months and years requires many observations spread over time rather than a large telescope, which suits a distributed network of observers far better than it suits a professional facility competing for limited nights. Organisations coordinating such observation have accumulated databases running back over a century and containing many millions of measurements, which are used in published research and which cover objects no professional programme was watching. Visual estimates by comparison with nearby stars were the traditional method and remain useful, and electronic detectors have raised the precision available to amateurs substantially. The arrangement works because the task is one of persistence and coverage rather than of instrument size.

The naming convention for these objects is a source of confusion that has a historical explanation. Variables within a constellation are labelled with letters starting at R and continuing through Z, then with double letters, and then with a V followed by a number once those are exhausted, which is why catalogues contain designations that look arbitrary. The scheme began when only a few were known and was extended repeatedly rather than replaced. Classes are named after a prototype star, so a whole category carries the name of the first member studied, which means the class names are also constellation-based designations and carry no descriptive information. Modern surveys detecting millions of variables have made the traditional scheme impractical for new discoveries, which now carry survey designations, and the older names persist for the well-studied objects.

Several fundamental results came from this work. The relationship between pulsation period and brightness in one class made it possible to measure distances to other galaxies, which established that the universe extends far beyond our own and led to the discovery of its expansion. Eclipsing binaries give the masses and radii of stars directly from geometry and orbital motion, which is the primary check on models of stellar structure and the source of most measured stellar masses. Pulsation periods and their changes constrain the interiors of stars, since the modes a star can support depend on its internal structure, and that has become a substantial field in its own right. And the brightness curves of a particular kind of exploding star, corrected using the shape of the curve, supply the distance measurements that revealed the accelerating expansion of the universe.

Stars vary because they pulsate, erupt or rotate with spotted surfaces, or because something blocks the light, and the shape of the brightness curve distinguishes those. Period and brightness in one class made extragalactic distances measurable, and eclipsing pairs give masses and radii directly. The field depends heavily on amateur observers, since the task needs persistence and coverage rather than large instruments.