Light curve

Lore & Background

Graphs of the apparent magnitude of a variable star over time are commonly used to visualise and analyse their behaviour. Although the categorisation of variable star types is increasingly done from their spectral properties, the amplitudes, periods, and regularity of their brightness changes are still important factors. Some types such as Cepheids have extremely regular light curves with exactly the same period, amplitude, and shape in each cycle. Others such as Mira variables have somewhat less regular light curves with large amplitudes of several magnitudes, while the semiregular variables are less regular still and have smaller amplitudes. The shapes of variable star light curves give valuable information about the underlying physical processes producing the brightness changes. For eclipsing variables, the shape of the light curve indicates the degree of totality, the relative sizes of the stars, and their relative surface brightnesses. It may also show the eccentricity of the orbit and distortions in the shape of the two stars. For pulsating stars, the amplitude or period of the pulsations can be related to the luminosity of the star, and the light curve shape can be an indicator of the pulsation mode.

Light curves from supernovae can be indicative of the type of supernova. Although supernova types are defined on the basis of their spectra, each has typical light curve shapes. Type I supernovae have light curves with a sharp maximum and gradually decline, while Type II supernovae have less sharp maxima. Light curves are helpful for classification of faint supernovae and for the determination of sub-types. For example, the type II-P (for plateau) have similar spectra to the type II-L (linear) but are distinguished by a light curve where the decline flattens out for several weeks or months before resuming its fade. In planetary science, a light curve can be used to derive the rotation period of a minor planet, moon, or comet nucleus. The time separation of peaks in the light curve gives an estimate of the rotational period of the object. The difference between the maximum and minimum brightnesses (the amplitude of the light curve) can be due to the shape of the object, or to bright and dark areas on its surface.

Reader's Guide

Light curves are a fundamental tool in astronomy, providing a means to study a wide variety of celestial phenomena. For variable stars, the periodicity, amplitude, and shape of light curves are key to classification and understanding the underlying physics, such as pulsation modes in Cepheids or the geometry of eclipsing binaries. In supernova research, light curves help distinguish types and sub-types, such as Type II-P versus Type II-L, even when spectra are faint. For minor planets and asteroids, light curves allow astronomers to infer rotation periods and shapes, despite the inability to resolve these objects directly. The Asteroid Lightcurve Database (LCDB) uses a quality code (U) from 0 to 3 to assess the reliability of period solutions derived from light curves. Occultation light curves, often binary in shape, can reveal double stars or asteroids, and atmospheric effects on bodies like Titan. The transit method for exoplanet discovery relies on periodic dips in a star's light curve caused by a planet passing in front. Light curve inversion is a mathematical technique used to model surfaces of rotating objects, such as starspots or asteroid albedos. Microlensing events produce brief brightness increases, and the shape of the lensing light curve allows inference of the properties of otherwise-invisible objects, such as the possible exoplanet in the PA-99-N2 event.

Did You Know?

• The Asteroid Lightcurve Database uses a quality code U from 0 (incorrect) to 3 (well-defined) for period solutions.
• Type II-P supernovae are distinguished from Type II-L by a light curve plateau lasting weeks or months.
• Occultation light curves can show steps when either the occulting or occulted body is double, such as a double star or double asteroid.
• Microlensing event PA-99-N2 may have been due to a star in the Andromeda Galaxy that has an exoplanet.