What Is a Light Curve?

A light curve is a plot of the visible light from stars over time, which is used to determine a star's behavior. The technique of plotting light is important for variable stars, which change brightness in both regular and unpredictable ways. Light curves can also be used to describe the action of binary stars, which are two stars that travel around each other.

Variable stars can change brightness in regular patterns, due to the rotation of the star or interaction with other stars. A variable binary occurs because the two stars are moving relative to each other, and one can block the light of the other as they move. The graphs or plots of visible light can be made from visual observations of astronomers, or by computer models using digital light equipment.

Some stars may show variable light patterns over several years, so repeated observations are required for a good curve to be developed. Though astronomers may assign different light values for the same star over time, the light curve can be accurate because the many observations are plotted together and averaged. Many astronomers feel that light curves from visual observations are as accurate as computer models.

Binary star behavior can be measured using this method, due to an effect called eclipsing. In the same way the moon can eclipse the sun and block its light when viewed from the Earth, one star of a binary pair can eclipse the other. When this occurs, the visible light viewed through a telescope will change, and a light curve can be used to plot the measurements. If the eclipsing behavior occurs in a regular pattern over time, the data can be used to determine the rotation period, or the time it takes for the binary stars to rotate around each other.

Another use of the light curve is observing a supernova, which is an exploding star. Certain stars reach a point in their life when the star rapidly collapses due to gravitational forces, then explodes. The result is a large increase in visible light as the star's gases expand outward at high speed, and observations can be used to estimate the outward speed of the star's gases and the distance of the supernova from Earth.

Supernova explosions are separated into different classes based on the way the explosion occurs. Some stars rapidly dim after the explosion, while others reach a certain light level and remain there for a time, called a plateau behavior. These differences would not be noticeable using individual observations, which can make a light curve useful for classification. Supernova classes also create different elements from the chemical reactions that occur during the explosions, and the curves can help determine the chemical compositions of the star's expanding gases and remaining core material.