Variable star

A **variable star** is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either: **Intrinsic variables**, whose luminosity actually changes; for example, because the star periodically swells and shrinks, and **extrinsic variables**, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.

Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

## Discovery

The most common kinds of variability involve changes in brightness, but other types of variability also occur, in particular changes in the spectrum. By combining light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable.

### Variable star observations

A photogenic variable star, Eta Carinae, embedded in the Carina Nebula
Variable stars are generally analysed using photometry, spectrophotometry and spectroscopy. Measurements of their changes in brightness can be plotted to produce light curves. For regular variables, the period of variation and its amplitude can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to the next. Peak brightnesses in the light curve are known as **maxima**, while troughs are known as **minima**.

Amateur astronomers can do useful scientific study of variable stars by visually comparing the star with other stars within the same telescopic field of view of which the magnitudes are known and constant. By estimating the variable's magnitude and noting the time of observation a visual lightcurve can be constructed. The American Association of Variable Star Observers collects such observations from participants around the world and shares the data with the scientific community.

From the **light curve** the following data are derived:

* are the brightness variations periodical, semiperiodical, irregular, or unique?
* what is the **period** of the brightness fluctuations?
* what is the **shape** of the light curve (symmetrical or not, angular or smoothly varying, does each cycle have only one or more than one minima, etcetera)?

From the **spectrum** the following data are derived:

* what kind of star is it: what is its temperature, its luminosity class (dwarf star, giant star, supergiant, etc.)?
* is it a single star, or a binary? (the combined spectrum of a binary star may show elements from the spectra of each of the member stars)
* does the spectrum change with time? (for example, the star may turn hotter and cooler periodically)
* changes in brightness may depend strongly on the part of the spectrum that is observed (for example, large variations in visible light but hardly any changes in the infrared)
* if the wavelengths of spectral lines are shifted this points to movements (for example, a periodical swelling and shrinking of the star, or its rotation, or an expanding gas shell) (Doppler effect)
* strong magnetic fields on the star betray themselves in the spectrum
* abnormal emission or absorption lines may be indication of a hot stellar atmosphere, or gas clouds surrounding the star.

In very few cases it is possible to make pictures of a stellar disk. These may show darker spots on its surface.

## Classification

Variable stars may be either *intrinsic* or *extrinsic*.

* **Intrinsic variable stars**: stars where the variability is being caused by changes in the physical properties of the stars themselves. This category can be divided into three subgroups.
	+ Pulsating variables, stars whose radius alternately expands and contracts as part of their natural evolutionary ageing processes.
	+ Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections.
	+ Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like novae and supernovae.
* **Extrinsic variable stars**: stars where the variability is caused by external properties like rotation or eclipses. There are two main subgroups.
	+ Eclipsing binaries, double stars where, as seen from Earth's vantage point the stars occasionally eclipse one another as they orbit.
	+ Rotating variables, stars whose variability is caused by phenomena related to their rotation. Examples are stars with extreme "sunspots" which affect the apparent brightness or stars that have fast rotation speeds causing them to become ellipsoidal in shape.

These subgroups themselves are further divided into specific types of variable stars that are usually named after their prototype. For example, dwarf novae are designated *U Geminorum* stars after the first recognized star in the class, *U Geminorum*.