Editing Variable star (section)

===Variable star observations===
[[Image:New View of the Great Nebula in Carina.jpg|right|thumb|A photogenic variable star, [[Eta Carinae]], embedded in the [[Carina Nebula]]]]
Variable stars are generally analysed using [[Photometry (astronomy)|photometry]], [[spectrophotometry]] and [[spectroscopy]]. Measurements of their changes in brightness can be plotted to produce [[light curve]]s. For regular variables, the [[Frequency|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 astronomy|Amateur astronomers]] can do useful scientific study of variable stars by visually comparing the star with other stars within the same [[Telescope|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.