Editing Variable star (section)

==Detecting variability==

The most common kinds of variability involve changes in brightness, but other types of variability also occur, in particular changes in the [[stellar spectrum|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===
[[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.

===Interpretation of observations===
Combining light curves with spectral data often gives a clue as to the changes that occur in a variable star.<ref>{{Cite web|url=https://www.aavso.org/files/Variable%20Star%20Classification%20and%20Light%20Curves%20Manual%202.1.pdf|title=Variable Star Classification and Light Curves|access-date=15 April 2020}}</ref> For example, evidence for a pulsating star is found in its shifting spectrum because its surface periodically moves toward and away from us, with the same frequency as its changing brightness.<ref>{{Cite web|url=https://tophat.com/marketplace/science-&-math/physics/textbooks/oer-openstax-astronomy-openstax-content/1200/34508/|title=OpenStax: Astronomy {{!}} 19.3 Variable Stars: One Key to Cosmic Distances {{!}} Top Hat|website=tophat.com|access-date=2020-04-15}}</ref>

About two-thirds of all variable stars appear to be pulsating.<ref>{{Cite book|last=Burnell|first=S. Jocelyn Bell|url=https://books.google.com/books?id=lb5owLGIQGsC&q=About+two-thirds+of+all+variable+stars+appear+to+be+pulsating.&pg=PA115|title=An Introduction to the Sun and Stars|date=2004-02-26|publisher=Cambridge University Press|isbn=978-0-521-54622-5|language=en}}</ref> In the 1930s astronomer [[Arthur Stanley Eddington]] showed that the mathematical equations that describe the interior of a star may lead to instabilities that cause a star to pulsate.<ref>{{Cite journal|url=http://adsabs.harvard.edu/full/2004JAHH....7...65M|title=2004JAHH....7...65M Page 65|journal=Journal of Astronomical History and Heritage|bibcode=2004JAHH....7...65M|access-date=2020-04-15|last1=Mestel|first1=Leon|year=2004|volume=7|issue=2|page=65|doi=10.3724/SP.J.1440-2807.2004.02.01 |s2cid=256563765 }}</ref> The most common type of instability is related to oscillations in the degree of ionization in outer, convective layers of the star.<ref>{{Cite journal|url=http://adsabs.harvard.edu/full/1967IAUS...28....3C|title=1967IAUS...28....3C Page 3|journal=Aerodynamic Phenomena in Stellar Atmospheres|bibcode=1967IAUS...28....3C|access-date=2020-04-15|last1=Cox|first1=J. P.|year=1967|volume=28|page=3}}</ref>

When the star is in the swelling phase, its outer layers expand, causing them to cool. Because of the decreasing temperature the degree of ionization also decreases. This makes the gas more transparent, and thus makes it easier for the star to radiate its energy. This in turn makes the star start to contract. As the gas is thereby compressed, it is heated and the degree of ionization again increases. This makes the gas more opaque, and radiation temporarily becomes captured in the gas. This heats the gas further, leading it to expand once again. Thus a cycle of expansion and compression (swelling and shrinking) is maintained.{{Citation needed|date=April 2020}}

The pulsation of [[Cepheid variable|cepheids]] is known to be driven by oscillations in the ionization of [[helium]] (from He<sup>++</sup> to He<sup>+</sup> and back to He<sup>++</sup>).<ref>{{Cite journal|url=http://adsabs.harvard.edu/full/1963ApJ...138..487C|title=1963ApJ...138..487C Page 487|journal=The Astrophysical Journal|bibcode=1963ApJ...138..487C|access-date=2020-04-15|last1=Cox|first1=John P.|year=1963|volume=138|page=487|doi=10.1086/147661}}</ref>