Editing Variable star (section)

===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>