Zz Ceti Stars

Class of variable star characterized by modest luminosity variations (from 0.001 to 0.2 magnitudes or 0.1% to 20% of their average luminosities) and periods of 30 to 1,200 seconds. Their location on the Hertzsprung-Russell diagram identifies them as white dwarf stars that have entered the instability strip as they evolve along the white dwarf track. ZZ ceti stars are also known as DAV stars. ZZ Ceti stars evolve from intermediate mass stars (6 to 8 Msun) that have managed to retain hydrogen in their outer envelopes. This type of white dwarf is classified as a DA white dwarf, as opposed to DB white dwarfs that have helium, but no hydrogen in their outer envelopes (hence the alternative name DAV stars). The surface temperatures of ZZ Ceti stars are ~12,000
K. The pulsations responsible for their variability are the result of the ionization and then recombination of hydrogen in the outer envelope.

Image source: http://astronomyonline.org/Stars/VariableStars.asp.

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