ZZ Ceti Variables

ZZ Ceti stars are white dwarfs that exhibit periodic changes in brightness. When most stars run out of nuclear fuel to burn, they expel their outer layers until only the core is left behind. This core then becomes an extremely hot and dense white dwarf. When such a white dwarf is in a narrow temperature range near 12,000 degrees Kelvin, minor pulsation events occur that are caused by periodic changes in the light-absorbing properties of hydrogen in a thin surface layer. The pulsations produce periodic brightness variations on short timescales ranging from 30 seconds to 20 minutes. As a ZZ Ceti star cools over millions of years, the variations become more complex, because the pulsations interact with convection deeper inside the star.

Below is a sonified video of observations of a ZZ Ceti white dwarf variable star. The video is 35 seconds long, but spans 46 minutes in real time and contains 276 observations. This means that the observations were taken with 10 second resolution. The periodic rise and fall in brightness is clearly exhibited. This ZZ Ceti variable repeatedly fluctuates by about 4% of its average brightness. The video scans over time (x-axis) and modulates pitch based on relative flux change (y-axis). Lower pitch represents dimmer observations and higher pitch represents brighter observations.

Shown below is the sonified spectrum of our ZZ Ceti variable. ZZ Ceti stars can be distinguished from other types of pulsating white dwarfs because they have hydrogen dominated atmospheres and don't exhibit any helium signatures. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), moving from blue to red wavelengths from 3650 to 5100 angstroms. The limited wavelength range covers only the bluer region of the spectrum. The spectrum begins fairly level, before increasing in brightness at longer wavelengths. This increase in brightness is interrupted repeatedly by very broad absorption lines, or dips, that correspond to various electron transitions in hydrogen. The reason the lines are so broad is because the intense surface gravity of white dwarf stars creates very high pressure in the surface hydrogen layer, so that the hydrogen atoms are moving at high velocities within the gas. The rapid motions create a "Doppler shift" of the absorption frequencies, both toward bluer and redder wavelengths, significantly broadening the lines compared to normal stars with lower surface gravity.

The ZZ Ceti light curve is provided thanks to JJ Hermes, Ph.D., Assistant Professor of Astronomy, Boston University.
The spectrum is from Gianninas, A. et. al. 2011, ApJ, 743, 138.