Cepheids
Cepheid-1
Type 1 or “Classical” Cepheid variables are pulsating giant stars. As giants, Cepheids have radii from a few tens to a few hundred times the size of our Sun. Their temperature and spectral type vary as they pulsate. Because their size changes as they pulsate, their luminosity varies as well - bigger is brighter! Cepheid variables have a well-defined relationship between pulsation period and luminosity, which makes them valuable for calculating distances. Because of this relationship (discovered in 1908 by Henrietta Swan Leavitt, a member of the Harvard Computers), measuring the period of a Cepheid tells you its intrinsic luminosity. Comparing that intrinsic luminosity to the apparent brightness tells you the distance to the Cepheid. Cepheids are luminous enough to be visible in other galaxies, providing the first reliable distances to them. Comparing those distances to the velocity of recession of the galaxies (most galaxies appear to be moving away from us in all directions) eventually led to the discovery of the expansion of space, otherwise known as the Big Bang.
Shown below is a sonified video of an observed Cepheid-1 variable. The light curve contains 174 observations that were taken over nearly 4 years, from early 2018 to late 2021. Each interval of time during the video corresponds to a 5 day span, so that the video plays in about 30 seconds. The light curve of the Classical Cepheid star ranges in R-band magnitude from 15.6 to 15.1. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
The next sonified video is a phased light curve for the Classical Cepheid variable star. This star was determined to pulsate on a short interval - about every 1 day and 5 hours. In the video, you should hear two full phases, each consisting of an extended dimming period followed by the pulsation which is heard as a rapid increase in brightness. The video scans over phase (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Below is a sonified video of the spectrum for the observed type 1 Cepheid variable. 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 4100 to 8800 angstroms. Lower pitch represents weaker flux. Because the Classical Cepheid star is a hot giant star, the spectrum video begins with a peak in the blue, short wavelength end, interrupted by some rapid fluctuations caused by absorption by calcium and hydrogen atoms in the atmosphere. The peak is followed by a gradual decrease in flux (pitch) as wavelength increases. Absorption lines towards the longer, red wavelengths in the spectrum are caused by hydrogen atoms.
This Cepheid-1 variable, J171031.11+282308.77 was targeted for SDSS-IV spectroscopy as a variable in the TDSS project (Roulston in prep.). The light curve is an optical r-band from the ZTF.
Cepheid-2
Type 2 Cepheid variables are pulsators with periods between 1 and 50 days. They are population 2 stars, which means that they are members of an older generation of stars formed during an earlier time in the universe. We know this because they are metal-poor, which means they are deficient in the elements created by previous generations of stars. Like all Cepheid variables, type 2 Cepheids have a well-defined relationship between pulsation period and luminosity, which makes them valuable for calculating distances.
Shown below is a sonified video of an observed Cepheid-2 variable. There are 42 observations that can be heard over a 500 day span, which is a fairly low sampling rate. One reason the observations are so sparse is the 150 day seasonal gap in observations that can be heard in the video, during which the Cepheid-2 variable was not visible from Earth. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
The next video is the phased light curve for the observed Cepheid-2 variable. The star was determined to have a pulsation period of about 26 days based on the 42 observations taken over 500 days. In the video, you should hear two full phases, each consisting of a slow dimming period followed by the pulsation which is heard as a rapid increase in brightness. The video scans over phase (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Below is a sonified video of the spectrum for the observed type 2 Cepheid variable. 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 3800 to 7200 angstroms. Lower pitch represents weaker flux. The spectrum begins at the blue, short wavelength end with some rapid fluctuations caused by absorption by calcium and hydrogen atoms in the atmosphere, followed by a gradual decrease in flux (pitch) as wavelength increases. Significant absorption lines towards the longer, red wavelengths in the spectrum are caused by different electronic level transitions in hydrogen atoms.
This Cepheid-2 variable, J013300.26+304645.9 was targeted for SDSS-IV spectroscopy as a variable in the TDSS project (Roulston in prep.). The light curve is an optical r-band from the ZTF.