Irregulars and Semi-Regulars

Irregular or semiregular variables are giants and supergiants that may have periodic pulsations interrupted by intervals of semiregular or irregular variability. Their periods range from 30 to over 1,000 days and amplitudes often up to several magnitudes, Alpha Orionis, or Betelgeuse is an example of a semi-regular (SRc type) variable star. Betelgeuse is an enormous red supergiant, with spectral type M1Ia. As it pulsates, the star's radius goes from about the size of the orbit of Mars and at maximum diameter may equal the orbit of Jupiter.

Below is a sonified video of observations of V0346CrA, a semi-regular variable that is nearly identical to Betelgeuse in magnitude and periodicity. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. The 177 observations span 927 days, and the video lasts around 37 seconds. Therefore, each second in the video corresponds to 25 days in real time, and each beat that plays represents 3 days. The periodic oscillations in brightness can be heard clearly in the video a few times, during spans when the sampling frequency is sufficiently high.

Shown next is a sonified video of the phased light curve for V0346CrA. This star has a period of 278 days and a variability amplitude of 2 magnitudes. Listen and hear two full phases corresponding to about a year and a half on Earth. The video scans over phase (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.

Below is the spectrum for Betelgeuse, an irregular pulsating variable star. 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 will gradually increase in flux (pitch) as wavelength increases. Betelgeuse's spectrum peaks at redder wavelengths, which matches it's description as a cool, red supergiant.

Betelgeuse spectral data courtesy of Krisciunas, K. et al. 2017 PASP, 129, 4504 (link). Light curve data is from ASAS-SN.