BY Draconis Variables
BY Draconis (BY Dra) variable stars have large spots on them, analogous to sunspots. While they may have a fairly steady overall luminosity, the brightness that we measure from the star changes as the cooler, darker spots rotate towards and away from Earth. The associated brightness fluctuation arising from star spots is usually less than 0.5 magnitudes (about a 50% change). The period of BY Dra variability depends on the star’s rotational rate. Some of these variables, including our example below, also exhibit flaring activity. This means that the quasiperiodic brightness fluctuations caused by star spots are punctuated by irregular flares of increased brightness.
Shown below is a sonified video of observations of a BY Draconis variable star. The video spans 400 days and comprises 67 observations. Each beat of time in the video corresponds to two days in real time. There are no noticeable trends to be heard in the video besides the quick fluctuations in brightness, which span about 0.3 magnitudes. 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 our BY Draconis variable. It was determined to have a period of about 6.5 hours. The two full phases plotted correspond to 13 hours on Earth. Notice outlying bright observations which don’t fit the periodic trend – these correspond to solar flares. 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 the observed BY Draconis 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 5580 to 7600 angstroms. Lower pitch represents weaker flux. During the first half of the video, the spectrum remains constant in flux (pitch) as wavelength increases. Two prominent emission lines are plotted – H-alpha around 6563 Angstroms, and Fe-II around 7030 Angstroms. This BY Dra spectrum then increases in flux to peak at redder wavelengths, which means it is cool and likely a dwarf star.
The spectrum for this BY Draconis variable came from the FAST spectral archive. The light curve is an optical r-band from the ZTF.