Spectroscopic observations allow observers to estimate the rotation velocity of superflare stars

 

Source: Subaru Telescope

Top: Four neutral iron (Fe I) absorption lines are shown. Spectral lines show some broadening because of the Doppler effect on the light from the rotating stellar surface. Slowly rotating stars like the Sun have a narrow line profile, while rapidly rotating stars have a wide line profile. Measuring these broadenings allows an estimate of the stellar rotation velocity. 

Bottom: The wavelength of light from the surface of a rotating star shifts because of the Doppler effect. For example, the wavelength of light from point A becomes a bit short (is blue-shifted) since this point is approaching us (the observer). By contrast, the wavelength of light from point C is a bit long (is red-shifted) since this point moves away from us. The wavelengths of light from point B have no shifts since this point moves perpendicular to the line of sight. Finally, this line-shift effect results in broadening of spectral lines.