Q: Can you figure out how fast the star is rotating?
Measure the Full-Width-at-Half-Minimum (FWHM) of the
line: its width at a point halfway to the bottom of the
absorption feature. Then use this formula to find the
effective shift due to rotational motion.
effective shift = FWHM / 2.35
We see from the spectrum that FWHM is about 0.5 nm.
So
effective shift = 0.5 nm / 2.35 = 0.21 nm
The doppler formula then gives a speed
0.21 nm
velocity = ---------- * (300,000 km/s)
492 nm
= 130 km/s