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