Observations have shown that the angular size of the Crab Nebula increases by 0.4 “per year. At the same time

Observations have shown that the angular size of the Crab Nebula increases by 0.4 “per year. At the same time, the lines in the nebula’s spectrum are bifurcated by 0.008. Why is a forked line visible instead of one displaced line? Estimate the distance to the nebula.

Since the Crab Nebula is almost transparent, both parts of it approaching and receding are observed. Therefore, the lines in the spectrum are split by the Doppler shift – blue and red. The speed of mutual removal of these parts is V = Cλ / λ, V = 2400 km / s If we assume that the apparent expansion of the nebula occurs with the same speed, then its angular velocity = V / R, whence the distance to the nebula is R = V / ( 0.4 “/ year). Then R = 1.2 kpc. (According to the most accurate measurements, this distance is close to 1.7 kpc.)