If the spectrum of a distant galaxy is observed to have a calcium K absorption line that occurs at a wavelength of 500.4 nm, what is this galaxy's distance if the rest wavelength of this absorption line is 393.4 nm? Assume the Hubble constant is 70 km/s/Mpc.
A) 720 Mpc
B) 950 Mpc
C) 1,200 Mpc
D) 2,600 Mpc
E) 3,700 Mpc

Question

Quizplus · Accepted Answer

The redshift z is calculated as (500.4 - 393.4) / 393.4 = 0.272. Using Hubble's law, v = H0 * d, where v = z * c (c is the speed of light, approximately 300,000 km/s), we find v = 0.272 * 300,000 = 81,600 km/s. Solving for distance d gives d = v / H0 = 81,600 / 70 = 1,165.7 Mpc, which is closest to 1,200 Mpc.

If the Spectrum of a Distant Galaxy Is Observed to Have