If you found a galaxy with an Hα emission line that had a wavelength of 756.3 nm,what would be the galaxy's distance if the Hubble constant is 70 km/s/Mpc? Note that the rest wavelength of the Hα emission line is 656.3 nm.
A)650 Mpc
B)760 Mpc
C)3,200 Mpc
D)6,400 Mpc
E)7,600 Mpc

Question

Quizplus · Accepted Answer

The redshift z can be calculated using z = (observed wavelength - rest wavelength) / rest wavelength = (756.3 nm - 656.3 nm) / 656.3 nm = 0.152. The velocity v = z * c = 0.152 * 3 x 10^5 km/s = 45,600 km/s. Using Hubble's law, distance = velocity / Hubble constant = 45,600 km/s / 70 km/s/Mpc = 650 Mpc.

If You Found a Galaxy with an Hα Emission Line