Astronomy
How Do Astronomers Measure the Rotation of Galaxies?
Quick fact
In the Andromeda Galaxy, stars at the visible edge orbit the center at about 250 km/s – far faster than the gravity of all visible matter alone could explain.
Why this is interesting
You have likely heard the pitch of a siren change as it passes you – could the same trick reveal what a galaxy is made of?
Read the full explanation
Understanding How Do Astronomers Measure the Rotation of Galaxies?
Astronomers measure galaxy rotation by analyzing light from its stars and gas. When a galaxy is tilted slightly, stars on one side are moving toward us and on the other side away from us. Using the Doppler effect, the light from approaching stars is blueshifted, while receding stars appear redshifted. By mapping these shifts across the disk, astronomers can calculate the orbital speed of material at many distances from the center, building a rotation curve that shows how fast stars move as a function of radius.
A deeper explanation
The measurement begins with spectroscopy: splitting the galaxy's light to identify spectral lines, which serve as precise markers. Each line's wavelength shift reveals the line-of-sight velocity. Combining many spectra across the disk, astronomers plot a rotation curve. In a typical spiral galaxy, the curve flattens instead of falling off as Newtonian gravity predicts. This discrepancy implies a massive, invisible component – dark matter – that surrounds galaxies in a halo, further confirmed by gravitational lensing and cosmic structure simulations.