Astronomers used two ways to find out. They obtained accurate distances by measuring the velocities of galaxies with ground-based telescopes. But many galaxies were beyond the range of ground-based observatories. That s when astronomers turned to the Deep Field exposures. By analyzing the colors in these exposures, astronomers were able to estimate distances.
Astronomers express distance in terms of redshift.
Redshift is a measure of how fast galaxies are moving away from Earth. Since the universe is expanding, the more distant galaxies are receding at a faster rate and thus have a higher redshift. Once astronomers establish a galaxy s redshift, they can make a rough estimate of its distance. The distance isn t exact because the universe s expansion rate is not yet precisely known. Using the 10-meter W.M. Keck atop Hawaii s Mauna Kea to measure the velocities of objects in the Deep Field, astronomers have obtained redshifts for 125 galaxies and roughly 500 more in the surrounding area.