Clusters of galaxies are gravitationally bound groups of about 1000 galaxies about 3 million light years across, with a combined mass 10,000 times the mass of our own Milky Way galaxy. The blueness, he explained, is caused by younger, hotter stars. When you look nearby, you find clusters of galaxies have hardly any blue galaxies. They're almost all red galaxies. But Ulmer and an undergraduate student, Anne J. Metevier, and postdoctoral researcher A. Katherine Romer focused on three clusters only 2 billion light years away.
Clusters emit X-rays from hot, ionized gas between their galaxies.
This turned out to be true, she said, as evidenced by the color of the clusters. In fact, one of the three clusters Ulmer's group studied is the bluest galaxy cluster ever. When the gas is compressed on the galaxies, that ignites star formation. The result is bluer galaxies within the cluster. The optical observations were made by Romer on the National Science Foundation's National Optical Astronomy Observatory's 36-inch telescope on Kitt Peak in Arizona.