This diagram illustrates this process. Galaxies in the early universe appear at the left. The blue regions are where star formation is in progress and the red regions are the “dead” regions where only older redder stars remain, and there are no more young blue stars being formed. The resulting giant spheroidal galaxies in the modern universe appear on the right.
A major astrophysical mystery has centered on how massive, quiescent elliptical galaxies, common in the modern universe, quenched their once furious rates of star formation. Such colossal galaxies, often also called spheroids because of their shape, typically pack in stars 10 times as densely in the central regions as in our home galaxy, the Milky Way, and have about 10 times its mass.
Astronomers refer to these big galaxies as red and dead as they exhibit an ample abundance of ancient red stars but lack young blue stars and show no evidence of new star formation. The estimated ages of the red stars suggest that their host galaxies ceased to make new stars about 10 billion years ago. This shutdown began right at the peak of star formation in the universe, when many galaxies were still giving birth to stars at a pace about 20 times faster than nowadays.
