The Universe we can see is made up of billions of galaxies, each containing anywhere from hundreds of thousands to hundreds of billions of stars.
Large numbers of galaxies are elliptical in shape, red and mostly made up of old stars. Another type is the spiral, where arms wind out in a blue thin disk from a central red bulge.
On average stars in spiral galaxies tend to be much younger than those in ellipticals.
Dr Asa Frederick Bluck from the University of Victoria in Canada and his colleagues used data from the Sloan Digital Sky Survey to group together over half a million galaxies of all different colors, shapes, and masses.
They then used pattern recognition software to measure the shape of each one, to see how the proportion of red stars in a galaxy varies with its other properties.
They found that the mass in the central bulge is the key to knowing the color of the whole galaxy.
Above a given bulge mass, galaxies are red and have no new young stars.
Almost all galaxies have supermassive black holes at their centers. The mass of the bulge is closely related to the mass of the black hole; the more massive the black hole the more energy is released into the surrounding galaxy in the form of powerful jets and X-ray emission. This can blow away and heat up gas, stopping new stars from forming.
“A relatively simple result, that large galaxy bulges mean red galaxies, has profound consequences. Big bulges mean big black holes and these can put an end to star formation,” Dr Bluck said.
Asa F. L. Bluck et al. 2014. Bulge mass is king: The dominant role of the bulge in determining the fraction of passive galaxies in the Sloan Digital Sky Survey. MNRAS, accepted for publication; arXiv: 1403.5269