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 (more familiar) 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.

Now a group of astronomers led by Asa Bluck of the University of Victoria in Canada have found a (relatively) simple relationship between the color of a galaxy and the size of its bulge – the more massive the bulge the redder the galaxy.


Images of a small fraction of the galaxies analyzed in the new study. The galaxies are ordered by total mass of stars (rising from bottom to top) and by ‘bulge to total stellar mass ratio’ (rising from left to right). Galaxies that appear redder have high values for both of these measurements, meaning that the mass of the bulge –and central black hole – determines their color. Credit: A. Bluck. 

Asa and his team 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 (regardless of how big the disk surrounding it may be) 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,” Asa said.

Published in Monthly Notices of the Royal Astronomical Society.