Alan Heavens, Benjamin Panter and James Dunlop of Edinburgh University and Raul Jimenez of the University of Pennsylvania analysed spectra from the Sloan Digital Sky Survey for the stellar populations of almost 100 000 stars to obtain a complete history of star formation over time. The astronomers built theoretical models of how the galaxy spectra should look and compared them to observations. “It is a bit like fingerprint identification, one looks for the best match,” Jimenez told PhysicsWeb.

Using a computer program called Multiple Optimised Parameter Estimation and Data Compression (MOPED), Jimenez and colleagues were able to compare the observed and predicted spectra of 96 545 galaxies in just four weeks. Without the program such a feat would have taken eight years.

The astronomers calculated that star formation in the universe reached its peak about five billion years ago. It has since declined by around a factor of 10 to its present-day value. Moreover, they found that galaxies with a high mass, which include our Milky Way, formed most of their stars much earlier than galaxies with a lower mass.

“The mass dependence of star-formation history explains why previous surveys showed a much earlier date for star formation, since those studies were only able to examine more massive galaxies,” said Jimenez.

The Pennsylvania-Edinburgh team now plans to look at more spectra and develop better theoretical models. “We need to understand how galaxies got the star formation history we find, we really do not know yet,” he said.