PHILADELPHIA -- The universe reached the climax of its star-building activity five billion years ago -- more recently than previously thought -- according to researchers at the University of Pennsylvania and the University of Edinburgh.

The astronomers sifted through the fossil record of 96,545 nearby galaxies to chronicle the complete history of star formation over time.  Their findings, reported in the April 8 issue of the journal Nature, also determined that the more massive a galaxy the earlier its stars were formed, indicating that galaxies form stars differently depending on weight.    

"If we want to understand how structure in the Universe formed and evolved, then we need to understand the history of the stars," said Raul Jimenez, an assistant professor in Penn Department of Physics and Astronomy. "Fortunately we can read the history of the stars.  By analyzing all of the light coming out of a particular galaxy that is, the entire spectrum of visible light we can effectively see the entire 'fossil record' of that galaxy at one glance."    

While paleontologists generally need to dig down to find their fossil record, Jimenez and his colleagues needed only to look up, in this case to look up data from the Sloan Digital Sky Survey.  To sift through data from nearly 100,000 galaxies, Jimenez and Edinburgh astronomer Alan Heavens created a program called Multiple Optimized Parameter Estimation and Data Compression (MOPED) that analyzes spectrum data quickly by compressing information into more manageable blocks.  

"Stars of different masses evolve with different luminosities, so by looking at the integrated spectrum of a galaxy we can track those different luminosities, their masses and, therefore, how long ago they were born," Jimenez said. "Our method takes into account all the stars that are present in the observed galaxies today and allows us to create the most complete history of star formation yet assembled."

According to the researchers' findings, star formation in the universe peaked, on average, about five billion years ago.  By the time our own sun was born, about 4.7 billion years ago, almost half of the stellar mass in the universe since the big bang was already created.  Star formation has drastically dropped off since then and, as new stars are not being created faster than old stars are dying, this will lead to the gradual dimming of the universe.

The findings also show a difference in star formation between low-mass and high-mass galaxies.  Galaxies with a higher mass, our own Milky Way among them, formed most of their stars well before galaxies of a lower mass did.

"The mass-dependence of the 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," Jimenez said.

In an accompanying paper submitted to the Monthly Notices of the Royal Astronomical Society, Jimenez and his colleagues have explored how these results shed light on the assembly of dark matter, the mysterious component of the universe that holds galaxies together.

Contributing astronomers also include Benjamin Panter and James Dunlop of the Institute for Astronomy at the University of Edinburgh.

Funding for this research was partly provided by the National Science Foundation with a grant to Jimenez.

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