Astronomers detect light from Universe’s earliest stars

Artist's illustration of NASA's Fermi Gamma-ray Space Telescope.Using the Fermi Gamma-Ray Space Telescope, a team of astronomers has detected the light from the very first stars that formed after the Big Bang.

Led by astrophysicist Marco Ajello, of the SLAC National Accelerator Laboratory at Stanford University, the team measured the extragalactic background light (EBL) — the sum of all the radiation generated in the universe outside our own galaxy, by star formation as well as active galactic nuclei (AGNs) such as black holes —  and filtered out the light from all the known stars that had already been measured.

"The EBL also includes the light of the first massive stars that ever shone. We have a fairly good knowledge of the light emitted by 'normal' stars. Thus, by measuring the EBL we are able to constrain the light of the first stars," said Ajello.

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However, the EBL is extremely difficult to measure, due to how faint it is. Given the immense distances involved, astronomers couldn't observe background light directly. Instead, they used the light from a specific type of quasi-stellar object (quasar) known as blazars. A quasar is the supermassive black hole at the core of a very distant (thus ancient) galaxy that has consumed so much matter it spills out in the form of intense jets of radiation in the directions of the black hole's 'poles.' These jets can reach for up to hundreds of thousands of light years. A blazar is essentially a more compact version of a quasar, where one of the jets points generally in the direction of our solar system, so that we look 'down' the jet. The team used these jets in their measurement of the EBL.

This plot shows the locations of 150 blazars (green dots) used in the EBL study. Image released Nov. 1, 2012.

"We use [blazars] as cosmic lighthouses," Ajello said. "We observe their dimming due to the EBL 'fog.' This allows us to quantify how much EBL there is between us and the blazars. As blazars are distributed across the universe, we can measure the EBL at different epochs."

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Using this method, Ajello and his team were able to isolate the light from stars that existed over 13.1 billion years ago, only 600,000 years after the Big Bang. The stars from this period in time are thought to have been up to hundreds of times the size of our Sun, burning brighter and hotter, and burning out far quicker. The measurement of this light should provide answers about how soon these stars formed after the big bang, and how quickly they formed. However, astronomers still haven't been able to see the stars themselves.

"Detecting these stars is very important, but currently impossible," said Ajello.

"The Webb Telescope in a few years might be able to see the first galaxies (not the first stars though). In this way we are already able to set constraints on the amount and role of these stars in the early universe."

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