Star Light, Star Bright: Measuring All the Starlight (Ever!)

But even though we can’t even see all of the existing stars, astronomers have found a way to “see” all of the starlight produced in the history of the 13.7-billion-year-old universe. Today in the journal Science, the Fermi-LAT Collaboration described how they did this, and revealed findings that shed light on the history of star formation and the evolution of the universe.

If you want to know how much starlight is in the universe, you might try something like measuring all of the starlight you can see, and then estimating how much is out there that you can’t see. Scientists have performed refined versions of this type of analysis, but the estimates require lots of assumptions that may or may not match reality.

The Fermi-LAT Collaboration explored this question using an entirely new approach that doesn’t rely on the same types of assumptions. Instead of measuring starlight directly, they looked at the influence of starlight on high-energy gamma rays detected by the Large Area Telescope (LAT), an instrument on the space-based Fermi Gamma Ray Telescope.

These high-energy gamma rays are very energetic particles of light produced in extreme conditions, like inside the powerful jets associated with blazars (large galaxies with massive black holes at their center). The Fermi-LAT collaboration is a group of 400+ scientists from 12 countries and more than 90 institutions. On the paper discussing this project alone, there are 129 authors!

In this analysis, scientists looked at the gamma rays detected by LAT from more than 700 active blazars over nine years. Here’s why.

You might be familiar with the cosmic microwave background, a low-level background of radio waves that fills the universe and helps us understand its evolution. Well, the universe also has a low-level background of ultraviolet, visible, and infrared light. This is called the extragalactic background light (EBL), and is the result of the light emitted by all of the galaxies over the entire history of the universe. Gamma rays produced in distant blazars travel through the EBL before reaching LAT and, it turns out, this journey leaves a detectable imprint on LAT observations.

As they travel through the extragalactic background light, high-energy gamma rays occasionally collide with EBL photons and spin out into a pair of charged particles—one matter, the other antimatter. This is more likely to happen when gamma rays travel an especially long distance through the EBL or through a high density of EBL photons. In LAT, these annihilations show up as a dip in a source’s signal when you look at it across energies and over time.

Here’s the key: Gamma rays above a certain energy that travel similar distances will annihilate at the same rate, as long as the density of the EBL is the same. So, if two blazars are the same distance away but one signal has a bigger dip, you know the density of the EBL is higher between it and us than in the direction of the other blazar. Similarly, by comparing signals from blazars at different distances from us, you can look back at how the EBL has changed over time.

In this research, the Fermi-LAT team analyzed and compared nine-year’s worth of data from 739 blazars at various distances from us. From this, they were able to create a map of the EBL in space and time, and then reconstruct how the EBL has evolved over most of the lifetime of the universe. Since the EBL at any given time reflects the total amount of starlight produced, the reconstruction sheds light on the history of star formation.

The team’s data suggests that, over the entire history of the universe, stars have produced a total of about 4*10⁸⁴ photons. For reference, the sun gives off about 3*10⁵² every year. Although a simple number doesn’t quite capture the history of star formation, the research does what science often does: zooming in closer and closer to an evidence-based understanding of how the universe works, using the best tools available to us at this time in history.

—Kendra Redmond