This faint glow, of course, pales in comparison with what’s out there.

Astrophysicists have calculated an estimate for all the starlight ever produced throughout the history of the observable universe, which is home to at least 2 trillion galaxies, each brimming with millions and millions of stars. The universe is about 13.8 billion years old, and scientists say that their measure goes back as far as the first billion years, when the first stars were popping into existence like popcorn in hot oil.

Their findings were published Thursday in the journal Science.

The scientists estimate that the number of photons—particles of visible light—emitted by stars into the observable universe since its first billion years of existence is about 4 × 1084. Written out, the total is 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000 photons.

It’s pretty much impossible to fathom this staggering number, with all its vision-blurring zeros, so the astrophysicists have offered up our own star for comparison: The sun emits about 3 × 1052 photons per year. That is 30,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000,000,000.

As you might have guessed from the parade of zeros, measuring the starlight in the entire observable universe is not a simple task. Astronomers need powerful telescopes stationed in space, where their instruments have an unobscured view, free of contamination by Earth’s atmosphere. They need telescopes such as Hubble, which, over nearly three decades of operation, has stared deep into the universe and captured photos of the earliest galaxies. But as powerful as Hubble is, the telescope can’t see them all, especially fainter galaxies. This blind spot makes it difficult to create a complete picture of the starlight in the universe.

Read: Hubble’s hardware woes and the painful era of aging spacecraft

So scientists decided to measure the starlight indirectly. For this, they turned to another NASA space observatory, the Fermi space telescope. Fermi is designed to measure gamma rays, the most energetic form of light. Like visible light, gamma rays are made of photons, but are invisible to the human eye.

Some of the best sources of gamma rays in the universe come from blazars: giant black holes, millions of times more massive than our sun, that sit in the center of their galaxy. These black holes feed on surrounding cosmic material. As they eat, the black holes burp jets of extremely energetic particles, including gamma rays, and send them shooting through space at nearly the speed of light. Despite their names, “black holes are the brightest sources in the universe, which is pretty spectacular,” says Marco Ajello, an astrophysicist at Clemson University in South Carolina and the lead author of the study.