Ancient Suns Set The Night Sky Aglow

Astronomers have uncovered vast cosmic reservoirs of atomic hydrogen surrounding distant galaxies. The exquisite sensitivity of MUSE allowed for direct observations of dim clouds of hydrogen glowing with Lyman-alpha emission in the early universe — revealing that almost the whole night sky is invisibly aglow. This is a composite image showing the emission in blue against a photo taken with the Hubble Space Telescope.
ESA/Hubble & NASA, ESO/ Lutz Wisotzki et al.

Nearly the entire night sky glows a midnight blue from vast clouds of hydrogen gas. New observations made with the MUSE (Multi Instrument Spectroscopic Explorer) instrument on the European Southern Observatory’s Very Large Telescope have uncovered enormous cosmic reservoirs of atomic hydrogen surrounding distant galaxies. The team of astronomers discovered that the light — called Lyman-alpha emission — covered nearly the entire field of view of the telescope, leading them to extrapolate that most of the sky must be aglow with the stuff.

Astronomers have been looking at the sky in many different wavelengths of light from X-rays to radio waves for decades now, but the extent of the Lyman-alpha glow was still surprising. “Realizing that the whole sky glows in optical (light) when observing the Lyman-alpha emission from distant clouds of hydrogen was a literally eye-opening surprise,” said Kasper Borello Schmidt, a member of the team.

Milestones of the universe. This graphic summarizes the history of the universe as we know it from its start at the Big Bang to the current era. Intergalactic gas was in a neutral state from about 300,000 years after the Big Bang until light from the first generation of stars and galaxies began to ionize (excite) it. NOAJ

The early days of the universe are known as the “cosmic dark ages” because the first stars had yet to form. All was dark matter and neutral gas, mostly hydrogen and helium. Over time, the dark matter clumped together through gravity, pulling in the ordinary matter you and I are familiar with. Compressed by dark matter’s gravitational pull, ordinary matter collapsed to form the first stars and galaxies. This happened sometime around 300 million years after the Big Bang, the moment when our universe burst onto the stage.

These fiery orbs gave off lots of ultraviolet light which excited and energized the remaining neutral hydrogen gas, setting it aglow as the thousands of blue blobs you see in the image. Remember that when we observe distant galaxies, we’re looking back in time to the early universe, when new star formation was rabid. That’s why we see so many patches literally everywhere in the photo and almost certainly across the entire sky.

This chart shows the location of the Hubble Ultra Deep Field in the constellation of Fornax (The Furnace). The map shows most of the stars visible to the unaided eye under good conditions, and the region of sky shown in this image is indicated.
ESO, IAU and Sky & Telescope

The team made their observation in the Hubble Ultra Deep Field, a small, otherwise unremarkable patch of “vacant” sky in the constellation Fornax the Furnace. Hubble photos revealed something like a thousand galaxies here. One last thought. Lyman-alpha light lies in the ultraviolet part of the spectrum and is completely invisible to the naked eye and typical optical telescopes because it’s absorbed by the atmosphere.

So how can we get a picture of the splotchy glow? You’ve heard that the universe is expanding, right? That expansion not only enlarges the space between groups of distant galaxies but also stretches the short waves of light like ultraviolet into longer ones like visible and infrared. Cosmic expansion over such remote distances has stretched UV into visible light that telescopes on the ground can see and photograph. Amazing, isn’t it?

Next time you’re out on a moonless night, look up and imagine all that glowing gas smeared across the sky illuminated by the earliest generation of stars in our universe. Stars that exist no more.