A vast cloud of hydrogen gas – one of the largest single structures ever observed in the sky – has been discovered surprisingly close to Earth.
Named Eos after the Greek goddess of the dawn, the cloud was detected because of its faint far-ultraviolet light naturally emitted by hydrogen molecules.
Known as molecular clouds, these enormous structures of gas and dust are the cradles where new stars are born.
Traditionally, astronomers have relied on using radio and infrared telescopes to find these clouds. These telescopes detect carbon monoxide signatures – but the scientists found Eos using an entirely different method.
“This is the first-ever molecular cloud discovered by looking for far ultraviolet emission of molecular hydrogen directly,” said Prof Blakesley Burkhart, who led the team and is an author on the study.
“The data showed glowing hydrogen molecules detected via fluorescence in the far ultraviolet. This cloud is literally glowing in the dark.”
Located just 300 light years from Earth on the edge of a gas-filled region known as the Local Bubble, Eos stretches across a patch of sky equivalent to the width of 40 full Moons and has a mass about 3,400 times greater than the Sun.
Despite its size and proximity to us, it had remained hidden because it is ‘CO-dark’ – in other words, lacking enough carbon monoxide to show up using conventional detection methods.
“The discovery of Eos is exciting because we can now directly measure how molecular clouds are forming and dissociating, and how a galaxy begins to transform interstellar gas and dust into stars and planets,” Burkhart said.
Dr Thavisha Dharmawardena, a NASA Hubble Fellow at New York University, in the US, and a co-first author of the study, added: “When I was in graduate school, we were told that you can’t easily directly observe molecular hydrogen. It’s wild that we can see this cloud in data that we didn’t think we would see.”
The data came from a far-ultraviolet spectrograph aboard the Korean satellite STSAT-1. The dataset was publicly released in 2023 – and Burkhart spotted the hidden structure soon after.
“The story of the cosmos is a story of the rearrangement of atoms over billions of years,” Burkhart said.
“The hydrogen that is currently in the Eos cloud existed at the time of the Big Bang and eventually fell onto our galaxy and coalesced nearby to the Sun. So, it’s been a long journey of 13.6 billion years for these hydrogen atoms.”
The findings were published in the journal Nature Astronomy.
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