Around 4.4 million years ago—long before humans existed—two massive, blazing-hot stars swept past our solar system.
They didn’t come close enough to pose any danger, but they came close enough to leave behind a chemical “fingerprint” that astronomers can still detect today in the clouds of gas and dust that surround us.
This discovery comes from new research led by astrophysicist Michael Shull at the University of Colorado Boulder.
His study, published in The Astrophysical Journal, helps explain long-standing mysteries about the space just beyond our solar system and reveals how past cosmic neighbors may have influenced Earth’s environment.
Our solar system sits inside a patchy region filled with “local interstellar clouds”—thin, wispy clusters of hydrogen and helium gas stretching roughly 30 light-years across.
Beyond these clouds lies the “local hot bubble,” a large cavity in the galaxy where gas is much hotter and far less dense. Scientists believe this hot bubble was created by 10 to 20 supernova explosions over millions of years.
The structure of this region matters more than one might think. These clouds help shield Earth from harmful cosmic radiation.
Without them, the surface of our planet could have been exposed to much higher levels of ionizing radiation—potentially affecting the conditions that allowed life to flourish.
To understand how these local clouds formed and evolved, Shull and his colleagues used models to reconstruct what the neighborhood around the sun looked like millions of years ago.
They focused on two hot, bright stars: Epsilon Canis Majoris and Beta Canis Majoris. Today, these stars lie hundreds of light-years away in the constellation Canis Major, forming the “legs” of the Great Dog.
But millions of years ago, they were much closer—only about 30 to 35 light-years from the sun. In cosmic terms, this was a near miss.
These stars are known as B-stars, enormous and extremely hot. They shine at temperatures between 38,000 and 45,000 degrees Fahrenheit, compared to the sun’s comparatively cool 10,000 degrees.
As they passed by, they blasted the surrounding clouds with intense ultraviolet radiation. This radiation ionized the gas—stripping electrons from hydrogen and helium atoms and leaving them with an electrical charge.
This lingering ionization is something astronomers have puzzled over for decades. Observations showed that an unusually high percentage of helium and hydrogen around the solar system was ionized, but no one knew exactly why.
Shull’s study identifies at least six contributors, including three white dwarfs and the hot bubble itself, but the passing B-stars appear to have played a major role.
Modeling this history was a challenge. As Shull puts it, “The sun is moving. Stars are racing away from us. The clouds are drifting away. It’s a jigsaw puzzle where all the pieces are moving.” But by working backward in time, the researchers were able to see how the bright sweep of these two massive stars shaped the local environment.
The effects won’t last forever. Over millions of years, those ionized atoms will grab loose electrons and return to normal. As for Epsilon and Beta Canis Majoris, they are nearing the end of their lives. Each will explode as a supernova in a few million years. The explosions will be bright—spectacular even—but far enough away to pose no threat to Earth.
If future beings are here to see it, the sky will light up with a cosmic firework show born from a pair of stars that once brushed past our solar system and left behind a trace of their passing.
