Nearly 4.5 million years ago, two large, hot stars brushed tantalizingly close to Earth's sun. They left behind a trace in the clouds of gas and dust that swirl just beyond our solar system—almost like the scent of perfume after someone has left the room.
That's one finding from new research led by Michael Shull, an astrophysicist at the University of Colorado Boulder, and published Nov. 24 in The Astrophysical Journal.
The study sheds new light on the details of Earth's neighborhood in space.
Earth's solar system is surrounded by what scientists call the "local interstellar clouds." These wispy clumps of gas and dust are made up mostly of hydrogen and helium atoms and stretch about 30 light-years, or roughly 175 trillion miles, from end to end.
Zoom past that and our sun exists in a region of the galaxy known as the "local hot bubble," where gas and dust are relatively scarce.
Shull noted that understanding these features is important because they may have influenced the evolution of life on Earth over millions of years.
"The fact that the sun is inside this set of clouds that can shield us from that ionizing radiation may be an important piece of what makes Earth habitable today," said Shull, professor emeritus in the Department of Astrophysical and Planetary Sciences at CU Boulder.
In the new research, he and his colleagues used a series of equations, or models, to catalogue the forces that have shaped our corner of the galaxy over time.
The group examined two stars in particular: Epsilon and Beta Canis Majoris.
Today, these stars sit in the front and rear legs of the constellation Canis Major, or the "Great Dog." Based on the team's calculations, they likely charged past our sun around 4.4 million years ago at a distance of 30 to 35 light-years, a close brush in cosmic terms.
In the process, those stars, which are much hotter than the sun, emitted powerful ultraviolet radiation. That radiation "ionized" the local clouds, stripping electrons from the hydrogen and helium atoms and leaving them with a positive charge—a mark that scientists can still see today.
"If you think back 4.4 million years, these two stars would have been anywhere from four to six times brighter than Sirius is today, far and away the brightest stars in the sky," Shull said.
Jigsaw puzzle
The research drills down on a mystery that has confounded scientists for decades.
When researchers first began peering at the region of space beyond our solar system decades ago, including with the Hubble Space Telescope, they discovered something strange: Around 20% of the hydrogen atoms and 40% of the helium atoms in the local clouds had been ionized—the amount of ionized helium, in particular, seemed unusually high.
In the current study, Shull and his colleagues set out to inventory the celestial phenomena that may have contributed to that ionization.
The team wound back time to simulate what Earth's neighborhood was like millions of years ago—a difficult task, in part because the sun is barreling through the local gas in the galaxy at a speed of 58,000 miles per hour.
"It's kind of a jigsaw puzzle where all the different pieces are moving," Shull said. "The sun is moving. Stars are racing away from us. The clouds are drifting away."
The group reports that at least six sources may have helped to ionize the clouds around our solar system. They include three small white dwarf stars. The hot bubble itself may also have played a role.
Shull explained that this void in space was likely created by10 to 20 stars going supernova—a bit like blowing bubbles into a glass of milk. Those explosions heated up gas within the hot bubble. These hot gases continue to churn out ultraviolet and X-ray radiation today, which bakes the clouds around Earth's solar system.
Feeling the heat
Epsilon and Beta Canis Majoris likely contributed just as much to the ionization of the sun's local clouds as from the hot gas in the local bubble.
These stars, which today sit more than 400 light-years from Earth, are B-stars, which tend to live fast and hard. Epsilon and Beta Canis Majoris will only burn for 20 million years at most. They are about 13 times more massive than our sun and blaze at about 38,000 and 45,000 degrees Fahrenheit—making the sun, at roughly 10,000 degrees Fahrenheit, look chilly in comparison.
Shull noted that the ionization of the local clouds will likely disappear over millions of years as those positively charged atoms pick up stray electrons in space.
Epsilon and Beta Canis Majoris themselves don't have much time. Shull estimates that these stars will likely spend the last of their fuel and go supernova in the next few million years.
They won't pose any danger to Earth, Shull said, but will produce an impressive light show—if anyone is around to see it.
"A supernova blowing up that close will light up the sky," he said. "It'll be very, very bright but far enough away that it won't be lethal."
Co-authors of the new study include Rachel Curran at the University of North Carolina; Michael Topping at the University of Arizona; and Jonathan Slavin at the Harvard and Smithsonian Center for Astrophysics.
