Sun’s Ancient Encounter With Two Hot Stars Left A Legacy In The Solar System’s Neighborhood

The puzzling electric charge on clouds of interstellar gas can be explained by the passage of two hot, bright stars close to the Sun. “Close”, in this case, is something of a relative term even by astronomical standards, given these stars were never within our few hundred closest stellar neighbors. Nevertheless, if a new study is right, they still left a mark on our Solar System’s neighborhood.

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The space between the stars is not entirely empty, containing diffuse gas and dust. Some relatively concentrated patches in our sector of the galaxy are known as “local interstellar clouds” and are around 30 light-years wide. Beyond them lies an area known as the “local hot bubble,” where the void of space is even closer to a true vacuum, thought to be the product of a series of supernova explosions powerful enough to push most of the gas from the area.

In an attempt to explain this mixed galactic geography, Professor Michael Shull and co-authors reached the conclusion that most of the credit, or blame, should go to two stars, which we know as Epsilon and Beta Canis Majoris. Despite their names, these are the second and fourth brightest stars in the constellation dominated by Sirius, forming the celestial dog’s legs. To an australopithecine leaving the forests and first seeing an uninterrupted sky, these two stars would have been the brightest objects on moonless nights.

"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 in a statement. 

That’s despite the fact that each was about four times more distant than Sirius at their closest approach, so massive and luminous are the pair.

Today, Epsilon Canis Majoris is 400 light-years from Earth, and Beta Canis Majoris is 500 light-years. However, we have a fair idea of their movements through the galaxy, and that of our own Solar System. Between 4 and 5 million years ago, the paths crossed to the extent of being just 30-35 light-years apart. 

This might seem like a mere historical curiosity, something to make us wish evolution had happened a little faster, but Shull and co-authors think it explains a long-standing mystery about the local interstellar clouds: their ionization. Some of the gas in interstellar clouds has typically been stripped of its electrons, but 20 percent of their hydrogen and 40 percent of the helium in the local clouds have been ionized, which is well above typical rates.

It was the question of this ionization that Shull and colleagues set out to resolve, identifying six sources of radiation powerful enough to make a notable contribution. One source is the legacy of the supernovae that created the surrounding hot bubble. There’s a reason for its name. When most gas was pushed out, the explosions heated up the material left behind, and radiation from metal ions within that gas continues to ionize surrounding material, including in the local clouds.

However, the team concluded the passage of Epsilon and Beta Canis Majoris through the region was at least as large a factor. As B-type stars, this pair is about four times as hot as the Sun, and tens of thousands of times more luminous. Hotter stars emit more of their radiation at shorter wavelengths, including X-rays and high-frequency UV, both of which are capable of ionizing gas.

There was a surprising obstacle to calculating these stars’ migrations: they’re both too bright for the Gaia space telescope to measure their movements properly. However, Gaia’s predecessor, Hipparcos, was more useful in this case, allowing the team to reconstruct their passage.

An even more surprising source of ionization comes from three stars too faint to see with the naked eye, despite now being two to three times closer than the B-type pair. These three are white dwarfs, tremendously hot from when they were main sequence stars, but now just the size of a rocky planet. These, and some other white dwarfs in our vicinity, have been proposed as sources of the ionizing radiation responsible for the local interstellar clouds. However, the authors conclude that even combined, they still had a smaller effect than either Epsilon or Beta Canis Majoris.

Gas clouds near the galactic plane have probably all encountered some high-energy radiation over billions of years, but the ionization is temporary. Eventually, the atoms capture other electrons to neutralize their charge. However, in the vastness of space, that process is slow, so the clouds retain a lot of the ionization induced less than five million years ago.

With masses about 13 times the Sun's, the two Canis Major giants are comfortably over the eight solar mass threshold at which stars end their lives as supernovae. At their closest, each might have posed a threat to Earth, but they are now a safe distance away. By the time they blow a few million years from now, that distance will be even greater. Instead, their passage has offered Earth some protection, with the ionization meaning the local clouds absorb more cosmic radiation than would normally occur.

When they do explore, our descendants will get a brief taste of something even brighter than what our ancestors saw. "A supernova blowing up that close will light up the sky," Shull said. "It'll be very, very bright but far enough away that it won't be lethal."

The study is published in The Astrophysical Journal.