Jingyi Zhang/NASA
Early human ancestors beheld stars far brighter than any we can see in the night sky today.
The intriguing electrical charge present in interstellar gas clouds can be explained by the passage of two hot stars and bright in the vicinity of the Sun.
“Upcoming”, in this context, is a relative term even by astronomical standards, as these stars have never been among the hundreds of closest stellar neighbors to our system.
Still, if a new , recently published in The Astrophysical Journalis correct, they left a lasting brand in the vicinity of the Solar System.
The space between the stars is not completely empty: Contains diffuse gas and dust. Some relatively concentrated regions in our sector of the galaxy are known as “local interstellar clouds” and are about 30 light-years wide, explains .
In addition to these, there is an area called “local hot bubble”, where the space void comes even closer to an absolute vacuum, presumably the result of a series of supernova explosions powerful enough to expel almost all the gas from that zone.
In an attempt to explain this mixed galactic geography, professor Michael Shull and his colleagues concluded that much of the credit – or blame – should go to two stars, known to us as Epsilon from Beta Canis Major.
Despite the names, these are the second and fourth brightest stars of the constellation dominated by Siriusforming the “legs” of the celestial dog.
The Canis Major constellation seen with the naked eye. The brightest star in the image is Sirius
For an australopithecus leaving the forest and contemplating for the first time a clear sky, these two stars would have been the brightest stars on moonless nights.
“If we go back 4.4 million years, these two stars would have been between four and six times brighter than Sirius is today — standing out largely as the brightest stars in the skydespite each of them being, at its closest point, about four times the distance from Sirius, such was its mass and luminosity”, said Shull, at the University of Colorado.
Currently, Epsilon Canis Majoris is 400 light-years from Earth, and Beta Canis Majoris is 500 light-years away. However, we know their movements through the galaxyas well as that of our Solar System.
Between 4 and 5 million years ago, their paths crossed to the point that they are only 30-35 light years away from each other.
It may seem like a simple historical curiosity, something that makes us wish evolution had happened a little faster, but Shull and his colleagues believe that This explains an ancient mystery on local interstellar clouds: their ionization.
Part of the gas in these clouds has been stripped of its electrons, but 20% of the hydrogen and 40% of the helium in the local clouds are ionized, values much higher than usual.
It was this issue of ionization that Shull and the team set out to solve, identifying six sufficiently intense radiation sources to contribute significantly.
One of these sources is the legacy of supernovae that created the enveloping hot bubble. The name is not by chance: When most of the gas has been expelled, the explosions heat the remaining material and radiation from the metal ions in that gas continues to ionize the surrounding material, including in local clouds.
However, the team concluded that the passage of Epsilon and Beta Canis Majoris through this region was also a major factor. As type B stars, they are about four times hotter than the Sun and tens of thousands of times more luminous.
Hottest stars emit more radiation at short wavelengths, including high-frequency X-rays and ultraviolet, both capable of ionizing gas.
There was an unexpected obstacle when calculating the trajectories of these stars: they are both too bright so that the Gaia space telescope can correctly measure its movements.
However, Gaia’s predecessor, Hipparcos, proved to be more useful in this caseallowing the team to reconstruct the stars’ path.
An ionization source even more surprising comes from three stars too faint to be seen with the naked eyealthough they are currently two to three times closer than the type B pair.
They are white dwarfs, extremely hot when they were main sequence stars, but now about the size of a rocky planet. These, and other nearby white dwarfs, have been identified as sources of ionizing radiation responsible for local interstellar clouds.
However, even together, they had a smaller effect than each of the giants Epsilon or Beta Canis Majoris, the study authors conclude.
Gas clouds close to the galactic plane have probably been exposed to high-energy radiation over billions of yearsbut the ionization is temporary. Over time, atoms capture other electrons and neutralize their charge.
However, In the vastness of space, this process is slowso clouds retain much of the ionization induced less than five million years ago.
Com masses about 13 times greater than that of the Sunthe two giants of Canis Major surpass, without difficulty, the threshold of eight solar masses from which stars end their lives as supernovae.
At its closest point, each could have posed a threat to Earth, but now they are at a safe distance. When they explode, in a few million years, they will be even further away.
On the contrary, its passage provided some protection to Earthas ionization causes local clouds to absorb more cosmic radiation than usual.
When this happens, our descendants will have a brief taste of something even brighter than what our ancestors saw. “A supernova exploding at that distance will light up the sky“, says Shull. “It will be very, very bright, but far enough away that it will not be lethal.”
