Our sun had a near miss with two huge, scorching hot stars that 'raced past' around 4.4 million years ago, boffins have discovered. Early human ancestors might have looked up and seen the bright balls of gas searing through the skies, as astronomers believe they were so close they would have been clearly visible from Earth.
The discovery was made when boffins examined a 'scar' left by the event in interstellar clouds - swirling mists of gas and dust just beyond the solar system, reports Space.com.
The research also shines a light on how conditions in that part of interstellar space may have been influential in the evolution of life on Earth.
The two 'intruder stars' are now in the front and rear 'legs' of the constellation Canis Major (the Great Dog) - a whopping 400 light years away from Earth (one light year is about six trillion miles - the distance light travels in one Earth year).
But 4.4 million years ago, they were much closer. The calculations are complicated by the fact that, with the universe constantly expanding, everything is always moving. Even the sun is hurtling through space at 8,000 miles per hour (93,000 km/h) - about 75 times as fast as the speed of sound.
As Michael Shull of the University of Colorado Boulder said in a statement: "It's kind of a jigsaw puzzle where all the different pieces are moving. The sun is moving. Stars are racing away from us. The clouds are drifting away."
Shull and his colleagues studied the two stars - known as Epsilon Canis Majoris, or Adhara, and Beta Canis Majoris, or Mirzam - and found that 4.4 million years ago they would have come within 30 light years of the sun.
That's still a vast distance (around 175 trillion miles), it counts as a near miss in a galaxy that stretches for 105,700 light years.
Shull said: "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."
The two stars are both about 13 times the size of our sun, and much, much hotter - busting the mercury at 45,000 degrees Fahrenheit compared to our sun's much more chilled 10,000 degrees.
When they raced past, they emitted powerful ultraviolet radiation that ripped away electrons from atoms in the local interstellar clouds, a process called "ionization" which left the 'scar' that scientists spotted.
These interstellar clouds, made up mainly of hydrogen and helium, stretch for around 30 light years themselves.
They may have played a crucial role in how life on Earth has evolved, as Schull explained: "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."
Meanwhile, the two 'intruder' stars Epsilon and Beta Canis Majoris are set to provide more exciting cosmic events in the future. Massive stars like these burn through their fuel quicker than our sun, and are set to go supernova in the next few million years.
They are thankfully too distant to pose a threat to Earth - but are close enough that anybody left on the planet will witness a truly spectacular light show.
Shull added: "A supernova blowing up that close will light up the sky. It'll be very, very bright but far enough away that it won't be lethal." The team's research was published at the end of November in The Astrophysical Journal.