About 4.5 million years ago, a great cosmic dog kicked past our Solar System – and its effects may still be seen today.

Astrophysicists calculated that two stars, now located some 400 to 500 light-years from Earth, swung through our neighborhood on their journey through the Milky Way, coming as close as 32 light-years. These stars now make up part of the legs of the constellation Canis Major , the "greater dog."

"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," says Michael Shull, astrophysicist at the University of Colorado (CU) Boulder.

While they've since dimmed as they've retreated into the distance, these stars could have left their mark in other ways. Their intense heat may have ionized

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