Looking up on a clear night, it isn’t immediately obvious that about half of the stars shining in the sky are actually double (or more) star systems.
By the numbers, this means that any given star is about twice as likely to have at least one companion.
The most famous naked-eye multiple-star system is the “Horse and Rider” in the Big Dipper, the star — visually, the pair of stars — that give the Dipper’s handle its crook. Astronomers have determined that the fainter of these two stars, Alcor, is itself a double star; and the brighter of the two, Mizar, is a double-double system.
The “Horse and Rider” is a sextuple star system that lies about 83 light-years from the sun.
If a star is part of a binary star system, then both stars orbit each other, going around in a time that depends on the stars’ masses and how far apart they are. The more massive the pair of stars and the closer they are to one another, the more quickly they orbit.
A reasonable separation distance for binary stars is about 1 light-year; a reasonable orbit time is about 25 million years.
How would we know if Sol (the sun) had a companion? Wouldn’t we expect to see a really bright star somewhere in the sky if Sol had a companion?
Not necessarily. The brightest star in the night sky — named Sirius, in the constellation Canis Major — is almost nine light-years away. It is not the closest star we know.
As far as astronomers have been able to determine so far, the star closest to Sol lies in the mostly southern-hemisphere constellation Centaurus.
At a distance of 4.2 light-years, the star named Proxima Centauri is the closest to Sol, yet it cannot be seen with the naked eye. Proxima Centauri can’t be seen because it is a so-called red-dwarf star. It is very cool, with a surface temperature about half that of Sol; and it is very small, only about one-seventh the diameter of Sol.
Sol’s nearest neighbor shines with an intrinsic brightness just 0.2 percent that of Sol.
This gets astronomers wondering: Are there any other stars like Proxima Centauri near the sun?
The idea that Sol has a companion derives from a periodic pattern some scientists claim to see in mass extinction episodes evident in the geologic record.
These scientists claim evidence that every 25 million years or so, life on Earth is challenged by episodes of heavy bombardment by meteors. They claim these episodes are induced by a death-star companion orbiting Sol with the same period.
But searches for Nemesis have not so far been revealing.
Surely, very cool, small, faint stars are abundant in the solar neighborhood; and in the Milky Way Galaxy at large.
If one was about a light-year or so from Sol, we should be able to find it. So far, we haven’t.
The results of a current NASA mission named Wide-field Infrared Survey Explorer should be definitive. It is surveying the entire sky for faint, cool objects; and is about half-way done, not yet finding any star closer than Proxima Centauri.
So, presently, I don’t believe Sol is a binary — with a death-star companion hypothetically named Nemesis that periodically pummels Earth with interplanetary debris.
— By Doug Furton, a member of the physics faculty at GVSU. Send questions and suggestions to firstname.lastname@example.org.