A solar binary, a system where two stars orbit each other, is actually quite common in our universe. Our closest neighbor, Alpha Centauri, is a binary (actually, it's a triple, with a third red dwarf star out far from the main pair). Star pairs, and double pairs, occur throughout the galaxy in high numbers.
The question becomes, can a planetary system exist in a binary (or higher) system? Or can only the lone stars, like our sun, manage to keep planets in a stable orbit?
The answer is, sometimes. The trouble is that many of the binary systems have stars that are very close to each other, so close that the two gravity wells are overlapping to a huge degree. This makes it highly unlikely that a planet could develop any sort of stable orbit about either sun. If a planet was circling one star, then each time it came near the other star the tidal effects would be large enough to shear the planet apart, or at least pull it out of orbit.
So, to have a planet orbiting a sun that's part of a binary system necessitates that the two suns are a good distance apart. How far apart? Well, there's a lot of math involved, but once it is all said and done it works out that the second sun must be at least five times the distance from the planet to the primary sun.
For example, Alpha Centauri's two main suns are 11 AU (Astronomical Unit: The distance from the Earth to the Sun, about 93 million miles) apart from each other on their closest approach. Meaning that a planet could be orbiting one of the suns as long as it was less than 2 AU from it. So, the Earth could exist in an orbit about Alpha Centauri A or B.
But, as viewed from the surface, the second sun would look a hundred times smaller and fainter than the primary. This is because that while the Earth is one AU from the primary sun, it would be ten AU from the secondary, and light drops off in brightness at the square of the distance (10 x 10 = 100). This is still much brighter than our moon, even during a full moon, and would lend an eerie orange glow to the landscape. The third sun, Proxima Centauri, would just be a little red dot, as its distance is 13,000 AU from the two pairs, and its less bright to begin with.So, as a fiction writer, binaries, trinaries, and more are certainly possible, but one needs to take into account that the planet will be circling a primary sun, and all the other suns will be progressively further out, by a factor of at least five.
But, Professor Thule, what if the second sun is a giant star, you ask? That's certainly another possibility. Suns come in many sizes, from the little red dwarf to the mighty blue super-giant. This implies a chance where a planet circles a medium-yellow star like our sun, but then have a second sun that is a giant, and while farther away, is many times bigger, thereby making it appear as if the two suns are equal in the sky. Now, giant stars have giant gravity wells, and our ballpark factor of five becomes a factor of ten or even twenty.
We can try this using Betelgeuse, a red giant some 400 light years from our solar system. It's a huge sun, 270 million miles across, big enough that if was dropped down in our system the Earth would be inside of it!
Assuming, for the moment, that one put our sun in orbit about Betelgeuse, could you get a double sunset? Yes. Betelgeuse is about 13,000 times brighter than our sun. So, if it was a bit more than a hundred AU from Earth, it would look as bright as our sun. So we have a factor of 100, more than enough to insure that the planet doesn't get pulled apart from the tidal effects of the second sun. Note that the Earth would now be receiving twice as much solar energy, and would therefore get much hotter. So, as a fantasy writer, you'd have to back off both suns a bit so that the total energy is about the same as one sun.
The problem, however, is that giants and super-giants have a tendency to be unstable. Meaning they often change in brightness, or go supernova into oblivion. This would be devastating for a planet's inhabitants.
In summary, a world can be part of a double sun system. In most cases, the second sun would be a dimmer, smaller sun to the primary. But, in a special case, both suns could appear to be the same size.
1 comment:
How about blue stars? Do they react differently than yellow? Say, if we have a blue star instead of a yellow one, how different would the earth be?
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