Stars in Weird Orbits

By Tony Helies • Illustrated by Tom Powers

An alien sunset, as seen from the imaginary planet described in the article. The two "suns" that make up Mizar A are setting. The bright stars in the sky are the three that make up Mizar B. The two white ones are Mizar B1 (left) and Mizar B2. The reddish star is Mizar B3.

Can you imagine a world with two suns? Our world orbits a single star we call the Sun. At night we can see hundreds, sometimes thousands of other stars. But half of those stars aren’t just single stars like our Sun.

They are binary star systems, which are made up of two or more stars orbiting each other. Because they are so far away from us, each system of stars looks like a single point of light.

If you can find the Big Dipper in the sky, you can locate a binary star tonight. In North America, the Big Dipper is visible in the night sky all year long. It is highest in the sky and easiest to see during the evening from February to August.

Go outside and face north. Look for the Big Dipper. The middle star on the “handle” of the dipper is Mizar, an amazing star.

If you have binoculars, use them to look at Mizar. On a clear night, you will see a smaller star next to it. People who have especially sharp vision may be able to see the second star without binoculars.

This star is called Alcor. It looks as if its close to Mizar, but it really isn’t. They are more than one million million miles apart. But because they are so far away from Earth, they look to us as if they are close together. Mizar and Alcor do not orbit each other, so they do not make up a binary star system.

But Mizar actually is such a system. With a good telescope you can see that Mizar is really made up of two stars. These stars, called Mizar A and Mizar B, orbit each other.

There are more surprises in the Mizar star system. With the use of a special instrument that analyzes the color of starlight, scientists learned that the objects they had been calling Mizar A and B are not single stars either. Each of them is really two stars very close together.

One Million Million Miles How far is a million million miles? It is such a long distance that light takes more than two months to travel that far. It’s ten thousand times the distance between Earth and the Sun. Mizar and Alcor are at least that far apart. If each were the size of a grain of salt, the distance between them would be greater than three football fields laid end to end.

And by carefully measuring the motions of those stars, scientists have learned that there is a very dim star orbiting around the two stars of Mizar B! So when you look up at the second star in the handle of the Big Dipper, you are really looking at five stars in one—with Alcor in the distance!

What would it be like to live on a planet in the Mizar binary star system?

Think about living on an Earth-like planet moving along a huge orbit around the two stars that make up Mizar A. (The picture below may help you.) We wouldn’t notice a big difference during the day, since our suns (the two stars of Mizar A) would be close together in the sky and too bright to look at. But our sunsets and sunrises would be spectacular. We would see two fiery disks on the horizon!

Our nights would be even more unusual. For half of our year, our planet would be between the two stars of Mizar A and the three stars of Mizar B. During our nights (when our planet faced away from Mizar A), we would see Mizar B. The three stars of Mizar B would shine in our night sky with the light of fifty full moons!

For the rest of the year, the five stars of Mizar A and B would be on one side of the planet (as shown below). They would all be in the daytime sky, and the night sky would be dark. Only then would we be able to look up and see other stars shining in the sky. Alcor would be the brightest star of all.

We don’t know if planets exist around binary stars. The gravitational forces of multiple suns might pull a planet out of orbit and eject it into space.

But a world with two suns is not impossible. If Jupiter, the largest planet in our solar system, had been one hundred times heavier, it would have evolved into a star. Then we could look up at night and see our second sun—Jupiter—shining down on us.

Look at Mizar tonight. Perhaps someone there is looking back at Earth and wondering what life would be like on a world with only one sun.

The Mizar binary star system, which is made up of two smaller binary star systems—Mizar A and Mizar B. The orbits have been drawn out of proportion to one another to show them all in one picture. Mizar A and Mizar B are really about 35 billion miles apart.| That’s 380 times the distance between Earth and the Sun. The imaginary planet is at a place in its orbit where a person standing on it could see the double sunset shown above.