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Dynamic stars

Think stars don’t move? Think again! When you look into the sky at night, the stars may appear to stay in place. But stars are always on the move – they’re just so far away that the motion is hard to see.

But some stars move in ways that appear too weird to be real. In 2005, astronomer Warren Brown found one of these space oddities. It was located on the outer edge of the Milky Way, the galaxy we live in. The star was speeding away like nothing he’d ever seen.

“It seemed absurd. No star in the galaxy has a speed like that,” Brown remembers. He’s an astronomer with the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. “I thought, oh my goodness, I have something kind of special here.”

No one had ever seen a star like this one. What does a scientist do when the results don’t make sense?

Brown didn’t give up. Instead, he kept looking. And he kept finding more of these strange and speedy stars, called hypervelocity stars. Hypermeans “above” and velocity has to do with speed. He and his team just found six more of these “above speed” stars, bringing the total to 16. And, he says, he’s found even more, and is just waiting for the scientific community to confirm them.

When he found the first hypervelocity star, Brown had a hard time believing his own results. And for good reason. He measured the star’s speed to be about 1.6 million miles per hour. That’s fast enough to escape the galaxy and fly away into deep space. If an airplane could fly that fast, it would be able to circle our planet in about one second.

To make matters worse, the star wasn’t flying around the galaxy, like most stars. Instead, it was flying out, like a pop fly that just keeps going up, up, up, out of the ballpark and out of sight. Not only was the star going faster than anyone had ever seen a star move, it was going in the wrong direction.

Brown estimates that there are about 10,000 hypervelocity stars on their way out of the galaxy. Ten thousand may seem like a lot, until you consider that there are hundreds of billions of stars in the Milky Way. If you were to pick a star at random, the chance that it is a hypervelocity star would be about 1 in 10 million.

Binary stars and black holes

How did the stars get to the outer edge of the galaxy in the first place? To find out, astronomers like Brown are piecing together a galactic puzzle with surprising pieces. One of these pieces is a giant black hole. More pieces come are pairs of stars called binaries. The idea holding these pieces together is gravity.

Gravity is a force that physically attracts any objects with mass. The more mass an object has, the more gravity it has. So more massive objects pull toward them lighter objects. The gravity of the Earth, for example, keeps your feet on the ground. The Earth’s large amount of mass also attracts the moon, keeping it in orbit. But the mass of the sun is large enough that its gravity attracts Earth, keeping it in orbit. The sun’s mass and gravity attracts everything else in our solar system. Gravity exists on smaller scales, too. There is a gravitational force between you and your pencil – or even you and your teacher! (But not enough for your pencil to go into orbit around you.