Wobbling Stars: Evidence That Gravity Pulls Both Ways

Why Do Stars Wobble?

1Astronomers have found thousands of planets around other stars. Most of these planets are far too dim to see next to their bright stars. One way astronomers find them is by watching the star itself. If a star moves back and forth in a small, regular circle, something unseen may be pulling on it. Eighth graders Saoirse Mbatha and Kenji Halvorsen read about this "star wobble" and made it the subject of their science-fair project.

2The pair used a free orbit simulator from a university website. In the simulator, they could set the mass of a star, the mass of one planet and the distance between them. The program then showed the star and the planet moving around each other. It also reported the pull of gravity between them and the speed at which the star moved in its small circle, called the wobble speed.

3First, Kenji set the planet's sideways speed to zero and pressed start. The planet and the star moved straight toward each other, slowly at first and then faster, until they collided. In every run like this, the two objects moved together and never apart. Then he gave the planet its normal sideways speed. This time, the planet circled the star, and the star moved in a much smaller circle on the side facing away from the planet. Both objects circled a shared center between them.

4Next, the pair ran six trials. They kept the distance between the star and the planet at 150 million km, the same as the distance between Earth and the Sun. They changed only the masses. Star masses are given in Suns, and planet masses in Jupiters, where 1 Jupiter is about 318 times the mass of Earth. To make the pulls easy to compare, they wrote each pull as a multiple of the pull in trial 1.

5For the last part of their poster, Saoirse looked up three real pairs of objects that circle a shared center. She wanted to know whether real moons and planets behave like the simulator, even though nobody can set their masses or rerun them in a lab.