Stars in Milky Way's Bulge Travel in Peanut Shell Shaped Orbit, Study Suggests

A new 3-D map of the stars located at the center of the Milky Way reveals they orbit in a peanut shell or figure-eight shaped orbit, according to a paper published in this week's Monthly Notices of the Royal Astronomical Society.

The orbits were previously believed to be more banana-esque, and the difference is important, according to the report's authors, not only for a better understanding of how today's stars move through our galaxy's center, but how the galaxy formed and evolves.

Led by Alice Quillen, a professor of astronomy at the University of Rochester, the researchers created a mathematical model they say accounts for what appears to be taking place in the Milky Way's center.

Unlike the solar system where most of the gravitational pull can be traced back to the Sun, the gravitational field found in the galaxy's hub is a complex maze dictated by millions of stars, huge dust clouds and dark matter. As the stars trace their orbits, they travel above or below the plane of the bar, a region of stars within the Milky Way named after its shape. In the middle of the bar is the galaxy's bulge, which expands out vertically.

When the stars cross the plane they get an extra push. At a certain point known as the resonance point, the timing of these pushes becomes such that stars are forced up higher above the plane. Like a child on a swing, every time these stars pass this point they are pushed a little higher, eventually forming the edge of the bulge.

While it is known that the resonance at this point means the stars undergo two vertical oscillations for every orbital period, the shape of the orbits in between has been unclear. Using computer simulations, Quillen and her team demonstrated that peanut shell shaped orbits match the effect of this resonance and could be the reason for the bulge's shape, which is also similar to a peanut shell.

"It is hard to look back into the past of our galaxy and know what was there, but simulations can give us clues," Quillen explained. "Using my model I saw that, over time, the resonance with the bar, which is what leads to these peculiarly shaped orbits, moves outwards. This may be what happened in our galaxy."

Ultimately, by understanding the distribution of speeds and the velocities of the stars in the bar and bulge, Quillen hopes to discover how much the bar has slowed down over time and whether the bulge "puffed up all at once or slowly."