How Stars Move at the Center of the Galaxy

Two months ago astronomers created a new 3D map of stars at the centre of our Galaxy (the Milky Way), showing more clearly than ever the bulge at its core. Previous explanations suggested that the stars that form the bulge are in banana-like orbits, but a paper published this week in Monthly Notices of the Royal Astronomical Society suggests that the stars probably move in peanut-shell or figure of eight-shaped orbits instead.

The difference is important; astronomers develop theories of star motions to not only understand how the stars in our galaxy are moving today but also how our galaxy formed and evolves. The Milky Way is shaped like a spiral, with a region of stars at the centre known as the “bar,” because of its shape. In the middle of this region, there is a “bulge” that expands out vertically.

In the new paper Alice Quillen, professor of astronomy at the University of Rochester, and her collaborators created a mathematical model of what might be happening at the centre of the Milky Way. Unlike the Solar System where most of the gravitational pull comes from the Sun and is simple to model, it is much harder to describe the gravitational field near the centre of the Galaxy, where millions of stars, vast clouds of dust, and even dark matter swirl about. In this case, Quillen and her colleagues considered the forces acting on the stars in or near the bulge.

As the stars go round in their orbits, they also move above or below the plane of the bar. When stars cross the plane they get a little push, like a child on a swing. At the resonance point, which is a point a certain distance from the center of the bar, the timing of the pushes on the stars is such that this effect is strong enough to make the stars at this point move up higher above the plane. It is like when the child on the swing has been pushed a little everytime he comes round and eventually he is swinging higher. These stars that are pushed out form the edge of the bulge.

The resonance at this point means that stars undergo two vertical oscillations for every orbital period. But what is the most likely shape of the orbits in between? The researchers showed through computer simulations that peanut-shell shaped orbits are consistent with the effect of this resonance and could give rise to the observed shape of the bulge, which is also like a peanut-shell.

Next month the European Space Agency will launch the Gaia spacecraft, which is designed to create a 3D map of the stars in the Milky Way and their motions. This 3D map will help astronomers better understand the composition, formation and evolution of our Galaxy.

“It is hard to look back into the past of our galaxy and know what was there, but simulations can give us clues,” explained Quillen. “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.”

“Gaia will generate huge amounts of data – on billions of stars,” said Quillen. This data will allow Quillen and her colleagues to finesse their model further. “This can lead to a better understanding of how the Milky Way might have evolved into the shape it has today.”

Quillen explained that there are different models as to how the galactic bulge was formed. Astronomers are interested in finding out how much the bar has slowed down over time and whether the bulge “puffed up all at once or slowly.” Understanding the distributions of speeds and directions of motion (velocities) of the stars in the bar and the bulge might help determine this evolution.

How Stars Move at the Center of the Galaxy

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