Milky Way Seen More Clearly Than Ever Thanks To New 3D Map Of The Stars
November 29, 2013

Infinity Can Be Seen At The Center Of Our Galaxy

Alan McStravick for - Your Universe Online

The Milky Way galaxy recently got a little easier to understand. A group of astronomers created a new 3D map of the stars that reside in the center of the Milky Way which shows, more clearly than ever, the bulge located at the core of our galaxy.

Prior to this new representation it was widely believed the stars in the core orbited in a sort of banana shape. However, scientists, publishing this week in the journal Monthly Notices of the Royal Astronomical Society, offer a new suggestion on how these core stars actually orbit. The researchers now believe they actually traverse the heavens in a figure eight-shape, similar to the symbol for infinity or the shape of a peanut.

This is no minor distinction either. Scientists require the most accurate theories of star motion in order to better understand current star movements and also  how the galaxy formed and has evolved. With its signature spiral shape, the Milky Way has a region of stars in its core referred to as the “bar.” In the middle of the bar, so named for its shape, is the “bulge”.

This new insight into star movement was arrived at with the creation of a new mathematical model developed by Alice Quillen, professor of astronomy at the University of Rochester, along with her colleagues.

Our solar system and the trajectories of bodies within it, in comparison to the region of the galaxy explored by this team, is relatively easy to map thanks to our own Sun exerting such gravitational force over its domain. By comparison, the gravitational field in the center of the galaxy, consisting of millions of stars, vast clouds of dust and even dark matter is far more difficult to describe. Quillen and team focused their attention on and near the bulge.

They were able to note how stars, traveling their orbit, would move both above and below the horizontal plane of the bar. Each time they passed the bar, the stars would receive a gravitational push. The team likens it to when you are pushing a child on a swing. As the stars reach their resonance point, a point a certain distance from the center of the bar, the team claims the timing of the pushes exerted on the stars becomes strong enough to push the star higher above the plane.

The amount of force ultimately leads to the stars undergoing two vertical oscillations for each orbital period. It was this fact that led the team to hypothesize that the peanut shell orbits would actually be consistent with the force of the resonance. They also claim this phenomenon is likely responsible for the shape of the bulge, itself resembling 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,” 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.”

In just over 3 weeks, the European Space Agency (ESA) is set to launch their Gaia spacecraft. Gaia was designed to 3D map the stars in the Milky Way and their motions. Once Gaia’s mission is complete, it is expected that astronomers will have a much clearer understanding of the composition, formation and evolution of 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.”

It is important to note that Quillen’s model is but one of many that try to explain how the galactic bulge was formed. Further study by Quillen and other teams of astronomers will likely learn how much the bar has slowed over time. With further knowledge of the speed and velocity of the stars in this region will come a greater likelihood we will definitively understand its evolution.

“One of the predictions of my model is that there is a sharp difference in the velocity distributions inside and outside the resonance,” Quillen said. “Inside – closer to the galactic center – the disk should be puffed up and the stars there would have higher vertical velocities. Gaia will measure the motions of the stars and allow us to look for variations in velocity distributions such as these.”

The model Quillen eventually developed required information on several different variables. To arrive at the conclusion of what occurs at resonance, she first had to have an understanding of the speed of the rotating bar combined with the mass density of the bar.

“Before I could model the orbits, I needed the answer to what I thought was a simple question: what is the distribution of material in the inner galaxy?” Quillen said. “But this wasn’t something I could just look up. Luckily my collaborator Sanjib Sharma was able to help out.”

Sharma was able to determine the speed of the circular orbits and how they changed as distance from the galactic center increased or decreased. This increase and decrease from galactic center is known as the rotation curve. With Sharma providing this key component, Quillen was able then to compute a mass density at resonance. This variable was critical in the formation of her model.

With that information in hand, Quillen then combined the new orbit models with the speed of the rotating bar. This, says Quillen, presented a much more accurate estimate of the mass density 3000 light years from the center of the Milky Way. That distance equates to approximately one eighth the distance from Galaxy center to Earth, where the outer edge of the bulge is located.

The Gaia spacecraft, set for launch on December 19, will be streamed live on the ESA portal.