Star Movements Have 'Deep Repercussions' for Understanding of Milky Way

Stars at the edge of our home galaxy appear to be moving more slowly than expected, scientists have revealed.

Based on the speed of the stars near the center of the Milky Way, astronomers assumed that the stars near the perimeter of the galaxy would move at a certain speed, but a new paper in the Monthly Notices of the Royal Astronomical Society journal shows that these edge stars are traveling at a slower rate than would be expected.

This implies that the galaxy itself may be structured differently from how scientists first thought, with the core of the Milky Way possibly containing less dark matter and, therefore, being lighter in mass than first assumed.

"There is something fishy going on somewhere, and it's really exciting to figure out where that is, to really have a coherent picture of the Milky Way," study co-author Lina Necib, an assistant professor of physics at MIT, said in a statement.

The Milky Way is a spiral galaxy with huge arms of stars and matter spinning in an enormous whirlpool. Our home galaxy is around 100,000 light-years across, with a single light-year measuring roughly 6 trillion miles. In the 1970s, astronomer Vera Rubin measured the speed of stars across other galaxies. She found that their circular velocity showed surprisingly flat rotation curves—a measure of how fast matter rotates at a given distance from the center of a galaxy—meaning that stars further out moved at around the same speed as those closer to the center.

This was thought to be impossible without another type of invisible matter driving the velocity of stars, which was eventually found to be dark matter.

Dark matter is a type of matter that does not interact with regular matter or the electromagnetic field except by gravity, making it incredibly difficult to observe with our current technology.

Around 27 percent of the known universe is thought to be comprised of dark matter, with only about 5 percent being "normal" matter and 68 percent being dark energy.

"Dark matter is a form of matter that appears to not interact with light, but we know must be there because of its gravitational influence on its surroundings (e.g., stars in galaxies rotate too fast)," Ellen Sirks, a postdoctoral researcher in dark matter at the University of Sydney's School of Physics, told Newsweek. "We also know that there has to be approximately 5-6 times more of it than 'ordinary' matter (stars, planets, gas, etc.)"

Now, the speed of the stars in our home galaxy has been measured, and it doesn't match up with what is seen in other galaxies.

"It turns out it's harder to measure a rotation curve when you're sitting inside a galaxy," Xiaowei Ou, study co-author and astrophysics researcher at MIT, said in the statement.

Data from 2019 gathered by the Gaia satellite showed that up to 81,000 light-years out from the galactic center, there was only a mild decline in star travel speed at the edges of the galaxy. However, new research that includes data up to 100,000 light-years out shows that stars start to travel increasingly slowly at the far reaches of the galaxy.

"What we were really surprised to see was that this curve remained flat, flat, flat out to a certain distance, and then it started tanking," Necib said. "This means the outer stars are rotating a little slower than expected, which is a very surprising result."

Using the new rotation curves of the outer stars, the astronomers determined that dark matter must be distributed differently from how they expected, with the center of the galaxy being less dark matter-dense than first thought.

"This result is in tension with other measurements," Necib says. "Really understanding this result will have deep repercussions. This might lead to more hidden masses just beyond the edge of the galactic disk, or a reconsideration of the state of equilibrium of our galaxy. We seek to find these answers in upcoming work, using high resolution simulations of Milky Way-like galaxies."

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