Our solar system is located in an offshoot of one of the Milky Way's spiral arms, so we don't have a clear sense of what our galaxy looks like edge on. Now, two groups of astronomers have pooled their data to create the most detailed map yet of the galaxy's core — and its shape comes as a surprise.
The galactic bulge, which contains a supermassive blackhole at the core, is a cloud at the center that consists of about 10,000 million stars. It encompasses an expanse thousands of light-years across. We're about 27,000 light-years from the bulge, and our vantage point is heavily obscured by dense clouds of dust. The only way we can really observe it is by looking at the wavelengths of light, like infrared radiation, which can pierce through the clouds of dust.
Previous efforts to observe the galactic core revealed an X-shaped structure. But the new observations, which were conducted by a group from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany and Pontificia Universidad Católica de Chile, Santiago, Chile/ESO, Santiago, Chile, reveal a shape that resembles a peanut shell or saddle.
The Max Planck Institute group conducted its work by using the VISTA telescope at ESO's Paranal Observatory in Chile. Their scans picked up a total of 22 million stars belonging to a class of red giants.
To nail down the bulge's exact shape, the Chilean team, led by Ph.D. student Sergio Vásquez, compared images taken 11 years apart with the MPG/ESO 2.2-meter telescope. This allowed his team to measure tiny shifts made by the motions of the bulge stars across the sky. This data was combined with measurements of the motions of the same stars moving towards and away from Earth. They then mapped out the motions of more than 400 stars in three dimensions.
“This is the first time that a large number of velocities in three dimensions for individual stars from both sides of the bulge [has] been obtained,” noted Vásquez in an ESO statement. “The stars we have observed seem to be streaming along the arms of the X-shaped bulge as their orbits take them up and down and out of the plane of the Milky Way."
Vásquez says that it all fit very well with predictions made from state-of-the-art models.
Scientists theorize that our Galaxy was originally a pure disc of stars that formed into a flat bar billions of years ago, but that the inner section then buckled to form the three-dimensional peanut shape.