WASHINGTON — Observing and studying our own galaxy, the Milky Way, can be a bit like trying to take in the view of an entire forest while standing inside it. But instead of trees obscuring the view, it’s gas and dust.
Astronomers have used stars called Cepheids to create a 3D model of the Milky Way and understand the forces that are warping and twisting its structure.
The Milky Way is a barred spiral galaxy, in which a bar-shaped region at the center is surrounded by a disk of stars and gas clouds that exist mainly in its four spiral “arms.” Our solar system is inside the disk, about 27,000 light-years from the center.
To understand the shape of the Milky Way, astronomers measured the distances of the Cepheids, which allowed them to create a 3D map of the galaxy. The map and accompanying study were published Thursday in the journal Science. The stars used to create the map are called classical Cepheids. These are young stars, between four and 20 times the mass of our sun and 100,000 times brighter. Given their mass and brightness, they probably burn through their fuel quickly and die after a few million years — young for the lifetime of a star.
The supergiant stars pulse regularly, their brightness changing in measurable patterns over hours and days. They act like a lighthouse beacon that cuts through the obscuring dust of our galaxy. The new 3D map relies on the measurements of more than 2,400 Cepheids, many of which were newly identified by data collected from the Optical Gravitational Lensing Experiment survey, known as OGLE. This survey more than doubled the known number of classical Cepheids.
“The OGLE survey is among the largest sky variability surveys, it monitors the brightness of almost two billion stars. OGLE collections of variable stars, including Milky Way Cepheids, are the largest in the world and are used by many researchers for various studies of the Universe,” said study author Andrzej Udalski, principal investigator for OGLE, in a statement.
The map revealed that the Milky Way is being warped by its stars at distances more than 25,000 light-years from its center. “Our map shows the Milky Way disk is not flat. It is warped and twisted,” said study co-author Przemek Mroz, a graduate student at the University of Warsaw, in a statement. “Warping of the Galactic disk has been detected before, but this is the first time we can use individual objects to trace its shape in three dimensions.”
The disk thickness varies as distance increases from the center. And the age of the Cepheids can be measured based on the pulsing patterns. The youngest of the Cepheids are close to the Milky Way’s center, and the oldest are on the edge.
“We found many elongated substructures in the disk composed of stars of similar age. This indicates that Cepheids located there must have formed around the same time in one of the spiral arms. However, Cepheids that were formed in a spiral arm do not currently follow the exact location of that arm, because rotation velocities of spiral arms and stars are slightly different,” study co-author Jan Skowron of the Ohio State University’s Astronomy Department said in a statement.
To model how this distortion happened, the researchers added known instances of star formation in the galaxy’s history during a simulation. Those times of star formation were added specifically to the spiral arms, and then motion and rotation was added to the stars. “The simulated and observed structures are strikingly similar. This shows that our idea about the recent history of the Galactic disk is plausible and can explain the structures we see,” Skowron said.
According to their findings, the distribution of the stars and their gravity is warping the galaxy’s disk to an S-like structure, the researchers said.
In February, a Nature study also measured Cepheids to create a 3D map of the Milky Way to show how it was warping and twisting. Using 1,339 large, pulsating stars to compile a 3D map of the Milky Way, the researchers discovered that the galaxy’s disk of stars is increasingly twisting, most likely due to the spinning of the disk. And the farther the stars are from the center, the more twisted it becomes.
Together, the studies show the importance of Cepheids when creating a map of our galaxy and revealing its changing structure.