Milky Way’s far side reveals some secrets

Beyond the galaxy’s center lie mostly uncharted swaths of space. But now astronomers have found some landmarks: five stars roughly 75,000 light-years from Earth. The discovery should help astronomers map the largely unexplored far side of the Milky Way and understand the nature of the enigmatic dark matter thought to hold galaxies together.

The five stars are all Cepheid variables, whose brightness fluctuates steadily. Astronomers use Cepheids as distance markers because the brighter the star, the slower it pulsates. By measuring a Cepheid’s period and how bright it appears from Earth, astronomers can calculate its distance; if the star is in another galaxy, then researchers know how far away the galaxy is as well.

Michael Feast, an astronomer at the University of Cape Town in South Africa, and colleagues found the stars in data from the Optical Gravitational Lensing Experiment, a telescope in Chile. The instrument records visible light from patches of the sky including in the direction of the center of the galaxy. “It was looking in that part of the sky,” says Feast, “not looking for these stars at all, but happened to find them.”

Feast’s team honed the stars’ locations, reported in the May 15 Nature, by observing the stars in infrared light with a telescope at the South African Astronomical Observatory. Infrared light cuts through intervening interstellar dust more effectively than visible light. Until now, all known galactic Cepheids were no farther than 28,000 light-years away; these five are almost three times as far.

“This is a beautiful piece of classical astronomy,” says Leo Blitz, an astronomer at the University of California, Berkeley, “where you find some stars, get some distances to them and try to infer something.” The stars, which are scattered well above and below the plane of the galaxy, confirm what earlier observations of hydrogen gas had suggested: The outer disk of the galaxy flares like a skirt. In this region, dark matter is thought to provide much of the gravity needed to hold the galaxy together.  Without the dark matter, Feast says, the stars and gas would flare even more. The Cepheids could be a useful laboratory, he says, for understanding how dark matter is distributed throughout the galaxy and what it’s made of. Stars at the outer edge of the Milky Way, where dark matter dominates, are hard to come by, so mapping their locations and tracking their motions could help pinpoint how much dark matter is out there.

Studying the edge of the galaxy is difficult, Blitz says, because the thick screen of dust in the plane of the Milky Way blocks a lot of the light from distant stars. Blitz likens it to trying to get a picture of the back of a house while standing in the front yard: “You just can’t do it.”

Most of what astronomers know about the far side of the galaxy comes from radio observations of hydrogen gas. Researchers use the velocities of the gas to infer distances; those inferences depend on complex calculations of how the galaxy rotates. The key advantage of Cepheids, says Giuseppe Bono, an astronomer at the University of Tor Vergata in Rome, is that they provide distances without relying on such calculations.

Since their discovery in 1908, Cepheids have helped astronomers map the universe. “They are a jack of all trades,” Bono says. In the 1920s, Edwin Hubble used Cepheids to show that what was then called the Andromeda Nebula was not a nebula but rather another galaxy. Hubble also relied on Cepheids to reveal that the universe is expanding uniformly in all directions, which provided the foundations for the Big Bang theory.