'Standard candles' reveal Milky Way galaxy's ancient core

POTSDAM, Germany, April 22 (UPI) -- The Milky Way hosts several generations of stars. Finding the really old ones -- the stars that witnessed the birth and early evolution of the galaxy -- isn't easy.

Luckily for astronomers, there exists an ancient class of stars, known as RR Lyrae variables, that flash out signals from the center of the Milky Way like lighthouses. The RR Lyrae variables are called "standard candles" because they send out a single pulse of bright light once daily.

Researchers used the AAOmega spectrograph on the Anglo Australian Telescope to record the velocities of hundreds of stars situated within the ancient halo globular clusters found at the center of the Milky Way. RR Lyrae variables are found only among stars that are at least 10 billion years old.

The latest research suggests RR Lyrae variables are accompanied by many stars closer in age to our own sun. They are metal-rich stars, suggesting they are younger. Because heavy metals are forged by stellar fusion, each generation of stars hosts more metal than the last. The oldest stars, like RR Lyrae variables, are metal-poor.

Hundreds of metal-rich stars gather in the center of the galaxy in a "bar" -- a football-shaped rotation. Hydrogen gas also coalesces and follows a similar path. But the new research, published in The Astrophysical Journal Letters, proves RR Lyrae variables take a divergent path around the Milky War's ancient core.

These more ancient stars exhibit more random patterns of movement, suggesting they formed far from the center of the Milky Way and have migrated over time. Their uniqueness underlines their more ancient origins.

"They account for only one percent of the total mass of the bar," study co-author Juntai Shen, of the Shanghai Astronomical Observatory, said in a news release, "but this even more ancient population of stars appears to have a completely different origin than other stars there, consistent with having been one of the first parts of the Milky Way to form."