Pulsing stars reveal ancient galactic core

An ancient population of stars has been discovered at the centre of the Milky Way, revealing a view into the formation of the Galaxy.

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The plane of our Galaxy as seen in infrared light by the WISE satellite.
Credit: NOAO/AURA/NSF/AIP/A. Kunder

The central 2,000 lightyears in the Milky Way hosts a group of stars over 10 billion years old that reveal the ancient core of the Galaxy, according to a new study.

A team of astronomers used the AAOmega spectrograph on the Anglo Australian Telescope to observe specimens of a type of star called an RR Lyrae variable at the centre of the Galaxy. Because RR Lyrae stars pulsate with regularity, astronomers can use these pulses to calculate distances within the cosmos. Such objects are known as ‘standard candles’.

RR Lyrae stars are also only found in populations of stars over 10 billion years old, meaning they can be used to pinpoint ancient areas of the Universe.

What shocked the astronomers behind the study was the orbital shape of the RR Lyrae stars. Heavy elements, or metals, are formed in stars, and further generations of stars become more metal-rich as a result. Because of this, older parts of the Milky Way are expected to be lower in metal content. Most of the central regions of our Galaxy are populated by stars rich in metals, and arranged in a bar structure. The team observed that the stars in this bar structure orbit in the same direction around the galactic centre, as does hydrogen gas. But the RR Lyrae stars were found to have random orbits, as though they had formed far from the centre of the Milky Way.

Study co-author Juntai Shen of the Shanghai Astronomical Observatory says: “They account for only one percent of the total mass of the bar, 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."

The team will now seek to measure the metal content of these ancient stars and gain additional information as to their history.


Front image: Hubble peers into the Arches Cluster close to the centre of the Galaxy
Credit: NASA/ESA
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