New estimate for the age of the Milky Way
Starquakes recorded by NASA’s now-defunct Kepler space telescope have helped answer a longstanding question about the age of the ‘thick disc’ of the Milky Way.
The Milky Way — like many other spiral galaxies — consists of two disc-like structures, known as thick and thin. The thick disc contains only about 20% of the galaxy’s total stars and, based on its vertical puffiness and composition, is thought to be the older of the pair.
To find out just how much older, 38 scientists used a method known as asteroseismology — a way of identifying the internal structures of stars by measuring their oscillations from starquakes. The study was led by Dr Sanjib Sharma, from Australia’s ARC Centre of Excellence for All Sky Astrophysics in Three Dimensions (ASTRO 3D) and the University of Sydney, and published in the Monthly Notices of the Royal Astronomical Society.
“The quakes generate soundwaves inside the stars that make them ring, or vibrate,” explained co-author Associate Professor Dennis Stello from ASTRO 3D and UNSW.
“The frequencies produced tell us things about the stars’ internal properties, including their age. It’s a bit like identifying a violin as a Stradivarius by listening to the sound it makes.”
Not that the researchers actually hear the sound generated by starquakes; instead, they look for how the internal movement is reflected in changes to brightness. This age-dating allows them to essentially look back in time and discern the period in the universe’s history when the Milky Way formed; a practice known as galactic archaeology.
“Stars are just spherical instruments full of gas, but their vibrations are tiny, so we have to look very carefully,” said Dr Sharma, who served as lead author on the study.
“The exquisite brightness measurements made by Kepler were ideal for that. The telescope was so sensitive it would have been able to detect the dimming of a car headlight as a flea walked across it.”
The data delivered by the telescope during the four years after it was launched in 2009 presented a problem for astronomers. The information suggested there were more young stars in the thick disc than models predicted.
In 2013, however, Kepler broke down, and NASA reprogrammed it to continue working on a reduced capacity — a period that became known as the K2 mission. The project involved observing many different parts of the sky for 80 days at a time, the first tranche of which represented a rich new source of data for Dr Sharma and his colleagues.
A fresh spectroscopic analysis revealed that the chemical composition incorporated in the existing models for stars in the thick disc was wrong, which affected the prediction of their ages. Taking this into account, the researchers found that the observed asteroseismic data now fell into agreement with model predictions, and calculated that the disc is about 10 billion years old.
“This finding clears up a mystery,” Dr Sharma said. “Earlier data about the age distribution of stars in the disc didn’t agree with the models constructed to describe it, but no-one knew where the error lay — in the data or the models. Now we’re pretty sure we’ve found it.”
According to Assoc Prof Stello, the results provide a strong indirect verification of the analytical power of asteroseismology to estimate ages. He added that additional data still to be analysed from K2, combined with new information gathered by NASA’s Transiting Exoplanet Survey Satellite (TESS), will result in accurate estimates for the ages of even more stars within the disc and this will help us to unravel the formation history of the Milky Way.
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