Understanding how galaxies form is still an open question in astronomy. Researchers think that galaxies grow in a bottom-up paradigm, with smaller objects forming first and then getting progressively bigger. And astronomers have now discovered the most conclusive evidence yet that this is the case.
An international team of astronomers from the Max Planck Institute and University of Virgina has charted the growth of our own galaxy, the Milky Way. They calculated the ages of 70,000 red giant stars halfway across our galaxy up to 50,000 light-years away. They found that the oldest stars were at the center, with younger and younger stars as one moves outwards.
“Close to the center of our galaxy, we see old stars that were formed when it was young and small,” said Melissa Ness, lead author of the study, in a statement. “Further out, we see young stars. We conclude that our galaxy grew up by growing out.”
Calculating the age of stars is not easy. The stars used in this study are red giants, older stars towards the end of their life cycle. Their age can be calculated using their mass, so the team had a huge task in calculating the mass for such a large number of stars.
They looked at stellar oscillations of the red giants using the planet-hunter spacecraft COROT and Kepler, which allowed them to estimate a mass and thus calculate an age. They compared the age estimation from the space telescopes with a different method, spectroscopy, that estimated the age by looking at the abundance of heavy elements such as nitrogen, carbon, and oxygen. Young stars have a smaller fraction of those elements compared to older ones.
The spectroscopy was obtained using the Apache Point Observatory Galaxy Evolution Experiment (APOGEE). “APOGEE is the ideal survey for this work because it can get high-quality spectra for 300 stars simultaneously over a large area of sky,” said Steve Majewski, Principal Investigator of the APOGEE survey and coauthor.
“Seeing so many stars at once means getting spectra of 70,000 red giants is actually possible with a single telescope in a few years’ time.”
Although evidence of bottom-up growth had already been found, this latest piece of research is the most detailed analysis produced on galaxy formation.
“Because we can see so many individual stars in the Milky Way, we can chart its growth in unprecedented detail. This unprecedented, enormous map really is one for the ages,” concluded Ness.
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