An international team of astronomers has discovered the oldest stars ever seen, dating from before the Milky Way Galaxy formed, when the Universe was just 300 million years old.
Chemical makeup of the oldest stars in the Milky Way points to huge explosions called hypernovae. Image credit: ESO.
“These stars formed before the Milky Way, and the galaxy formed around them. These pristine stars are among the oldest surviving stars in the Universe, and certainly the oldest stars we have ever seen,” said team member Louise Howes of the Australian National University.
To find the most pure and therefore oldest stars in the Milky Way, the astronomers sifted through about 5 million stars observed with Australian National University’s SkyMapper telescope.
“The SkyMapper telescope has a unique ability to detect the distinct colors of anaemic stars – stars with little iron – which has been vital for this search,” said team member Prof. Martin Asplund, also from Australian National University.
The astronomers selected 14,000 promising stars to look at in more detail, with a spectrograph on the Anglo-Australian Telescope near Coonabarabran in New South Wales, Australia. The instrument, the AAOmega spectrograph, breaks up the light of the star, much like a prism, allowing the team to make detailed measurements.
Their best 23 candidates were all very metal-poor, leading the team to the more precise instrument, the MIKE high-resolution spectrograph on the 6.5-m Magellan Clay telescope in Chile.
From the new data Howes and co-authors identified nine stars with a metal content less than one-thousandth of the amount seen in the Sun, including one with one-ten-thousandth the amount. The latter, SMSS J181609.62-333218.7, is now the record breaker for the most metal-poor star in the center of our galaxy.
The nine stars, which have been at the very center of our Galaxy for billions of years, contain chemical fingerprints which indicate that the very first stars may have died in spectacular deaths known as hypernovae, which were 10 times more energetic than a regular supernova.
“The stars have surprisingly low levels of carbon, iron and other heavy elements, which suggests the first stars might not have exploded as normal supernovae,” said Howes, who is the lead author of a paper in the journal Nature.
“Perhaps they ended their lives as hypernovae – poorly understood explosions of probably rapidly rotating stars producing 10 times as much energy as normal supernovae.”
However, knowing that the stars have low amounts of metal wasn’t enough to be certain that they formed very early in the Universe. They could be stars that formed much later in other parts of our Galaxy that weren’t as dense, and they are just now passing through the center.
To separate those possibilities, Howes and her colleagues measured distances and used precise measurements of the stars’ movement in the sky to predict how the stars were moving, and where they had been in the past.
They found that while some stars were just passing through, seven of the stars had spent their entire lives in the very center of the Galaxy. Computer simulations suggest that stars like these must have formed in the very early Universe.
“This work confirms that there are ancient stars in the centre of our Galaxy,” said team member Dr Andrew Casey of the University of Cambridge, UK.
“The chemical signature imprinted on those stars tells us about an epoch in the Universe that’s otherwise completely inaccessible. The Universe was probably very different early on, but to know by how much, we’ve really just got to find more of these stars: more needles in bigger haystacks.”
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L.M. Howes et al. Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way. Nature, published online November 11, 2015; doi: 10.1038/nature15747
