“These pristine stars are among the oldest surviving stars in the Universe, and certainly the oldest stars we have ever seen,” said astronomer Louise Howes currently at Lund University, who was a member of a 2015 team along with the University of Cambridge who discovered stars that date from before the Milky Way Galaxy formed, when the Universe was just 300 million years old.
The stars, found near the center of the Milky Way, are surprisingly pristine, containing only trace amounts of carbon, oxygen and other heavy elements produced by an even earlier star, which died in an enormous hypernova explosion. “Our Milky Way galaxy formed around them,” she added.
“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, which implies that planets orbiting these ancient objects with low metallicity will have low to zero probability of harboring life. “Perhaps they ended their lives as hypernovae – poorly understood explosions of probably rapidly rotating stars producing 10 times as much energy as normal supernovae.
Three Astrophysical Criteria Governing Habitability
The three key astrophysical criteria governing habitability are found to be intricately linked through the “fundamental metallicity relation” as shown by Sloan Digital Sky Survey observations of more than a hundred thousand galaxies in the local Universe, which showed that metal-rich, shapeless giant elliptical galaxies at least twice as massive as the Milky Way (with a tenth of its star formation rate) can potentially host ten thousand times as many habitable, earth-like planets, making them the most probable “cradles of life” in the Universe.
Fast forward to April 2019, the journal Monthly Notices of the Royal Astronomical Society reported that researchers analyzed a cluster of ancient, dim stars called HP1, located about 21,500 light-years away from Earth in the heart of the Milky Way’s central bulge. Using observations from Chile’s Gemini South telescope and archival Hubble Space Telescope data, the researchers calculated the age of the stars to be roughly 12.8 billion years old — making them some of the oldest stars ever detected in either the Milky Way or the entire observable universe.
“HP1 is one of the surviving members of the fundamental building blocks that assembled our galaxy’s inner bulge,” said Leandro Kerber of the University of São Paulo and Brazil’s State University of Santa Cruz.
Project leader of the 2015 ANU team, professor Martin Asplund, said finding such rare relic stars among the billions of stars in the Milky Way center was like finding a needle in a haystack. “The ANU 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.”
A Window on the Big Bang
After the discovery in 2014 of an extremely old star on the edge of the Milky Way, the team focused on the dense central parts of the Milky Way, where stars formed even earlier. “This primitive star surprises us for its high lithium content, and its possible relation to the primordial lithium formed in the Big Bang,” notes David Aguado, a researcher with the team at the Instituto de Astrofísica de Canarias (IAC) and the University of Cambridge. This discovery, which could give crucial information about the creation of atomic nuclei (“nucleosynthesis”) in the Big Bang, was made at the VLT, at the Paranal Observatory of ESO in Chile.
Similar to Our Sun
This star is similar to our sun, but with a much poorer metal content, less than one thousandth part of that of the solar metallicity. This composition implies that we are dealing with a star which was formed in the first 300 million years of the universe, just after the first supernovae from the most massive stars exploded in our galaxy.
“The lithium content of this primitive star is similar to that of other metal poor stars in the halo of our galaxy, and they define, roughly, a constant value, independent of the value of the metal content of the star,” explains Jonay González Hernández, a Ramón y Cajal researcher at the IAC, a co-author of the article. Lithium, synthesized in the Big Bang, is a very fragile metal which is easily destroyed in the interiors of stars by nuclear reactions at a temperature of 2.5 million degrees or above. The center of our Sun, for example, reaches a temperature of 15 million degrees Celsius or 27 million degrees Fahrenheit.
Long-Term Residents of the Milky Way Center
The team sifted through about five million stars observed with SkyMapper to select the most pure and therefore oldest specimens, which were then studied in more detail using the Anglo-Australian Telescope near Coonabarabran in New South Wales and the Magellan telescope in Chile to reveal their chemical make-up.
The team also demonstrated that the stars spend their entire lives near the Milky Way center and are not just passing through, a further indication that the stars really are the oldest known stars in the Universe.
Image at the top of the page shows a new picture of the Milky Way, created by an international team of astronomers who discovered that the Milky Way’s disc of stars becomes increasingly ‘warped’ and twisted the further away the stars are from the galaxy’s center. “We usually think of spiral galaxies as being quite flat, like Andromeda which you can easily see through a telescope,” says Richard de Grijs, an astronomer from Australia’s Macquarie University.