An ancient star has revealed traces thought to be from one of the Milky Way’s first supernovae. The discovery may help us understand the nature of the giant stars that once populated the universe.
The first stars were formed almost entirely from hydrogen and helium produced from the Big Bang. These stars created carbon and oxygen and, when the largest exploded as supernovae, made the other elements in the periodic table. These then became incorporated in a subsequent generation of stars and their planets.
The cycle has run several times in the inner parts of galaxies since the universe formed, but out on the galactic edges the pace is slower. Now a star, SDSS J001820.5-093939.2, has been found whose make-up bears the fingerprint of being formed from the debris of one of the original stars.
SDSS J001820.5-093939.2 has only 1/300th the sun’s iron, a hallmark of stars born before several cycles of star formation had taken place. More unusually, there are substantial differences in the quantities of elements neighboring each other on the periodic table, such as cobalt and nickel or scandium and titanium. In each case there is more of the even numbered element than the odd one next door.
SDSS/NAOJ. SDSS J001820.5-093939.2 looks like an ordinary, if lonely, star at first glance.
Writing in Science Wako Aoki of the Japanese National Astronomical Observatory points out, “Such features have been predicted by nucleosynthesis models for supernovae of stars more than 140 times as massive as the Sun.”
The elemental rations suggest that SDSS J001820.5-093939.2 is made up entirely from the remnants of a single massive star combined with hydrogen and helium that had not been through a previous round of star formation. Even very old stars, usually get their heavier elements from a mix of previous suns. It appears the star that forged SDSS J001820.5-093939.2’s metals was the only one to have lived and died in that region when SDSS J001820.5-093939.2 formed..
The finding sheds light on how big the largest of the early stars were. Today the only known stars over about 150 solar masses are believed to have formed through the merger of several stars, rather than being formed at this size.
However, some theories suggest that in the early universe it was possible for stars to form at 200 solar masses. Detailed analysis of the elements that make up SDSS J001820.5-093939.2 may help us learn how large its predecessor was. It appears the final catastrophe was a pair-instability explosion 10-100 times as bright as normal supernovae. Early estimates are 130-260 solar masses, and there is speculation some of the first generation of stars might have contained 1000 times the mass of the sun.
Usually, astronomers need to look into other galaxies, or at least to globular clusters many thousands of lights away to gain hints to the early universe. However, SDSS J001820.5-093939.2 lies below the galactic plane at a relatively modest distance of 1000 light years. Appropriately, this giant, long-dead star lay in what we call Cetus, the whale.