A team of astronomers from MIT has observed evidence that the Universe’s first stars exploded as asymmetric supernovae, strong enough to scatter heavy elements across the early Universe. The findings appear in the Astrophysical Journal.
An artist’s impression of a supernova. Image credit: NASA / CXC / M.Weiss.
Several hundred million years after the Big Bang, the very first stars flared into the Universe as massively bright accumulations of hydrogen and helium gas. Within the cores of these stars, thermonuclear reactions forged the first heavy elements, including carbon, iron, and zinc.
These first stars were likely immense, short-lived fireballs, and astrophysicists have assumed that they exploded as similarly spherical supernovae.
But now astronomers at MIT and elsewhere have found that these stars may have blown apart in a more powerful, asymmetric fashion, spewing forth jets that were violent enough to eject heavy elements into neighboring galaxies. These elements ultimately served as seeds for the second generation of stars, some of which can still be observed today.
“When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner,” said MIT’s Dr. Anna Frebel.
“Only when you have some kind of mechanism, like a jet that can yank out material, can you observe that material later in a next-generation star.”
In 2005, Dr. Frebel and colleagues found that a star called HE 1327-2326 is an ancient, surviving star that is among the Universe’s second generation of stars.
At the time, the star was the most metal-poor star ever observed, meaning that it had extremely low concentrations of elements heavier than hydrogen and helium — an indication that it formed as part of the second generation of stars, at a time when most of the Universe’s heavy element content had yet to be forged.
“The first stars were so massive that they had to explode almost immediately,” Dr. Frebel said.
“The smaller stars that formed as the second generation are still available today, and they preserve the early material left behind by these first stars. Our star has just a sprinkle of elements heavier than hydrogen and helium, so we know it must have formed as part of the second generation of stars.”
An artist’s rendering of how the first stars in the Universe may have looked. Image credit: N.R. Fuller, National Science Foundation.
In 2016, the team used the Cosmic Origins Spectrograph onboard the NASA/ESA Hubble Space Telescope to observe the star.
The astronomers made a list of heavy elements that they suspected might be within such an ancient star, that they planned to look for in the Hubble data, including silicon, iron, phosphorus, and zinc.
“We found that, no matter how we measured it, we got really strong abundance of zinc,” said MIT’s Dr. Rana Ezzeddine.
The researchers then ran over 10,000 simulations of supernovae and the secondary stars that form in their aftermath.
They found that while most of the spherical supernova simulations were able to produce a secondary star with the elemental compositions they observed in HE 1327-2326, none of them reproduced the zinc signal.
As it turns out, the only simulation that could explain the star’s makeup, including its high abundance of zinc, was one of an aspherical, jet-ejecting supernova of a first star.
Such a supernova would have been extremely explosive, with a power equivalent to about a nonillion times that of a hydrogen bomb.
“We found this first supernova was much more energetic than people have thought before, about 5-10 times more,” Dr. Ezzeddine said.
“In fact, the previous idea of the existence of a dimmer supernova to explain the second-generation stars may soon need to be retired.”
The results may shift scientists’ understanding of reionization, a pivotal period during which the gas in the Universe morphed from being completely neutral, to ionized — a state that made it possible for galaxies to take shape.
“People thought from early observations that the first stars were not so bright or energetic, and so when they exploded, they wouldn’t participate much in reionizing the Universe,” Dr. Frebel said.
“We’re in some sense rectifying this picture and showing, maybe the first stars had enough oomph when they exploded, and maybe now they are strong contenders for contributing to reionization, and for wreaking havoc in their own little dwarf galaxies.”
These first supernovae could have also been powerful enough to shoot heavy elements into neighboring ‘virgin galaxies’ that had yet to form any stars of their own.
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Rana Ezzeddine et al. 2019. Evidence for an Aspherical Population III Supernova Explosion Inferred from the Hyper-metal-poor Star HE 1327-2326. ApJ 876, 97; doi: 10.3847/1538-4357/ab14e7
