May 9 2019
The very first stars evolved into the universe as enormously bright aggregations of helium and hydrogen gas several hundred million years following the Big Bang. Inside the cores of these first stars, extreme thermonuclear reactions produced the first heavier elements, such as iron, carbon, and zinc.
Rana Ezzeddine and Anna Frebel of MIT have observed evidence that the first stars in the universe exploded as asymmetric supernova, strong enough to scatter heavy elements such as zinc across the early universe. (Image credit: Melanie Gonick).
These first stars were probably huge, short-lived fireballs, and researchers have supposed that they burst as similarly spherical supernovae.
However, currently, astronomers at MIT and other institutions have discovered that these first stars may have ripped apart in a more asymmetric, powerful manner, throwing up jets that were sufficiently fierce to expel heavy elements into nearby galaxies. Eventually, these elements served as seeds for the next generation of stars, a few of which can be detected even today.
In a paper published in the Astrophysical Journal on May 8th, 2019, the scientists reported a higher abundance of zinc in HE 1327-2326, an ancient, surviving star and one of the second generation of stars in the universe. They presume the star could only have obtained such a huge amount of zinc when an asymmetric explosion of one of the very first stars had enriched its birth gas cloud.
When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner. 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. And we believe that’s exactly what could have happened here.
Anna Frebel, Associate Professor of Physics, MIT
Frebel is also a member of MIT’s Kavli Institute for Astrophysics and Space Research.
“This is the first observational evidence that such an asymmetric supernova took place in the early universe,” added MIT postdoc Rana Ezzeddine, the lead author of the study. “This changes our understanding of how the first stars exploded.”
“A Sprinkle of Elements”
Frebel discovered HE 1327-2326 in the year 2005, a period when the star was the most metal-poor star detected ever, implying that it had very low concentrations of elements that are heavier than helium and hydrogen—indicating that it was formed as part of the second generation of stars, when the majority of the universe’s heavy element content had yet to be formed.
“The first stars were so massive that they had to explode almost immediately,” stated Frebel. “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.”
In May 2016, the researchers could observe the star that orbits close to Earth, merely 5000 light-years away. They gained time on NASA’s Hubble Space Telescope over two weeks and were able to record the starlight over a number of orbits. They made use of the Cosmic Origins Spectrograph, an instrument aboard the telescope, to quantify the minute abundances of different elements inside the star.
The spectrograph is designed with high precision to pick up faint ultraviolet light. Some of those wavelengths are absorbed by certain elements, such as zinc. The researchers made a list of heavy elements that they suspected might be within such an ancient star, that they planned to look for in the UV data, including silicon, iron, phosphorous, and zinc.
“I remember getting the data, and seeing this zinc line pop out, and we couldn’t believe it, so we redid the analysis again and again,” Ezzeddine recalls. “We found that, no matter how we measured it, we got this really strong abundance of zinc.”
A Star Channel Opens
Later, Ezzeddine and Frebel got in touch with their collaborators in Japan, specializing in the development of simulations of supernovae and the secondary stars formed in their aftermath. The scientists ran more than 10,000 simulations of supernovae, each with distinct configurations, explosion energies, and other parameters. They discovered that whereas a majority of the spherical supernova simulations could produce a secondary star that had the elemental compositions observed by the researchers in HE 1327-2326, not even one of them reproduced the zinc signal.
Apparently, the only simulation that was able to give an explanation regarding the composition the star, such as the high abundance of zinc in it, was one of an aspherical, jet-ejecting supernova of a first star. A supernova such as this would have been highly explosive, with a power comparable to nearly a nonillion times (that is, 10 followed by 30 zeroes) that of a hydrogen bomb.
We found this first supernova was much more energetic than people have thought before, about five to 10 times more. In fact, the previous idea of the existence of a dimmer supernova to explain the second-generation stars may soon need to be retired.
Rana Ezzeddine, Study Lead Author and postdoc, MIT
The outcomes of the team may shift researchers’ understanding of reionization, a crucial period during which the gas in the universe transformed from being totally neutral to ionized—a state that enabled the formation of galaxies.
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. 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.
Anna Frebel, Associate Professor of Physics, MIT
It is also possible that these first supernovae were fierce enough to eject heavy elements into nearby “virgin galaxies” that had yet to form any stars of their own.
“Once you have some heavy elements in a hydrogen and helium gas, you have a much easier time forming stars, especially little ones,” stated Frebel. “The working hypothesis is, maybe second generation stars of this kind formed in these polluted virgin systems, and not in the same system as the supernova explosion itself, which is always what we had assumed, without thinking in any other way. So this is opening up a new channel for early star formation.”
The National Science Foundation partially funded this study.