The Universe’s First Supernovae May Have Been Powerful, Asymmetric Jets

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When it comes to the history of the universe, figuring out how we got what we currently exist in based on our understanding of what the earliest conditions were is something of a fun scientific puzzle, if the phrase “fun scientific puzzle” translates to “a major field of study you can dedicate a lifetime to solving.” As our understanding of star formation has improved, we’ve separated stars into three major categories based on how much metal they contain.

Based on our current understanding of the Big Bang, the two major elements created were hydrogen (76 percent) and helium (24 percent), with minute traces of lithium and beryllium. Heavier elements did not yet exist. Today, however, elements heavier than beryllium are common. (In astronomy, anything higher on the periodic table than hydrogen or helium is considered a metal.) The question is, how did we get from a post-Big Bang environment in which metals were all but nonexistent, to our current universe?

Reion_diagram

The very first Population III stars may have contributed to the reionization of the universe. The degree to which they contributed is still a matter of research.

The current prevailing theory in astronomy is that the early universe was dominated by enormous stars containing virtually no metals to speak of. These stars are known as Population III stars. They have yet to be directly observed. Population III stars are believed to have been extremely unstable, with a lifespan measured in hundreds of thousands to millions of years. The enormous pressure and conditions at the core of these stars created the higher-numbered elements on the periodic table. When they went supernova, they seeded these metals throughout the universe, planting the seeds of the universe we see around us today.

Population III stars have not yet been directly observed. They existed almost immediately after the Big Bang, and the James Webb Space Telescope is intended to search for evidence of their formation. But we have found potential candidates for their successors, the so-called Population II stars. Population II stars are stars that have far less metal in them than the more recent stars we see around us today but contain more metal than their hypothesized Population III progenitors. One such star is HE 1327-2326, a star with 0.8 solar masses and just 1/400,000 as much iron in its core as our own Sun.

Here’s where things get interesting. Astronomers Rana Ezzeddine and Anna Frebel have conducted a study of how HE 1327-2326 inherited its own metals based on its current spectrographic signature. (Frebel discovered HE 1327-2326 back in 2005.) What makes HE 1327-2326 interesting is that it contains an unusual amount of zinc — far more than the researchers were able to account for using standard, symmetrical supernova models.

“When a star explodes, some proportion of that star gets sucked into a black hole like a vacuum cleaner,” says Anna Frebel, an associate professor of physics at MIT and a member of MIT’s Kavli Institute for Astrophysics and Space Research. “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.”

“This is the first observational evidence that such an asymmetric supernova took place in the early universe,” adds MIT postdoc Rana Ezzeddine, the study’s lead author. “This changes our understanding of how the first stars exploded.”

The idea that supernovae can explode asymmetrically has been proposed as a means of explaining how Type II supernovae often eject a compact object at a high rate of speed. The physics of exactly how these early stars would have supernova’d is uncertain because we don’t have any Population III stars around to examine. Some theories suggest their metal-poor nature allowed them to be much, much larger than stars today, with correspondingly shorter lifetimes. Understanding how they died and the way they distributed material in the process tells us about the conditions of the early universe.

Out of more than 10,000 simulations, none of the symmetrical explosions produced a star with the strong zinc signature HE 1327-2326 exhibits. An asymmetrical supernova with a primary jet, on the other hand, does. The total energy released in the explosion is estimated at roughly a nonillion (that’s 1030) times that of the Hiroshima bomb.

“We found this first supernova was much more energetic than people have thought before, about five to 10 times more,” Ezzeddine says. “In fact, the previous idea of the existence of a dimmer supernova to explain the second-generation stars may soon need to be retired.”

If Frebel and Ezzeddine are correct, some of our assumptions about how the first stars seeded heavy elements may need to be rethought. The explosions that birthed the building blocks of early galaxies may have seeded “pristine” clouds of hydrogen and helium with metals that made it much easier for the next generation of Population II stars to form, contributing to the overall spread of heavy metals across the universe and, eventually, seeding the material that our own Sun coalesced from.

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