A Billions of years ago, something in the swirling darkness of space erupted with a fury that obscured the glow of entire galaxies. Eventually, the light of this cataclysm reached the Earth and, in November 2016, it was captured by a group of intrepid humans on the Gaia satellite of the European Space Agency. They discovered that the conflagration was not just an unfathomable energy, but that it looked like a solitary bonfire, it continued to burn, fading so slowly that its glow could still be seen years after its beginning.
It was not a typical supernova, the fireworks at the end of a massive star's life. This event came from a star so gigantic - at least 100 times the mass of our own sun - that his death was different from anything scientists had ever seen. Stars of this size were probably common in the early universe, but they have become extremely rare. Even his location was strange; it appeared 54,000 light-years away from the knot of its dwarf galaxy, far from any place where you could hope to find a mysterious flash.
As reported in The Astrophysical Journal in August, the surprising nature of the phenomenon makes it a strong candidate for one of two highly elusive and hypothetical types of stellar pyres. And because the supermassive star that exploded is an excellent substitute for the ephemeral stars that existed in the early chapters of the universe, the event could help astronomers better understand this chapter of our cosmic history, created there long ago.
At the beginning, SN 2016iet - as the explosion was called - was supposed to be a very bright supernova, nothing unusual at a glance.
But this event was remarkably energetic. "We were waiting, waiting, and waiting for this thing to be gone," said Edo Berger, an astronomer at Harvard University and co-author of the new study. "But every time we went to see the telescope and we watched it again, it was still there, fading so slowly."
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| Figure 1. The Supernova SN 2016iet was found 54,000 light-years away from the host galaxy, raising questions about the initial formation of the star. |
According to the logic of stellar convulsions, such an explosion must have come from such a huge star. Supermassive stars are rare; they burn furiously and are short-lived. "These are the brightest stars in the sky, and like celebrities, they demand our attention," said Brian Fields, an astrophysicist at the University of Illinois, Urbana-Champaign, who did not participate in the event. study.
The team discovered that SN 2016 had taken place in a metal-free area - the brilliant term used by astronomers to refer to anything heavier than hydrogen or helium. Metals in the stars help them reject a significant amount of matter in their lifetime, so that at the end of their life, their mass is generally lower than their beginnings. Stars without metals can not easily lose material and tend to remain supermassive.
The star who made SN 2016iet was certainly prominent. Using stellar explosion models, the team estimates that the star of origin was between 120 and 260 times the mass of our sun when it began to seriously burn its hydrogen fuel.
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| Figure 2. The oldest documented supernova was observed by Chinese astronomers in 185 AD This image combines information from four space telescopes to show how this explosion heated the surrounding gas and dust - X-Ray: NASA / CXC / SAO and ESA; Infared: NASA / JPL-Caltech / B. Williams (NCSU) |
The primitive universe contained virtually no metal, as the stars had not yet made them. The primordial stars, the early metallurgists, made the heavy elements that helped to create future generations of stars and possibly planets. Fields said that these first stars were actually the "first seeds of life". They were also, he added, "one of the first beacons that illuminated the universe and ended the dark ages".
They were probably also supermassive, which makes the star behind SN 2016i and an overview of their appearance and behavior. They may also be responsible for the first black holes, said Stan Woosley, an astrophysicist at the University of California at Santa Cruz. These first black holes may have become the monsters that exist today in the heart of galaxies.
The size of the star will play a determining role in determining the type of calamity of the explosion. It could be one of two types of extremely energetic events that at the moment only exist in theory.
All stars, including our sun, play a gravitational balancing game: an immense star-like gravity attempts to reduce it to a point, but the energy of the thermonuclear furnace at the heart of the star generates an external pressure who pushes her away.
In some of the most supermassive stars, where such furnaces heat to extremely high temperatures, many material-antimatter pairs are created. Part of the energy that would otherwise contribute to the fight against gravity is absorbed by the making of these pairs. Outside pressure can not follow gravity, which dominates and shrinks the star.
Collapse engenders cataclysm. The star contracts so violently and the nucleus burns so vigorously that "with a single pulse, nuclear combustion completely destroys the star," said Woosley, who played a pivotal role in developing the theory of these "supernovas" with pair instability ". probably the most violent thermonuclear explosion in the modern world, "he said. The explosions are so complete that the entire star is erased and there is nothing left to form a black hole.
If a star has a lower total mass but remains massive enough for these annoying pairs to interfere, it contracts and burns, but not aggressively enough to tear. The star bounces, dropping a shell of giant matter moving thousands of kilometers per second into the universe. The process is repeated over time. Newly ejected shells collide with older shells, producing huge bursts of light. In the end, the lost mass is such that the creation of new pairs does not significantly affect the star and it dies in a classic black hole formation scenario.
This is called a "pulsating pair instability supernova". To make one, the star of origin during its hydrogen burning phase must have a mass at least 90 times greater than that of the sun, said Woosley. A pair of supernova with maximum instability requires a star whose mass during its hydrogen combustion phase was 140 solar masses. With a minimum of 120 solar masses, SN 2016iet could match any of these stories. Berger explained that the longer the SN 2016, which continues to produce afterglow, the higher the estimates of the star mass will be.
Yet, no model of stellar ruin fits perfectly.
The event had two brightness spikes, which can represent shells of colliding material. But models say that the time between peaks should be on time scales of several centuries, not 100 days. In addition, if it was a type of heartbeat, said Woosley, the event was too bright too long. The SN 2016iet also lost a lot of mass just a decade before the final explosion, Berger said - too much mass far too late to fit the models.
Another troubling question is how this explosion occurred 54,000 light-years away from the host dwarf galaxy, a region that seems to lack a lot of star building material. "How can a star like this form effectively?" Berger said.
One of the ideas is that the event took place in a galaxy, but we can not see it because it is currently overshadowed by the blast, said Sebastian Gomez, Harvard graduate student and lead author of the new study. The team is now using the venerable Hubble Space Telescope to search for it.
With the difficult to explain characteristics of SN 2016iet, two possibilities exist. The first is that the theoretical models of pairwise instability supernovas must be adjusted to fit the observations. Alternatively, the event is neither type of death, but if so, "it will have to be something really complicated," said Gomez, "or something we do not know.
The team will continue to observe SN 2016iet to see if it's really either type of supernova. In any case, it will probably not be an event as rare as this one.
Fields explained that the large synoptic telescope, capable of seeing all the available sky in just three nights, is currently under construction in Chile. When it is ready, around 2022, it will allow scientists to "see everything that moves, flickers, fires or explodes".
Astronomers will find more supernovas each year than in any human history to date. More animated stars with complicated deaths will also be identified, and our understanding of them will be a giant leap.
Until then, the quest for these hypothetical hypothetical monstrous dead flames. "We know these things must be out there," Woosley said. "At least, I do it."