During 130 days of observation, astronomers discovered for the first time a series of explosions leading up to the star’s final collapse.
For the first time since we observed the Universe, a team of astronomers has managed to take real-time images of the last days of a star and follow its evolution step by step until the moment it exploded as a supernova. It is a red supergiant called SN 2020tlf, 120 million light-years from Earth, and the researchers observed it during its last 130 days of existence, until its final violent collapse.
As explained in an article just published in ‘The Astrophysical Journal’, during that previous period, scientists could see how the dying star emitted intense flashes as huge bubbles of gas exploded low above it.
These intense pyrotechnics before the supernova was a big surprise, as previous observations of red supergiants about to explode showed no trace of these violent emissions.
“This is a major advance in our understanding of what massive stars do moments before they die,” said Wynn Jacobson-Galán of the University of California at Berkeley and the lead author of the study. “Direct detection of pre-supernova activity in a red supergiant star has never been observed before in an ordinary type II supernova. For the first time, we saw a red supergiant star explode!”
The biggest stars in the Universe
In terms of volume, but not mass and brightness, red supergiants are the largest stars in the Universe, hundreds or even more than a thousand times larger than the Sun. Like our star, red supergiants generate energy through the nuclear fusion of elements in their nuclei. But because they are so large, they can forge elements much heavier than the hydrogen and helium that stars normally burn. And as they burn more and more massive elements, the temperature and pressure of their cores increases. Ultimately, as they begin to fuse iron and nickel, these stars run out of energy, their cores collapse and they expel their gaseous outer atmospheres into space in a violent explosion, known as a type II supernova.
The authors of the new study began observing SN 2020tlf in the summer of 2020 when the star began to ‘flicker’ with bright flashes of radiation that the team later interpreted as huge gas bubbles exploding near its surface. Using two telescopes in Hawaii, Pan-STARRS1 at the University of Hawaii Institute for Astronomy and the WM Keck Observatory on Mauna Kea, astronomers followed the dying star’s evolution step by step over 130 days. After which, the star exploded.
Surrounded by gas
In their paper, the researchers detail the observation of a dense cloud of gas surrounding SN 2020tlf at the time of its explosion, likely the same gas it had been expelling for months before. This suggests that the star began to experience violent explosions long before the final collapse of its core, which occurred in the fall of 2020.
“We’ve never seen such violent activity in a dying red supergiant,” says study co-author Raffaella Margutti of UC Berkeley. “We’ve seen it generate very luminous emissions, then collapse and burn, until now.”
The discovery challenges previous ideas of how red supergiants evolve just before they explode. Until now, all-stars of this type observed at the end of their lives remained relatively inactive: they did not show evidence of violent eruptions or luminous emissions, as SN 2020tlf did.
“I am very excited about all the new unknowns that have been unlocked by this discovery,” says Jacobson-Galán. The detection of more cases like SN 2020tlf will have a dramatic impact on the way we define the last months of stellar evolution, uniting observers and theorists in the quest to solve the mystery of how massive stars spend the last moments of their lives.”
