From NASA
April 21, 2021
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Scientists have found fragments of titanium blasted from a famous supernova. This discovery, made with NASA’s Chandra X-ray Observatory, could be an important step in determining exactly how some giant stars explode.
This work is based on Chandra observations of the remains of a supernova called Cassiopeia A (Cas A), which is located in our galaxy about 11,000 light years from Earth. This is one of the youngest known supernova remnants, around 350 years old.
For years, scientists have struggled to understand how massive stars – those with a mass about ten times that of the Sun – explode when they run out of fuel. This result provides an invaluable new clue.
“Scientists believe that most of the titanium used in our daily lives – like in electronics or jewelry – is made in a massive stellar explosion,” said Toshiki Sato of Rikkyo University in Japan, who led the study, which in the journal Nature appears. “Until now, however, it has never been possible for scientists to capture the moment immediately after the manufacture of stable titanium.”
When the nuclear power source of a massive star runs out, the center collapses under gravity to form either a dense star core called a neutron star or, less commonly, a black hole. When a neutron star is created, the interior of the collapsing massive star ricochets off the surface of the star’s core, reversing the implosion.
The heat of this catastrophic event creates a shock wave – similar to the sonic boom of a supersonic jet – that rushes out through the rest of the doomed star, creating new elements through nuclear reactions. However, in many computer models of this process, energy is quickly lost and the shock wave travels outward, preventing the supernova explosion.
Recent three-dimensional computer simulations suggest that neutrinos – subatomic particles of very low mass – that are created during the creation of the neutron star play a crucial role in driving bubbles as they move away from the neutron star. These bubbles propel the shock wave forward to trigger the supernova explosion.
With the new study from Cas A, the team discovered strong evidence of such a neutrino-driven explosion. In the Chandra data, they found that finger-shaped structures pointing away from the explosion site contain titanium and chromium, which coincides with iron scraps previously discovered with Chandra. The conditions required for the creation of these elements in nuclear reactions, such as temperature and density, are consistent with those of bubbles in simulations that drive the explosions.
The titanium that Chandra found in Cas A and that is predicted by these simulations is a stable isotope of the element, which means that the number of neutrons its atoms contain implies that radioactivity will not convert it to another , lighter element changes. Previously, astronomers had discovered an unstable titanium isotope at various locations in Cas A using NASA’s NuSTAR telescope. Every 60 years, about half of this titanium isotope converts to scandium and then to calcium.
“We have never seen this signature of titanium bubbles in a supernova remnant, a result that was only possible with Chandra’s incredibly sharp images,” said co-author Keiichi Maeda of Kyoto University in Japan. “Our result is an important step towards solving the problem of how these stars explode as supernovae.”
“When the supernova happened, titanium fragments were produced deep inside the massive star. The fragments penetrated the surface of the massive star and formed the edge of the supernova remnant Cas A, ”said co-author Shigehiro Nagataki from the RIKEN cluster for groundbreaking research in Japan.
These results strongly support the idea of a neutrino-driven explosion to explain at least some supernovae.
“Our research could be the most important observation investigating the role of neutrinos in the explosion of massive stars since the detection of neutrinos from supernova in 1987A,” said co-author Takashi Yoshida of Kyoto University in Japan.
Astronomers used Chandra for over a million seconds, or more than 18 days, to observe the time of the supernova Cassiopeia A (Cas A) between 2000 and 2018. The amount of stable titanium that is produced in Cas A exceeds the total mass of the earth.
These results were published in the April 22, 2021 edition of Nature. In addition to Sato, Maeda, Nagataki and Yoshida, the authors of the paper are Brian Grefenstette (California Institute of Technology in Pasadena, California), Brian J. Williams (NASA Goddard Space Flight Center in Greenbelt, Maryland) and Hideyuki Umeda (University) from Tokyo in Japan), Masaomi Ono (RIKEN cluster for groundbreaking research in Japan) and Jack Hughes (Rutgers University in Piscataway, New Jersey).
NASA’s Marshall Space Flight Center administers the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science from Cambridge, Massachusetts and flight operations from Burlington, Massachusetts.
Photo credit: NASA / CXC / RIKEN / T. Sato et al .; NuSTAR: NASA / NuSTAR)
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Last updated: April 23, 2021
Editor: Lee Please
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