(CNN) – A group of astronomers discovered a new type of supernova, or stellar explosion, which offers us a new window into the violent life cycle of stars. The new research, focused on supernova 2018zd, confirms a prediction made by University of Tokyo astronomer Ken’ichi Nomoto more than 40 years ago.
Amateur astronomer Koichi Itagaki of Japan observed supernova 2018zd in March 2018, prompting astronomers to use telescopes to study it about three hours after it occurred. The supernova occurred about 31 million light-years from Earth, and archival images from the Hubble and Spitzer space telescopes allowed scientists to see the faint star before the explosion. This was the first time astronomers were able to see a star like this before and after it went supernova.
The main support that prevents stars from sinking under the weight of their own gravity is the energy of their nuclei.
Normally, supernovae occur in two ways. During a core collapse supernova, also called a gravitational supernova, a massive star (more than 10 times the mass of our Sun) consumes its fuel and the star’s core sinks forming a black hole or dense remnant called a neutron star. The other type is called a thermonuclear supernova, and it occurs when a low-mass stellar remnant called a white dwarf, typically less than eight times the mass of our Sun, explodes after dragging matter from a companion star toward itself.
But what about stars between eight and 10 solar masses, like the star involved in supernova 2018zd? They explode a little differently.
This third type, which had not been observed until now, is called an electron-capturing supernova, and was originally described by Nomoto in 1980. As the star’s core loses fuel, gravitational forces push electrons out of the core and they fuse with atomic nuclei. This sudden drop in the pressure of the electrons triggers a collapse and the star bends under its own weight.
What remains is a dense neutron star with slightly more mass than our Sun.
A study based on the new research is published this Monday in the journal Nature Astronomy.
“One of the main questions in astronomy is comparing how stars evolve and how they die,” said Stefano Valenti, co-author of the study and professor of Physics and Astronomy at the University of California, Davis, in a statement. “There are still a lot of links missing, so this is very exciting.”
Understanding the electron capture supernova
Daichi Hiramatsu, a graduate student at the University of California, Santa Barbara, and Las Cumbres Observatory, led an observing team that collected data on supernova 2018zd for two years after it was first observed.
The more data they collected, the more the researchers realized that it could be the first example of an electron-capturing supernova.
Nomoto’s theory of these supernovae suggests that they carry an unusual chemical signature after they are produced, which the researchers observed in the 2018zd data. It also matched the other five criteria of Nomoto’s theory required for the proposed supernova type. These include a strong mass loss before the supernova, a weak explosion, a small radioactivity, a nucleus rich in elements such as oxygen, neon, and magnesium, and a Super Asymptotic Giant Branch (SAGB) star. in English). These rare SAGB stars are bloated old red giant stars.
“We started by asking ‘what is this weirdo?’ Then we looked at all aspects of SN 2018zd and realized that all of them can be explained in the electron capture scenario, ”Hiramatsu said.
Because these stars exist within a limited mass range, they are not light enough to prevent their cores from collapsing, but neither are they heavy enough to create heavier life-prolonging elements, such as iron.
‘This is the best known case of this interesting category of supernovae that lie between the mass range of the exploding white dwarf and the iron core of a massive star that collapses and then rebounds into an explosion, the so-called core collapse supernovae, “said Alex Filippenko, professor of astronomy at the University of California, Berkeley, in a statement. “This study significantly increases our understanding of the final stages of stellar evolution.”
Filippenko said that the fact that the researchers had access to Hubble images showing the star before and after the explosion helped them confirm the type of supernova that occurred.
According to the researchers, this type of supernova is likely responsible for a nebula that lit up the skies nearly 1,000 years ago.
Creation of the Crab Nebula
In 1054, a supernova occurred in our Milky Way so bright that it could be seen in the sky during the day around the world for 23 days, and it remained visible in the night sky for almost two years.
The result of this supernova was the famous Crab Nebula, a point of fascination for astronomers over the years who now, as a result of the new study, believe was created by an electron-capturing supernova.
Although the Crab Nebula has long been considered the best-known example of an electron-capturing supernova, if it existed, there were some doubts because the event occurred a long time ago.
It is likely that the supernova’s brightness was enhanced by material discarded by the explosion colliding with material previously released from the star, something that was also observed during the 2018zd supernova.
“I am very happy that the electron capture supernova was finally discovered, which my colleagues and I predicted existed and had a connection to the Crab Nebula 40 years ago,” Nomoto said in a statement. “I very much appreciate the great efforts made to obtain these observations. This is a wonderful case of combining observations and theory. ‘
“It was a ‘Eureka moment’ for all of us to be able to contribute to closing the theoretical circle of 40 years ago, and for me personally because my career in astronomy began when I looked at the impressive images of the universe in the institute library, one of which it was the iconic Crab Nebula taken by the Hubble Space Telescope, ”added Hiramatsu.
Astronomers will continue the search to see if they can find more examples of electron-capturing supernovae.
“The term ‘Rosetta Stone’ is used too often as an analogy when we find a new astrophysical object, but in this case I think it is appropriate,” said Andrew Howell, scientist at Las Cumbres Observatory and adjunct professor at the University of California, Santa Barbara, in a statement.
“This supernova is literally helping us decipher millennial records from cultures around the world,” said Howell, who was also involved in the study. “And it’s helping us associate one thing we don’t fully understand, the Crab Nebula, with something else that we have incredible modern records for, this supernova. In the process, he is teaching us about fundamental physics: how some neutron stars are created, how extreme stars live and die, and how the elements that we are made of are created and dispersed throughout the universe. ‘