Two types of star explosions have been known for a long time, but there has been no clear observation evidence for a third one. Now, for the first time, astronomers have identified an example of this third type, known as the electron capture supernova. This star explosion, observed in 2018, was triggered because the enormous force of gravity pushed electrons in the star’s core into their atomic nuclei. This caused the star to collapse and thus the supernova. The new data now also shed new light on the famous Crab Nebula – the glowing remnant of a supernova visible in the sky in the Middle Ages.
There are typically two main triggers for a star to explode: In a core collapse supernova, a massive star of more than ten times the mass of the Sun has largely used up its fusion fuel. As a result, the nuclear fusion subsides and there is no longer enough outward radiation pressure to compensate for the gravitational effect of the star material. As a result, the star core collapses and an explosion occurs. The second type, a thermonuclear supernova of type Ia, occurs in binary star systems. When a star with less mass of up to eight solar masses becomes a white dwarf there, it often sucks material from its companion. If it exceeds a mass limit, an explosion occurs. But what happens to stars that are between eight and ten solar masses heavy?
Electron capture collapse
A theory about the end of such medium-weight stars was formulated around 40 years ago by Kenichi Nomoto of the University of Tokyo and others. According to this, these stars initially expand into giants, while the nuclear fusion of atoms in their core fuses until a large part of the core consists of oxygen, neon and magnesium. Then, however, the fusion stops because the pressure in the core is insufficient to initiate the next step, the fusion to form iron atoms. As a result, the radiation pressure decreases and gravity compresses the atoms in the nucleus so strongly that electrons are pushed into the atomic nuclei. This process, known as electron capture, causes the nucleus to become unstable and explode in a supernova. So much for the theory. So far, however, it has not been possible to clearly prove the existence of this type of supernova on the basis of astronomical observations. “No supernovae could be identified as being caused by electron capture – also because the theoretical predictions were partly unclear,” reports Daichi Hiramatsu from the Las Cumbres Observatory in California.
Now, however, it may be the first time that an electron capture supernova has been detected. The stellar explosion was observed in a galaxy 31 million light-years away in March 2018 and was followed up with various telescopes on Earth and in orbit over the next two years. “It gives us a really excellent, complete data set that goes from emanating to fading,” explains co-author Azalee Bostroem of the University of California at Davis. In addition, the team of astronomers succeeded in comparing images of the previous star with older images from the Hubble and Spitzer space telescopes. These suggest that the predecessor star is actually a so-called super AGB star – a red giant with an atypical oxygen-neon core.
From the combination of all the data, the astronomers determined six features that suggest that the 2018zd supernova is an electron capture supernova. In addition to the predecessor star, this also includes severe mass losses before the explosion, an unusual composition of the light spectrum, a comparatively weak explosion as well as little radioactivity and a nucleus rich in neutrons. “All of these properties of SN 2018zd can be explained by an electron capture supernova,” says Hiramatsu.
Explanation also for the Crab Nebula
In the opinion of the astronomers, their observations therefore suggest that the 2018zd supernova is actually this third type of supernovae, which has so far only been theoretically postulated. “That was a real ‘Eureka’ moment for us as we can now help confirm the 40-year-old theory,” says Hiramatsu. Nomoto commented: “I am very pleased that the electron capture supernova has finally been discovered. This is a wonderful example of how theory and observation complement each other. “
The discovery of the electron capture supernova could also shed new light on one of the most famous supernova relics of all: the Crab Nebula. This structure of glowing gases, located around 6,300 light years away in the constellation Taurus, was created by a star explosion in 1054. According to records from Chinese astronomers, this supernova could even be seen in the sky for 23 days during the day, at night the afterglow of the explosion shone for two years. But although the remnant of this supernova is one of the best-studied of all, it has been debated what kind of stellar explosion caused it. However, the knowledge gained through the observation of SN 2018zd now helps to assign some of the previously difficult to explain features of the Crab Nebula and suggest that it was also created by an electron capture supernova. “The term ‘Rosetta Stone’ is often used as an analogy, but in this case it really fits,” says co-author Andrew Howell of the Las Cumbres Observatory. “Because the Supernova 2018zd is now helping us to decode records from all over the world that are thousands of years old. And it helps us to link the phenomenon that we did not understand before, the Crab Nebula, with a well-documented event like this supernova. “
Source: Daichi Hiramatsu (Las Cumbres Observatory, Goleta, California) et al., Nature Astronomy, doi: 10.1038 / s41550-021-01384-2