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Supernova 2018zd

Discovered third type of supernova – electron capture The first observation of a supernova also clarifies the appearance of the crab nebula

Proved 40 years later: Astronomers first observed an electron capture supernova – one-third, the only type of explosion ever theoretically predicted. A star falls because its massive gravity forces the electrons inside it into the nucleus. This type of supernova can also explain how the famous Crab Nebula formed – a monument to a star explosion visible in the sky even in daylight in 1054.

Two main types of star eruptions are known so far. The massive stars of more than ten solar masses are called an eruption at the end of their life cycle Core collapse supernova. Due to the decreasing nuclear fusion the radiation pressure is no longer sufficient to compensate for the enormous force of gravity, causing the center to collapse and cause an explosion. A Supernova Vom Type Ia Occurs on the other hand in binary star systems with white dwarf. This monument of a star with eight solar masses will absorb matter from its companion until it crosses a boundary and the white dwarf explodes.

The nucleus of a super AGP star consists primarily of oxygen, neon and magnesium. The electrons of these elements are pushed into the nucleus by pressure. © S. Wilkinson / Los Compress Laboratory

What happens to the stars in between?

However, it is not yet clear what will happen to the stars that are exactly between eight and ten solar masses. These stars are too heavy to turn into white dwarfs, but not heavy enough to allow nuclear fusion to continue until iron atoms are formed – just like atomic decay stars. At this point, a supernova model proposed by the Japanese astronomer Kenichi Nomoto about 40 years ago is coming into operation – the electro-capture supernova.


According to this theory, these star explosions occur with so-called super AGP stars – red giants whose core consists primarily of oxygen, neon and magnesium. At this point the fusion inside stops and the pressure inside increases so much that the electrons are pressed into the nucleus. This process, called electron capture, causes the embryo to become unstable and explode in a supernova. So much for theory. However, so far there is no evidence for observations for this type of supernova.

All the characteristics of the long-awaited supernova type

Now this has changed: for the first time, astronomers have discovered such an electron-capturing supernova in space. In March 2018, the first telescopes recorded a flash of explosion in the outer part of a galaxy 31 million light years away. Various laboratories on Earth and in orbit then followed the development of the Supernova 2018zd in two years.

A team led by Daisy Hiramatsu from the Los Compress Laboratory in California has now evaluated the data – and found many more abnormalities. These include strong mass losses before the actual supernova, abnormal composition of the light spectrum, relatively weak eruption and small radiation processes and a nucleus full of neutrons. “All of these properties of the SN 2018zd can be explained by the electron capture supernova,” says Hiramatsu.

In addition, a team of astronomers was able to find images of the previous star compared to older images from the Hubble and Spitzer Space Telescopes. These suggest that the precursor star is actually a super AGP star – a red giant with a distinct oxygen-neon core.

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The “missing link” to star eruptions

According to the research team, all of this suggests that the 2018zd supernova is the long-awaited third type of star eruption. “This is a real‘ Eureka ’moment for us, which can help confirm the now 40-year-old theory,” says Hiramatsu. Observations show for the first time that this type of eruption has been present for a long time and is actually in stars – thus closing the gap between core collapse and type IA supernova.

“I’m very happy that the electron capture supernova has finally been discovered,” commented Kenichi Nomoto, the founder of the theory. “This is a great example of how theory and observation complement each other.”

Cancer Nebula
The Crab Nebula is one of the most famous and most photographed supernova remnants. © NASA / ESA, NROO / AUI / NSF and G. Daphne / University of Buenos Aires

Description of the Crab Nebula

Observations also shed new light on the celebrity Cancer Nebula – The glowing remnant of a supernova visible in the sky in the Middle Ages. This system of luminous gases was formed by a stellar explosion in 1054, about 6,300 light-years away. According to Chinese astronomers, the supernova was visible in the sky for 23 days during the day, and shone at night for two years after the eruption.

But while the remnant of this supernova is one of the most well-educated of all, it is debatable what kind of star eruption it caused. However, the knowledge gained from the 2018zd supernova now helps to assign some features previously difficult to describe to the crab nebula, and suggests that it was created by an electron-capturing supernova.

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The term “Rosetta Stone” is often used as an analogy, but in this case it really does apply, “says Andrew Howell, co-author of the Los Compress Laboratory. It helps us connect the Crab Nebula to a well-documented event like this supernova. ”(Natural Astronomy, doi: 10.1038 / s41550-021-01384-2)

Quell: Los Compress Laboratory Global Telescope, University of California – Davis