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A massive explosion in space solved a thousand-year-old mystery

When the stars aligned: A star that ended its life in a distant galaxy and exploded with great force solved an 11th-century astronomical mystery

Description + supernova image credit: A composite of images from Las Cumbers Observatory and the Hubble Space Telescope of the electron-capture supernova SN2018zd (bright circle on the right), and the galaxy in which it occurred, NGC 2146. Credit: NASA/STScI/J. DePasquale; Las Cumbres Observatory
Composite of Las Cumbers Observatory and Hubble Space Telescope images of the electron-capture supernova SN2018zd (bright circle on the right), and the galaxy where it occurred, NGC 2146. Credit: NASA/STScI/J. DePasquale; Las Cumbres Observatory

A new study with the participation of a researcher from Tel Aviv University discovered a new type of supernova - an electron capture supernova. While such supernovae were theoretically predicted about 40 years ago, so far no convincing case of one has been observed in the universe. Electron capture supernovae are created from the explosions of stars 8-9 times heavier than the Sun. This discovery also sheds light on the mystery of the supernova from 1054 that was observed by ancient astronomers, and whose remains appear today as the Cancer Nebula.

The researcher, Dr. Yair Harkabi, is an astrophysicist from the Sackler Faculty of Exact Sciences at Tel Aviv University. The research was published in the prestigious journal Nature Astronomy.

The violation of the balance between opposing forces

What is a supernova? It is an explosion of a star following the violation of the balance that exists in the stars during their life between two opposing forces.

Gravity tries to shrink every star. Our Sun, for example, is held against such contraction by the existence of nuclear combustion at its core, which produces a pressure that opposes gravity. As long as there is nuclear combustion, gravity will not be able to cause the star to collapse. However, in the end, the nuclear combustion ends, just like a car runs out of gas, and the star collapses. For stars like the Sun, the collapsed core is called a white dwarf. The material is so compressed that quantum forces between the electrons prevent it from collapsing further.

For stars 10 times heavier than the Sun, the quantum forces are not enough to stop the compression, and the core continues to collapse until a neutron star or black hole is formed, accompanied by a huge explosion. In the intermediate mass range, there are stars that, when they collapse, electrons are compressed (or rather, trapped) into the nuclei of atoms. In such a situation, the quantum forces cannot stop the compression and the star collapses and explodes.

So far, two main types of supernovae have been observed: one type is the nuclear explosion of a white dwarf absorbing material from a companion (a nearby star or when two white dwarfs collide). A white dwarf is a compressed core left when a star up to 8 times the size of the Sun ends its life. The second type is an explosion following the collapse of the core of the heavy star 10 times (or more) than the Sun, which has exhausted its nuclear "fuel", and collapses in on itself. Theoretical research in astrophysics hypothesized that stars in the intermediate mass range (between 8 and 10 solar masses) will explode in a different way, related to electron capture.

This is the theory formulated in the 80s by Kenichi Nemoto of the University of Tokyo, and others. Over the past decades, astrophysicists have formulated predictions of what an explosion caused by the capture of electrons in the core of a collapsing star should look like. The star is supposed to lose material of a certain chemical composition in the years before its collapse, and the explosion itself is supposed to be relatively weak, produce little nuclear fallout, and scatter neutron-rich elements.

Supernova of the third type

Yair Harkabi: Photography: Israel Hadari
Yair Harkabi: Photography: Israel Hadari

In the new study, published in Nature Astronomy, shows the supernova SN2018zd discovered in 2018 by amateur astronomer Koichi Itagaki from Japan. Dr. Yair Hakabi from the Department of Astrophysics at Tel Aviv University also participated in the study. This supernova, located in the galaxy NGC 2146, has exactly all the properties expected of an electron capture supernova, which have not been observed in any other supernova so far. Also, since this supernova is relatively close to us - only about 31 million light years from us - the researchers were able to identify the star as it appeared before the explosion in archival images taken by the Hubble Space Telescope. As evidence, the star itself also fits the predictions of stars that are supposed to go through an electron capture supernova, and is different from stars that exploded as supernovas of the known types.

While other previously discovered supernovae have exhibited some of the properties expected of an electron-capture supernova, only SN2018zd exhibits all six of the appropriate properties: a star in the appropriate pre-explosion mass range; loss of material before the explosion; special chemical composition; Relatively weak explosion; Scattering of little nuclear fallout; Scattering of neutron-rich elements. "At first, we wondered what this strange supernova was," says Daichi Hiramatsu of the University of Santa Barbara in California, who led the study. "Little by little, we realized that all the properties of supernovae can be explained using the electron capture scenario."

Cancer Nebula

The Cancer Nebula: These supernova remnants glow with energy from a spinning neutron star driving a vortex of magnetized plasma. Source: NASA.
Cancer Nebula: These supernova remnants glow with energy from a swirling neutron star driving a vortex of magnetized plasma. Source: NASA.

The discovery also sheds light on one of the most famous supernovae of the past. In 1054, a star exploded in our galaxy, the Milky Way. According to Chinese records from that time, the explosion was so bright that it was visible during the day, and cast a shadow at night. The remnants of that supernova, now called the "Cancer Nebula", were studied in depth and found to have an unusual composition. That supernova is thought to have been caused by electron capture, but since it happened almost 1000 years ago, this could not be proven. Now, with the discovery of SN2018zd, the hypothesis that the supernova of the year 1054 was also of the electron capture type is being strengthened.

"It's amazing that using modern tools we are also shedding light on historical events in the universe" says Dr. Harkabi. "Today, with robotic telescopes that scan the sky with unprecedented efficiency, we can discover more and more rare but critical phenomena for understanding the laws of nature, without having to wait another 1000 years between one event and another."

Dr. Harkabi is a member of the global supernova project, and uses the Las Cumbers telescope network to study changing and rare phenomena such as supernovae, neutron star mergers, and the tearing apart of stars by black holes.

Link to the scientific article

More of the topic in Hayadan:

9 תגובות

  1. Collapse and explosion are opposites!
    Collapse is inward, and explosion is outward.
    Please consider this an oxymoron.
    Thanks.

  2. Regarding the mass loss around the collapse. I will explain according to my intuition, but I may be wrong. I am indeed a physicist but in a different field. If someone from the field is reading - I hope they will correct me if necessary. As a matter of fact - when electrons fuse with nuclei, the number of particles in the gas decreases - instead of two, an electron and a nucleus, there is now one - the nucleus and the trapped electron. Following this, the gas will contract under the force of gravity and the compression will cause a rise in temperature and an increase in nuclear activity. This will cause an explosion that will blow some of the material out and some will compress in.

  3. To the question about the loss of the material near the collapse. First of all, a disclaimer - I am indeed a physicist, but not from the field. Now to the point (based on my intuition): when electrons are trapped in nuclei then where there were two particles (nucleus and electron) there is now one particle (the nucleus fused with the electron). So now we have a smaller particle density, and therefore less pressure, and now the star will collapse internally, heat up (the compression of the star causes heating), the heating increases the nuclear activity, this causes an explosion that blows matter out but also compresses matter in the center inward. I hope I'm not wrong, and if I am, I hope someone from the field will correct me.

  4. Maybe there are also irresponsible people there who started an atomic war with advanced technology

  5. Thank you for the clear and clear explanation like a star that even someone like me who does not understand anything on the subject was able to understand. It's just wonderful!

  6. The collapse in a super nova is known after the explosion of a star (with a diameter of up to about 10,000,000 km. and above that creates a collapse into a black hole. For comparison, the diameter of the sun is 1,200,000 km). This answer to the commenter who asked that he did not understand the meaning of "collapse"

  7. Very nice writing except for the use of the word collapse!!! Check the definition of this word!!

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