In 1054 CE, supernova stellar explosions lit up the sky with enough brightness to be visible from Earth during daytime for 23 consecutive days.Its remains are still visible today as the Crab Nebula. New research provides us with our best understanding of what caused this phenomenon.Astronomers believe that both SN 2018zd (and the 1054 CE supernova) are electron-capture supernovae, a rare type of supernova that is not found alongside type I (thermonuclear), and type II (core collapsing).Although experts have been speculating about the existence of this third type, it has been hard to find physical evidence. SN 2018zd, which is only 31 million light years away, may have unique characteristics that we haven't yet identified.Andrew Howell, an astrophysicist at the University of California, Santa Barbara, (UCSB), says that "this supernova literally helps us decode thousand year-old records from cultures around the world." "It is helping us to associate one thing that we don't understand, the Crab Nebula with another thing that we have amazing modern records of, the supernova."In the process, it is teaching us fundamental physics: How neutron stars are made, how extreme star live and die, and how elements get created and scattered throughout the Universe.Each star is constantly fighting gravity. Stars are either undergoing ongoing fusion or densely packed electrons that prolong their lives. Supernovae are characterized by either an increase of mass leading to a runaway thermonuclear blast (type I) or star exhaustion and its iron core collapsing (type II).Scientists suspected that there was a third possibility. This could happen when electrons from a star's oxygen/neon-magnesium center get smashed into nuclei, which causes it to fall under its own weight. This requires a precise balance, as the star may be too heavy or too light to be pulled into its final moments in this way.Artist impressions of a super-asymptotic, giant branch star and its core. Las Cumbres Observatory; S. WilkinsonScientists knew that an electron-capture supernova would form from a rare, massive super-asymptotic big branch (SAGB star). It also must meet five other criteria: a large mass loss before the supernova phase and unusual chemical composition.Astronomers used archive images from the Hubble Space Telescope that were taken before SN 2018zd exploded as well as more recent readings after explosion to determine that SN 2018zd had all the indicators needed for an electron-capture Supernova, the first one ever recorded."We began by asking the question, "What's this strangeo?" Daichi Hiramatsu (UCSB) is an astrophysicist. "Then, we looked at every aspect of SN 2018zd. We realized that they could all be explained by the electron-capture scenario."Although it was thought that the Crab Nebula was caused by an electron-capture supernova it is much more difficult to piece together the physics behind an explosion that was seen from Earth over a thousand years. New research suggests that the 1054 CE explosion was likely to be a type III explosion. This partly explains the brightness.Because SN 2018zd had exactly the same effect, the team believes that the supernova remnants and the material from the exploding star collided, increasing the supernova's luminosity.Ken Nomoto, an astronomer from Japan's University of Tokyo, made the first prediction of supernovae electron-capture back in 1980. He has seen his hypothesis supported by new discoveries more than 40 years later.Nomoto says, "I'm very happy that the electron-capture Supernova was finally discovered. Which my colleagues and I predicted would exist and have a link to the Crab Nebula forty years ago.""I appreciate your efforts in collecting these observations. This is a great example of the combination theory and observations.Nature Astronomy published the research.
