Around July 4, In 1054, Chinese astronomers recorded a “guest star” that shone so brightly that it was visible in broad daylight for 23 days. The remnants of that ancient supernova now form Crab Nebula, which has long been of great interest to astronomers. Some assume that SN 1054 (as it is now known) is a new, rare type of supernova first described by a physicist some 40 years ago. A team of astronomers has now identified another recent supernova – called SN 2018zd – that meets all the criteria for this new type, according to new work published in a journal Astronomy of nature, thus providing a vital link missing in our knowledge of stellar evolution.
“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 fits.” said co-author Andrew Howell Las Cumbres Observatory (LCO). “This supernova literally helps us decode a thousand-year-old records from cultures around the world. And it helps us connect one thing we don’t fully understand, the Crab Nebula, to another thing we have amazing modern records about, this supernova. In the process, she it teaches us about fundamental physics: how some neutron stars form, how extreme stars live and die, and how the elements we are made of are created and scattered throughout the universe. “
There are two types known supernova, depending on the mass of the original star. The supernova collapse of the iron core occurs with massive stars (larger than 10 solar masses) collapsing so violently that they cause a huge, catastrophic explosion. Temperatures and pressures become so high that the carbon in the star’s core begins to melt. This stops the collapse of the nucleus, at least temporarily, and this process continues, over and over again, with increasingly heavy atomic nuclei. (Most of the heavy elements in the Periodic Table are born in intense furnaces of supernova explosions that were once massive stars.) When the fuel is finally completely depleted, the (by then) iron core collapses into a black hole or neutron star.
Then there is a thermonuclear supernova. Smaller stars (up to about eight solar masses) gradually cool and become dense ash cores known as white dwarfs. If a white dwarf that has run out of nuclear fuel is part of a binary system, it can siphon away matter, adding mass to it until its core reaches a high enough temperature for carbon fusion to occur.
In 1980, Japanese physicist Ken’ichi Nomoto of the University of Tokyo speculated that there might be a third middle type: the so-called “electron-trapping” supernova, in which a star is not heavy enough to produce an iron core – crashing a supernova, yet not light enough to prevent its core from completely collapsing. Instead, such stars stop the fusion process when their nuclei are composed of oxygen, neon, and magnesium. In this scenario, neon and magnesium eat electrons in the nucleus, causing the nucleus to bend under its own weight. The end result is a supernova.
Ever since Nomoto first proposed supernovae to capture electrons, theorists have followed up on his work to identify six key characteristics: stars should have a lot of mass; they should lose most of that mass before the explosion; that mass should have an unusual chemical composition; the resulting supernova should be weak; there should be little radioactive fallout; and the nucleus should contain neutron-rich elements.
SN 2018zd was first discovered in March 2018, at just 31 million light-years in a galaxy known as NGC2146. The team was able to identify the probable star of the ancestor by reviewing archival images taken by the Hubble Space Telescope and the Spitzer Space Telescope. They continued to collect data on SN 2018zd over the next few years. Astronomers from UC Davis provided spectral analysis that proved to be key evidence that this was indeed an electron-catching supernova.
When they combed through the published data on supernovae to date, the team noticed a few that met some of the predicted criteria. But only SN 2018zd marked all six fields. Because of this discovery, astronomers are even more certain that the supernova in 1054 that gave birth to the Cancer Nebula was also a supernova for capturing electrons, even though it happened too long ago to get final confirmation. This would also explain why SN 1054 shone so brightly: It is likely that the ejected matter in the explosion collided with material spilled by its genealogy star – the same thing that happened with SN 2018zd.