The Crab Nebula has always fascinated me, albeit amazed me that it doesn’t look anything like a crab! It’s the result of a star that exploded at the end of its life back in 1054 CE, leaving behind what is known as a supernova remnant. Back then the explosion would have been visible to the naked eye, even in daytime. It was thought that the supernova that led to the cloud was from a less evolved star with a core made from oxygen, neon and magnesium. Recent studies by the James Webb Space Telescope reveals that it may actually be the core collapse of an iron rich star.
The Crab Nebula can be found in the constellation Taurus measuring 11 light years across. Deep inside the cloud, which expands at a rate of 1,500 kilometres per second, lies a rapidly rotating neutron star known as a pulsar. It emits a beam of electromagnetic radiation that sweeps across space much like a lighthouse sweeping out across the ocean. It has been the subject of many studies to learn about the dynamics of stellar evolution.
Previous studies have attempted to understand the total kinetic energy of the original explosion based upon the velocity of the expanding cloud. The data suggested that the supernova was relatively low energy so the progenitor star was likely to be in the range of 8 to 10 times the mass of the Sun. If it had been more massive it would have experienced a more violent supernova which would be revealed in higher velocity of the expanding gas cloud. But there was a problem.
Hiramatsu et al. 2023/Sebastian Gomez (STScI)
The observations of the Crab Nebula, particularly the high rotational speed of the pulsar, seemed to conflict with current supernova theory. In the model for lower mass stars like that which was the progenitor star of the Crab Nebula, the oxygen in the core ignites as the core collapses. This process does not have sufficient energy to generate such a fast rotating pulsar.
A team of astronomers have addressed this curiosity using MIRI (Mid-Infrared Instrument) and NIRCam (Near-Infrared Camera) onboard the James Webb Space Telescope to collect data from the Crab Nebula. The team was led by Tea Temim from the Princeton University in New Jersey. They report that the gas composition of the cloud suggests the star may have been more evolved with some iron in the core which could have led to a higher energy supernova than previously thought.
With Webb’s sensitive infrared instruments, the iron and nickel emission lines can be seen with more clarity than ever before. Studying the bright lines in the spectrum of the nebula has allowed a much more reliable estimate of the iron and nickel ratio to be deduced. They found it was a higher percentage compared to the Sun which was expected for a more energetic supernova.
The results are promising but the readings were taken from two small regions of the nebula so to rule out variations across the entire 11 light years further readings are needed. If the data from Webb is representative from the entire nebula then it’s possible one of the mysteries of the nebula may finally be solved.
Source : Investigating the Origins of the Crab Nebula With NASA’s Webb