The Crab Nebula, the remnant of a supernova observed in 1054 AD by Chinese astronomers, has long been one of the most famous space structures. Now, with the James Webb Space Telescope (JWST), astronomers have imaged the nebula in unprecedented detail. The new data provides insights into the pulsar wind, dusty filaments, and the elemental composition of the nebula.
An iconic structure
The Crab Nebula is one of the most studied objects in the sky, with over 30,000 published papers mentioning it. It lies some 6,500 light-years away in the constellation Taurus, and it’s been observed for almost a thousand years: the supernova that created the structure was observed by Chinese astronomers in 1054 AD. This explosion left behind a star known as the Crab Pulsar.
A pulsar is a rapidly rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles. The Crab pulsar has a diameter of only 28 kilometers or so, but is extremely dense, with a mass equivalent to that of our Sun. It spins at an incredible rate of 30 times per second, emitting beams of electromagnetic radiation that sweep across the sky like a lighthouse.
The Crab Nebula has also produced some of the most iconic space photographs, and it’s also helped researchers understand such systems in greater detail. For instance, the nebula’s current structure can reveal clues about the star and the explosion itself. But studying it in multiple wavelengths can provide even more useful information — and this is where the JWST comes in.
Space in many spectra
One of the key advantages of JWST is its ability to observe in the infrared spectrum. Infrared observations are essential for studying dusty regions of space, as dust absorbs and scatters visible light but emits infrared radiation. The Crab Nebula contains a complex network of dusty filaments. And JWST’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) facilitated detailed study of these filaments.
The telescope’s spectrometer further detected multiple nickel and iron lines in the ejecta filaments. These are crucial for understanding the processes that occurred during the supernova. The data suggest a low-mass iron-core-collapse supernova, refining our understanding of stellar evolution and explosion mechanisms. While the possibility of another type of explosion (an electron-capture explosion) is not entirely ruled out, the observed properties align more closely with a core-collapse event.
Astronomic forensics
Figuring out what happened with the supernova is not an easy feat. In a way, astronomers are like detectives, looking for clues that can help shed light on the past processes.
Supernovae can result from two types of stars: white dwarfs and massive stars. During so-called “Type Ia supernovae”, which happens with a white dwarf, gases accumulate onto a white dwarf until it reaches a stability limit, triggering a runaway nuclear fusion reaction that destroys the star. In contrast, Type Ib/c and Type II supernovae occur when a massive star exhausts its nuclear fuel, leading to the collapse of its core and a subsequent explosion that ejects the star’s outer layers.
The presence of a pulsar in the Crab Nebula affirms that it was formed from a core-collapse supernova, as Type Ia supernovae do not produce pulsars. This was known before this study, but the new data comes in to finesse our understanding of what happened.
New understanding of the nebula
The precise measurements of elemental abundances, combined with updated photoionization models, have refined our knowledge of the supernova mechanism and will help astronomers create better models of such processes.
For instance, the JWST’s high-resolution images have provided a clearer view of the large-scale morphology of the ejecta filaments. The observations show an inner ‘cage’ of bright filaments energized by the synchrotron emission from the pulsar. This structure suggests that the emission expands outward through gaps in the cage, extending beyond the outer edges of the ejecta filaments.
Overall, the JWST’s advanced capabilities have provided a wealth of new data on the Crab Nebula and in turn, this will help us better understand such nebulas in general.
The study was published in the pre-print journal arXiv.
