James Webb investigates origins of the mysterious Crab Nebula

20 Jun 2024

An image of the Crab Nebula, captured by the James Webb Space Telescope. Image: NASA, ESA, CSA, STScI, Dr Tea Temim (Princeton University)

By analysing portions of the nebula, a new study suggests that the Crab Nebula could have been created by a rare electron-capture supernova, though further research is needed to rule out other possibilites.

Scientists have used the powerful instruments of the James Webb Space Telescope to learn more about the unusual Crab Nebula.

This nebula is the result of a supernova – a star exploding at the end of its life cycle, which results in a massive cloud of hot gas and space dust. The Crab Nebula is (relatively) nearby, being 6,500 light years away in the Taurus constellation.

The supernova explosion itself was seen on Earth in the 11th century and was bright enough to be seen during the daytime. But NASA says this nebula is also a highly unusual type of supernova aftermath, as it has a strange composition and very low explosion energy.

This nebula has drawn attention from astronomers for centuries as a way to learn more about supernovae. Webb was used last year to uncover the mysteries of this cosmic phenomena. Now, researchers have used readings from Webb to create new interpretations of the Crab Nebula’s origins.

A rarer type of supernova?

The low energy of the Crab Nebula has previously been theorised to have come from an electron-capture supernova – a rare type of explosion from a star that has a less-evolved core made of oxygen, neon and magnesium, rather than the more typical iron core seen in other stars.

But NASA says there are inconsistencies between the electron-capture theory and observations of the nebula itself, such as the rapid motion of the pulsar that astronomers have observed.

To lower the level of uncertainty surrounding the explosion that birthed the Crab Nebula, researchers used Webb’s spectroscopic instruments – which analyse the absorption and emission of light – to hone in on two areas located within the nebula’s inner filaments.

The team used Webb’s mid-infrared instrument (MIRI) to measure the nickel and iron emission lines in these filaments, to get a more reliable estimate of the nebula’s nickel to iron abundance ratio. They found that the ratio was still elevated compared to our own sun, but only modestly so – and much lower in comparison to prior estimates.

The researchers said these revised values are consistent with an electron-capture supernova, but that the results don’t rule out a more traditional iron core-collapse explosion from a similar low-mass star. The results have advanced our understanding of the Crab Nebula, but more studies are needed to distinguish between these two possibilities.

“At present, the spectral data from Webb covers two small regions of the Crab, so it’s important to study much more of the remnant and identify any spatial variations,” said astrophysicist Martin Laming, a co-author of the study. “It would be interesting to see if we could identify emission lines from other elements, like cobalt or germanium.”

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Leigh Mc Gowran is a journalist with Silicon Republic