Stars

Prelude to a Supernova: The James Webb Captures a Rare Wolf-Rayet Star

Massive stars are sprinters. It might seem counterintuitive that stars 100 or 200 times more massive than our Sun could only survive for as few as 10 million years. Especially since smaller stars like our Sun can last 10 billion years. Massive stars have huge reservoirs of hydrogen to burn through, but their massive size means fusion eats through their hydrogen much more quickly.

These massive stars are destined to reach the finish line quickly and explode as supernovae. There’s no other conclusion for them. But before they explode, some of them become Wolf-Rayet stars. That stage doesn’t last long, and the James Webb Space Telescope caught one in the act.

Wolf-Rayet (WR) stars exhibit powerful stellar winds that have blown away much of their mass, their surfaces are enriched with heavy elements, and they’re much hotter than most other stars. Some of them have lost their outer hydrogen layer and are fusing helium and other heavier elements in their cores. WR stars are rare, and though there are different types and sub-classes, they all have one thing in common: they’re stars in transition.

WR 124 is a well-studied Wolf-Rayet star about 15,000 light-years away in the constellation Sagitta. The star is visually stunning and is surrounded by a nebula of expelled material called M1-67. M1-67 is about six light-years across and is about 20,000 years old.

This Hubble Space Telescope image shows the spectacular cosmic pairing of the star Hen 2-427 — more commonly known as WR 124 — and the nebula M1-67 which surrounds it. WR 124 shines brightly at the very centre of this explosive image, and around it, the hot clumps of gas are ejected into space at over 150,000 kilometres per hour. Wolf–Rayet stars are super-hot stars characterized by a fierce ejection of mass. Image Credit: By Judy Schmidt – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=28186676

The James Webb Space Telescope imaged WR 124 as one of its first images in 2022. The JWST’s infrared observing capability revealed more detail in the nebular halo of gas and dust that surrounds the doomed star than other telescopes have. The star’s extreme stellar winds are at work blasting material away into space, creating the short-lived nebula. The beautiful nebula is a warning sign, heralding WR 124’s explosion as a supernova in a few hundred thousand years.

But WR 124’s demise also marks a new beginning. The star and its massive brethren are responsible for the heavy elements in the Universe. Elements like carbon, oxygen, and nitrogen are created by massive stars like WR 124 stars and ejected into the cosmos when they explode as supernovae.

WR 124 and its nebula teeter on the brink of massive—and in astronomical terms—rapid change. While it teeters, it’s an irresistible object for astronomers. Researchers have observed it over the years with multiple telescopes.

In 2016, a paper based on Herschel Space Telescope images of WR 124 showed that it had an initial stellar mass of 32 solar masses. It also showed that the nebula was ejected during a previous phase of the star’s evolution when it was either a Red Supergiant or a Yellow Supergiant.

While the nebula from other WR stars is more uniform, M1-67 is knotted and clumpy, probably from interactions with the interstellar medium. The nebula is both gaseous and dusty, with clumps of material 30 times more massive than Earth. The clumps are so large they would reach from the Sun to Saturn if they were in our Solar System. The gas in M1-67 is moving rapidly and is also extremely hot. It moves at about 160,000 km/h (100,000 mph.) So far, WR 124 has ejected about 10 solar masses of material to create the nebula.

The luminous, hot star Wolf-Rayet 124 (WR 124) sits in the centre of this NASA/ESA/CSA James Webb Space Telescope’s composite image combining near-infrared and mid-infrared wavelengths of light. The star displays the characteristic diffraction spikes of Webb’s Near-infrared Camera (NIRCam) caused by the physical structure of the telescope itself. NIRCam balances the star’s brightness with the fainter gas and dust surrounding it, while Webb’s Mid-Infrared Instrument (MIRI) reveals the nebula’s structure. The nebula’s structure reveals the star’s past episodes of mass loss. Rather than smooth shells, the nebula is formed from random, asymmetric ejections. Bright clumps of gas and dust appear like tadpoles swimming toward the star, and the stellar wind forms tails streaming out behind them. Image Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

A 2008 paper based on Very Large Array (VLA) observations of WR 124 and its nebula found a pair of cavities in the gas surrounding the star. The star is situated in the middle of one of the cavities while the other is offset. Like other cavities around other stars, they result from the bow shock created by the star’s stellar wind. Though they appear disconnected, they’re not. Instead, their unusual arrangement is because of WR 124’s rapid velocity through space, according to the paper.

These images from the Very Large Array show the location and morphology of the two cavities in M1-67. Cavity A is centred on the star, while Cavity B is offset. The arrangement is due to the star/nebula’s high speed through space and the resulting bow shock in the ISM. Image Credit: S. Cichowolski et al. 2008

The massive amount of dust coming from WR 124 is of great interest to scientists. Stars like WR 124 play a role in the Universe’s dust budget, something that researchers are keen to understand more thoroughly. Without dust, there are no planets like Earth and no life. One of the JWST’s science goals is to understand the dust budget more clearly, and the space telescope’s images of Wolf-Rayet stars are part of that effort.

Cosmic dust makes only a tiny contribution to the Universe’s baryonic mass, only about 0.1%. But it plays an outsize role in the Universe’s physics and chemistry. In particular, dust plays an important role in star formation, where it’s sometimes called ‘hydrogen’s wingman.’

When a cloud of gas and dust collapses and forms a star, it all happens inside a whirling maelstrom of matter. Hydrogen atoms find each other and bond together to form molecular hydrogen. But as the cloud collapses, the pressure and the temperature rise and the hydrogen atoms start moving too quickly to bond with one another. Inside all that chaos, the individual atoms have an easier time latching onto a speck of relatively cool, slow-moving dust. Multiple hydrogen atoms find each other on the surface of the dust, where they can bond together into molecular hydrogen, leading to star formation.

This is a two-panel mosaic of part of the Taurus Giant Molecular Cloud, the nearest active star-forming region to Earth. The darkest regions are where stars are being born. The dust grains in the cloud help stars form by providing a surface where individual hydrogen atoms can bond into molecules. Image Credit: Adam Block /Steward Observatory/University of Arizona

Dust plays another role in star formation, too. Once a new young star bursts to life in fusion, its powerful UV radiation can prevent gas in nearby clouds from forming the necessary hydrogen bonds, stopping more new stars from forming. But dust can act as a shield, absorbing UV and emitting it as infrared light. In this way, the UV can’t stop the hydrogen from forming molecules and, eventually, stars.

The problem is there’s a dust budget crisis in cosmology. Observations show that there’s far more dust in galaxies than theories can explain. One of the JWST’s jobs is to shed light on this mystery, and by imaging WR 124 and other WR stars, the telescope should start to explain why dust is so abundant.

Some evidence shows that WR stars could be responsible for this abundance of dust, partly through interactions with binary companions. (WR 124 doesn’t have a binary companion, but it still holds clues to the dust mystery.) But because these stars are so hot and so luminous, it’s difficult to observe the dust in great detail. That’s where the JWST comes in.

“What we refer to as the ‘dust budget crisis’ is the major problem in astronomy of not being able to account for all the dust that’s observed in galaxies, both in the nearby and distant, early universe,” said Ryan Lau of the Japan Aerospace Exploration Agency. “The mid-infrared light that Webb can detect is exactly the wavelength of light we want to look at to study the dust and its chemical composition.” Lau is part of the JWST’s effort to study dust-producing WR stars.

Wolf-Rayet stars are known to be efficient dust producers, and the Mid-Infrared Instrument (MIRI) on the NASA/ESA/CSA James Webb Space Telescope shows this to great effect. In this MIRI image, cooler cosmic dust glows at the longer mid-infrared wavelengths, displaying the structure of WR 124’s nebula. As MIRI demonstrates here, Webb will help astronomers to explore questions that were previously only available to theory, like how much dust stars like this create before exploding in a supernova and how much of that dust is large enough to survive the blast and go on to serve as a building block of future stars and planets. Image Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

“Understanding the formation of dust is critical for us to trace our own cosmic origins,” Lau says. “Webb is one of the most powerful scientific tools ever built in the quest to find answers to these fundamental questions.”

Wolf-Rayet stars have blown away most of their hydrogen, which can’t form dust. Instead, they shed other elements from deeper inside their structure, like carbon, which can form dust. As the JWST gives scientists a better look at WR stars like WR 124, they should gain a better understanding of WR stars and the dust they create and eject into the Universe.

This is a JWST image of another Wolf-Rayet star, WR 140, a part of a binary pair of stars. The rings in this image are episodic ejections of dust from the star. WR 140 is a prototypical example of cosmic dust production. Image Credit: By NASA, ESA, CSA JWST MIRI & Ryan Lau et al.; Processed by Meli thev – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=121325992

JWST’s images of WR 124 are snapshots in an ever-changing view of the massive star. When it eventually explodes as a supernova, it’ll be similar to stars that exploded in the early Universe. Those stars seeded the Universe with the heavy elements necessary for rocky planets to form and for life to eventually arise. Maybe one day, somewhere in the Milky Way, future life can trace its beginnings back to stars like WR 124.

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Evan Gough

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