Before going supernova, the biggest stars in the universe may evolve into a rare Wolf-Rayet star, creating strong stellar winds, majestic nebulae, and the building blocks for new stars.
The James Webb Space Telescope (JWST) recently observed in stunning detail a Wolf-Rayet star, the prelude to a supernova —a vast, cosmic explosion that marks the death throes of a massive star. This brief stage of the impending death of a gigantic star, veiled in a distinctive shroud of gas, was a rare sight for humanity.
The doomed Wolf-Rayet star (pictured above), designated WR 124 and located 15,000 light-years away in the constellation Sagitta, has a mass equivalent to 30 suns, even after shedding material equalling 10 suns in the form of the gas and dust that now surrounds it. WR 124 was spotted in infrared by the powerful space telescope.
A Wolf-Rayet star doesn’t just represent the conclusion to a massive star’s life, but it also perfectly exemplifies the endless cycle of stellar life and death that has molded the development of galaxies over billions of years. This is because the supernova explosion that follows a Wolf-Rayet star enriches the cosmos with material forged by the nuclear furnace at the star’s heart during its lifetime.
This material becomes the building blocks of the next generation of stars, planets, and in the case of our planetary system, even our bodies. Thus, Wolf-Rayet stars are fascinating targets for astronomers who want to study the evolution of the universe as a whole.
According to the Swinburne Center for Astrophysics and Supercomputing, a Wolf-Rayet star is a massive star that is at an advanced stage of its evolution and is in the process of shedding tremendous amounts of mass at an incredibly high rate. This class of star is named after French astronomers Charles-Joseph-Étienne Wolf and Georges-Antoine-Pons Rayet who first classified them in 1867.
The typical size of a Wolf-Rayet star is over 20 times the mass of the sun, and they have temperatures that range around 45,000 degrees Fahrenheit to 90,000 degrees Fahrenheit (or 25,000 to 50,000 degrees Celsius). This is much hotter than our sun, which has an average temperature of around 10,000 degrees Fahrenheit (5,500 degrees Celsius). The luminosity of Wolf-Rayet stars is greater than that of the sun by as much as a million times.
What Is Luminosity?
According to the Australia Telescope National Facility, luminosity “is a measure of the total amount of energy radiated by a star or other celestial object per second. This is therefore the power output of a star.” Luminosity is different than brightness, which is referred to as “apparent brightness” in astronomy. Apparent brightness is how bright a star looks to an observer, which is determined both by luminosity and how far the star is from the observer.
To maintain these incredible temperatures and luminosities, Wolf-Rayet stars must burn through the fuel for nuclear fusion incredibly quickly, thus hastening their destructive demise. When their fuel for nuclear fusion is exhausted the outward pressure that supports massive stars against the inward force of gravity also halts. This causes a complete gravitational collapse that will birth a black hole or a neutron star—also triggering a supernova explosion that blows away what remains of a Wolf-Rayet star’s outer layers.
Yet, even before going supernova, Wolf-Rayet stars are losing this outer material. It is being blown away at a rate of 10 solar masses every million years in powerful solar winds that can travel at speeds as great as 7 million miles per hour. To put this into context, the equivalent of three Earths’ worth of material each year is launched from Wolf-Rayet stars at a speed that is about 3,500 times as fast as a bullet fired from a rifle.
As a result of their high fuel-burning rate and their rapid shedding of outer material, Wolf-Rayet stars are incredibly short-lived, with massive stars lasting around a million years or even as little as just a few 100,000 years.
Types of Wolf-Rayet Stars
Wolf-Rayet stars are believed to start life as O-type stars with at least 25 times the mass of the sun. These high-mass, hot white-blue stars are often seen at sites of intense star formation, likely because as massive stars they don’t live that long, meaning their stellar siblings are still being born around them. O-type stars are commonly found in the spiral arms of galaxies and are responsible for the blue-white coloration of these galactic features.
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Wolf-Rayet stars themselves are distinguished from other stars by their unique spectra, the patterns of light they emit which reveal the chemical compounds they are composed of. The spectra of Wolf-Rayet stars show features created by ionized nitrogen and carbon and other heavy elements at their surface with an absence of hydrogen. These elements would usually be below outer layers of hydrogen, so the spectra of these stars reveal their stripped nature.
Wolf-Rayet stars come in three distinct classes, according to Harvard University, which are based on their spectra: WN, WC, and WO. The spectra of WN stars reveal the presence of dominant nitrogen with some carbon, while WC stars are dominated by carbon and show no signs of nitrogen. WO is a rare class of Wolf-Rayet stars that emit light showing the fingerprint of roughly equal amounts of carbon and oxygen.
How Do We See Wolf-Rayet stars?
The short life of Wolf-Rayet stars contributes to how difficult they are to spot, and astronomers have to be extremely fortunate to catch sight of one. Thus far we’ve sighted just over 500 of these doomed stars in the Milky Way, despite the fact there is an estimated population of between 1,000 and 2,000 in our galaxy.
The high luminosity of Wolf-Rayet stars doesn’t help us spot them because they emit most of this in ultraviolet light (UV), and they aren’t actually very bright at visible wavelengths of light. There are just two Wolf-Rayet stars that can be seen with the unaided eye over Earth.
The first is the closest Wolf-Rayet star to our solar system, Gamma 2 Velorum, which is located around 1,000 light-years away in the southern constellation Vela. This is one of the brightest stars in the sky and is composed of at least four individual stars, one of which is a Wolf-Rayet star in a binary system with a blue supergiant star.
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The other Wolf-Rayet star visible with the naked eye from Earth is Theta Muscae, a three-star system located around 7,400 light-years away in the southern constellation Musca (“the Fly”). The system contains a Wolf-Rayet star and two massive stellar companions.
It isn’t just the short lifetimes of Wolf-Rayet stars and their UV dominance that make these stars difficult to spot, however. The material they blow away also surrounds them, forming a shroud of gas and space dust that obscures them from sight. This dust in itself makes for important and fascinating scientific investigations.
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The Role of Cosmic Dust
Hydrogen is the most common element in Wolf-Rayet stars, but in isolation hydrogen—the universe’s lightest element—can’t form dust, according to NASA’s Jet Propulsion Laboratory. Wolf-Rayet stars have burned through most of their hydrogen, creating helium and then burning this helium to create heavy elements like carbon.
As the outer layers where hydrogen dwells are blown away in powerful stellar winds, these deeper, heavy materials are exposed and can then be shed as space dust, which then surrounds these massive stars. In particular, this cosmic dust is rich in carbon and carbon-based material. But, it isn’t just the composition of Wolf-Rayet dust that is interesting, these dust clouds can be shaped by the stars that sit in them into some fascinating shapes.
For example, in October 2022 the JWST observed a Wolf-Rayet star in a binary system located around 5,000 light years from Earth surrounded by dust that had taken on spectacular and striking features. As the Wolf-Rayet star at its heart shed vast amounts of dust, both it and its stellar companion orbited through the dust, carving out paths. A complete ring is created in this dust roughly once every 8 Earth years. As the dust spread out, this resulted in an incredible spiral pattern.
NASA/ESA/CSA/STScl/JPL-Caltech
By aiming their investigations at the shells of this dusty “bullseye,” scientists can determine how the proximity of the stars to each other affects the amount of dust shed by the Wolf-Rayet star.
The dust around Wolf-Rayet stars in general is also riven through with knots that are created by intense periods of material outflow; this tells the story of historic explosive ejections and is crucial to understanding stellar evolution.
Of course, this dust will eventually be joined by more material from the star’s outer layers after it erupts as a supernova and forms a neutron star or black hole. This material will go on to form large clouds, and dense, cool regions within these clouds will collapse to birth new stars. These stars will be enriched with more heavy elements than their predecessor stars, thanks to the explosive deaths of Wolf-Rayet stars, meaning that the study of the composition of this dust shows how stars have evolved from generation to generation.
ESO/P. Crowther/C.J. Evans, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
What Is the Biggest and Brightest Wolf-Rayet Star?
One of the most striking examples of a Wolf-Rayet star astronomers have discovered thus far is also one of the most massive and luminous stars ever spotted.
RMC 136a1, or R136a1, is located around 163,000 light years from Earth in a region of the Milky Way satellite galaxy, the Large Magellanic Cloud, called the Tarantula Nebula. R136a1 has around 315 times the mass of the sun with a temperature of around 95,000 degrees Fahrenheit (53,000 degrees Celsius). The star’s diameter is so great—around 30 million miles—that according to Nine Planets, it could fit over 4 billion suns within it.
What makes R136a1 really amazing, however, is the fact that it is an incredible 9 million times as luminous as our star, radiating more energy in just 5 seconds than the sun does in an entire Earth year. If this Wolf-Rayet star replaced the sun in our solar system, Earth would be destroyed within a day as the result of its energy output, and the rest of the planets would last only around a week longer.
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It is estimated that R136a1, which was discovered in 1960, is around 1 million years old. That’s incredibly young for a star and in comparison to our 4.6 billion-year-old sun. R136a1 is estimated to have already lost as much as 15 percent of its total mass, or around 50 solar masses. It won’t be long (in cosmic terms at least) until R136a1 goes supernova, leaving a black hole in its place.
Even before its explosive end, how long R136a1 holds on to its distinction as one of the most massive and luminous stars we know of remains to be seen. As the JWST observation of WR 124 demonstrates, there are many of this doomed class of stars yet to be observed in detail.
As astronomers continue to study Wolf-Rayet stars, their observations will also reveal more about how the earliest stars spread the building blocks for the next generation of stars throughout the cosmos, allowing them to piece together the story of stellar life and death that defines the universe. https://www.popularmechanics.com/space/deep-space/a43563625/what-is-a-wolf-rayet-star/?utm_campaign=socialflowFBPOP&utm_source=facebook&utm_medium=social-media&fbclid=IwAR0OXJSd_oAmqF193h7tP7FYQ-Qf06Z1Tm3FsN0Vfh_U1l5nZSV_wAu6y4A