How do stars die? The astronomers’ mantra is “Mass determines fate.”
The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority (80%) of stars today are M-class stars, with only 1-in-800 being an O-class or B-class star massive enough for a core-collapse supernova. Our Sun is a G-class star, unremarkable but brighter than all but ~5% of stars, by number. Earlier on, when there were no heavy elements, virtually all of the stars that formed were O-and-B stars: the hottest, bluest, most massive type.
Credit: LucasVB/Wikimedia Commons; Annotations: E. Siegel
If you’re born with over 8 solar masses, you’re
fated for a core-collapse supernova.
The anatomy of a very massive star throughout its life, culminating in a Type II (core-collapse) Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. The most massive core-collapse supernovae typically result in the creation of black holes, while the less massive ones create only neutron stars.
Credit: Nicolle Rager Fuller/NSF
Below that threshold, you’ll only form a white dwarf when you’ve exhausted your core’s fuel.
When our Sun runs out of fuel, it will become a red giant, followed by a planetary nebula with a white dwarf at the center. The Cat’s Eye nebula is a visually spectacular example of this potential fate, with the intricate, layered, asymmetrical shape of this particular one suggesting a binary companion. At the center, a young white dwarf heats up as it contracts, reaching temperatures tens of thousands of Kelvin hotter than the surface of the red giant that spawned it. The outer shells of gas are mostly hydrogen, which gets returned to the interstellar medium at the end of a Sun-like star’s life.
Credit: Nordic Optical Telescope and Romano Corradi (Isaac Newton Group of Telescopes, Spain)
But this oversimplifies a key aspect of stellar evolution: massive stars
expel matter as they age.
This portion of the Cosmic Reef composition highlights the blue reflection nebula created by winds blown off of a hot, massive, giant blue star that are then illuminated in reflected light by the original star that created it. The Wolf-Rayet star that powers it may be destined, in short order, for a stellar cataclysm such as a core-collapse supernova, but we can only see the presence of the cold, expelled gas from its outer layers.
Credit: NASA, ESA and STScI
Particularly in the later, giant stages of life,
strong winds blow off their expansive outer layers.
Imaged in the same colors that Hubble’s narrowband photography would reveal, this image shows NGC 6888: the Crescent Nebula. Also known as Caldwell 27 and Sharpless 105, this is an emission nebula in the Cygnus constellation, formed by a fast stellar wind from a single Wolf-Rayet star. The fate of this star: supernova, white dwarf, or a direct collapse black hole, is not yet determined.
Credit: J-P Metsävainio (Astro Anarchy) NASA recently featured Wolf-Rayet star WR 124, touting it as a “future supernova” within the Milky Way.
This Wolf–Rayet star is known as WR 31a, located about 30,000 light-years away in the constellation of Carina. The outer nebula is expelled hydrogen and helium, while the central star burns at over 100,000 K. In the relatively near future, many suspect that this star will explode in a supernova much like WR 124, enriching the surrounding interstellar medium with new, heavy elements. It cannot be predicted which evolved, massive star in our galaxy will be the Milky Way’s next supernova.
Credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt
Although the central star is ~30 solar masses, it’s already expelled at least 10 solar masses of material.
The Wolf-Rayet star WR 102 is the hottest star known, at 210,000 K. In this infrared composite from WISE and Spitzer, it’s barely visible, as almost all of its energy is in shorter-wavelength light. The blown-off, ionized hydrogen, however, stands out spectacularly, and reveals a series of shells to its structure.
Credit: Judy Schmidt; data from WISE, Spitzer/MIPS1 and IRAC4
With no stellar hydrogen remaining, it’s
already begun fusing heavier elements in its core.
The extremely high-excitation nebula shown here is powered by an extremely rare binary star system: a Wolf-Rayet star orbiting an O-star. The stellar winds coming off of the central Wolf-Rayet member are between 10,000,000 and 1,000,000,000 times as powerful as our solar wind, and illuminated at a temperature of 120,000 degrees. (The green supernova remnant off-center is unrelated.) Systems like this are estimated, at most, to represent 0.00003% of the stars in the Universe but could lead to supernovae if the conditions are right.
But, like many Wolf-Rayet stars, it might not be ultimately destined for a supernova.
The Wolf-Rayet star WR 124 and the surrounding nebula M1-67, as imaged by Hubble, both owe their origin to the same originally massive star that blew off its hydrogen-rich outer layers. The central star is now far hotter than what came before, as Wolf-Rayet stars typically have temperatures between 100,000 and 200,000 K, with some stars cresting even higher. Could a star like this, rather than Betelgeuse, be our galaxy’s next naked-eye supernova? Only time will tell.
Credit: ESA/Hubble & NASA; Acknowledgement: Judy Schmidt (geckzilla.com)
Many Wolf-Rayet stars lose too much mass over time, leaving a core that contracts to a white dwarf.
The planetary nebula NGC 5315, formed from a dying star that blows off its outer layers, has the temperature and ionization profile of a Wolf-Rayet star at its core. It is not yet known whether this planetary nebula arose from a Wolf-Rayet star that lost enough mass, or whether it arose from a normal star that achieved a Wolf-Rayet phase as its contracting down to a white dwarf.
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Numerous planetary nebulae possess Wolf-Rayet-like central stars.
This planetary nebula, NGC 2867, has at its core a stellar remnant with Wolf-Rayet properties. Although this couldn’t have arisen from a Wolf-Rayet progenitor, it’s possible that some white dwarf/planetary nebula combinations indeed do.
Credit: NASA/Hubble and Judy Schmidt/flickr
Other Wolf-Rayet stars will indeed collapse, but directly: into a black hole with no accompanying supernova.
The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time.
Credit: NASA/ESA/C. Kochanek (OSU)
WR 124 isn’t done losing mass or evolving.
This mid-infrared view of the star WR 124 and its surrounding material shows the copious production of gas and dust from the expelled material. It isn’t only Wolf-Rayet stars that produce this, but many evolved, “puffy” stars. The presence of a massive, close-in companion can enhance this effect.
Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team
With direct collapse and extreme mass loss still possible, WR 124 might not ever go supernova.
A supernova observed in 2019, SN 2019hgp, was an unusual type of supernova: the first of its kind ever seen. It’s the only supernova ever to be linked to having come from a Wolf-Rayet progenitor, despite some ~500 Wolf-Rayet stars known within our Milky Way alone. The percentage of Wolf-Rayet stars that do or don’t go supernova has not yet been established, raising doubts as to the eventual fate of WR 124.
Credit: SDSS (main), A. Gal-Yam et al., Nature, 2022 (inset)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.