Astronomers have found evidence that one of the most massive white dwarfs known has an oxygen-neon core instead of the more common carbon-oxygen core. The finding is important because the composition of a white dwarf’s core determines how it will evolve. A paper outlining this discovery was published in The Astrophysical Journal.
Ultramassive dead stars
Typically, white dwarfs have a mass of 0.5–0.7 times the sun’s mass. Such objects have a core made up mainly of carbon and oxygen (C/O core). When they have stellar companions, these dense objects can accumulate matter from them and eventually produce a Type Ia supernova. Ultramassive white dwarfs, with masses above roughly 1.05–1.1 times the sun’s mass, tell a different story that is not yet fully understood.
These more massive white dwarfs are thought to form from “ancestor” or progenitor stars in the range of about 8–10 times the sun’s mass. In these heavier progenitors, the core reaches higher temperatures and densities, allowing carbon to ignite and fuse further into oxygen and neon (O/Ne core). This does not happen in the cores of lower-mass stars, which stop fusing once they have built up carbon and oxygen, lacking the required core temperatures.
These higher-mass white dwarfs are interesting because, with oxygen-neon cores, they are thought to end their lives differently than typical carbon-oxygen white dwarfs, which end up producing a Type Ia supernova.
An indirect method
In this work, a team led by Stefan M. Arseneau of Boston University studied an ultramassive white dwarf, SDSS J060851.44-005950.3 (SDSS J0608−0059 for short). This white dwarf is in a binary system with a main-sequence companion star that is 2,684 astronomical units away.
To understand what its core is made of, the team had to weigh and measure the white dwarf SDSS J0608−0059 precisely. There was some difficulty because the photosphere—the visible surface layer astronomers actually observe—makes up only a tiny fraction of a white dwarf’s mass, while the core beneath it accounts for more than 99%. Since light spectra carry information only about that thin outer layer, they cannot directly reveal what the hidden core is made of.
Instead, the team used gravitational redshift to estimate the white dwarf’s mass. A photon climbing out of a massive, compact object’s gravity well loses energy, which stretches its wavelength—the more massive and compact the star, the greater this gravitational redshift.
In addition to this shift in wavelength, the white dwarf’s own motion through space produces a shift in the wavelength of spectral lines.
To untangle the two effects, the team used its companion star’s independently measured velocity—essentially the same velocity as the white dwarf’s—and subtracted it from the white dwarf’s measured velocity. Therefore, whatever was left over was attributed to its gravitational redshift.
The gravitational redshift alone reveals only the white dwarf’s mass-to-radius ratio, not each value separately. To separate them, the team combined the redshift with brightness measurements from several photometric surveys (Gaia, SDSS, PanSTARRs, SkyMapper), then ran a statistical simulation to find the mass, radius, distance, extinction and temperature that best fit all the data together. This yielded a mass of around 1.226 solar masses and a radius roughly half that of Earth.
A rare core
Next, the team tested what the core was made of. Carbon-oxygen and oxygen-neon cores produce slightly different mass-radius relationships at this extreme mass. Comparing their measurements with theoretical models for each composition, the data favored an oxygen-neon core.
The team also ruled out an alternative explanation that suggested the star’s high mass came from a merger of two white dwarfs rather than normal single-star evolution, since mergers can also produce unusually massive remnants. Its ordinary galactic velocity, absence of a magnetic field and survival within a wide binary all pointed away from a merger origin.
“By comparing to state-of-the-art mass-radius relations for ultramassive white dwarfs, we find preference for an O/Ne core over a C/O core, with a Bayes factor of 2.7,” the team writes in the paper. Researchers note that better Gaia data or additional ultraviolet observations could sharpen the result further.
Written for you by our author Shreejaya Karantha, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.
Publication details
Stefan M. Arseneau et al, An Ultramassive White Dwarf with a Likely Oxygen–Neon Core, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae7f0b
© 2026 Science X Network
Citation:
An ultramassive white dwarf half Earth’s size may hold a rare oxygen-neon core (2026, August 7)
retrieved 7 August 2026
from https://phys.org/news/2026-08-ultramassive-white-dwarf-earth-size.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.