26,000 Dead Stars Offer Clues to Dark Matter Mysteries

Researchers have unlocked a fascinating cosmic mystery: hotter white dwarf stars — remnants of dying suns — are puffier than their cooler counterparts, even when they weigh the same.

By studying over 26,000 of these ultra-dense stars, scientists are getting closer to using them as natural test labs for exploring extreme gravity and the elusive particles that make up dark matter. This discovery could rewrite our understanding of stellar physics and help uncover hidden forces in the universe.

Extreme Stellar Behavior Confirmed by 26,000 Dead Stars

Researchers have confirmed a long-predicted but elusive phenomenon in white dwarf stars, the ultra-dense remnants of dying stars. Analyzing over 26,000 white dwarfs, the study found that hotter stars are slightly larger, or “puffier,” than cooler ones, even when they share the same mass.

These findings, published on December 18 in The Astrophysical Journal by a team at Johns Hopkins University, bring scientists closer to using white dwarfs as natural laboratories. These stars offer unique opportunities to study the effects of extreme gravity and potentially uncover evidence of exotic dark matter particles.

Gravitational Effects and Stellar Density

“White dwarfs are one of the best-characterized stars that we can work with to test these underlying theories of run-of-the-mill physics in hopes that maybe we can find something wacky pointing to new fundamental physics,” explained Nicole Crumpler, a Johns Hopkins University astrophysicist who led the work. “If you want to look for dark matter, quantum gravity, or other exotic things, you better understand normal physics. Otherwise, something that seems novel might be just a new manifestation of an effect that we already know.”

White dwarfs are cores of stars that were once like our sun but that have exhausted all the hydrogen once used as nuclear fuel. These stripped-down stars are so dense that a teaspoon of their material weighs upward of a ton, far heavier than ordinary matter. With that mass packed so tightly, their gravitational pull can be hundreds of times stronger than Earth’s.

Advancing White Dwarf Research

The research relied on measurements of how those extreme conditions influenced light waves emitted by white dwarfs. Light traveling away from such massive objects loses energy in the process of escaping its gravity, gradually turning redder. This “redshift” effect stretches light waves like rubber in ways telescopes can measure. It results from the warping of spacetime caused by extreme gravity, as predicted by Einstein’s theory of general relativity.

By averaging measurements of the white dwarfs’ motions relative to Earth and grouping them according to their gravity and size, the team isolated gravitational redshift to measure how higher temperatures influence the volume of their gaseous outer layers.

The research continues efforts by the same Johns Hopkins group. Their 2020 survey of 3,000 white dwarfs confirmed the stars shrink as they gain mass because of “electron degeneracy pressure,” a quantum mechanical process that keeps their dense cores stable over billions of years without the need for nuclear fusion, which typically supports our sun and other types of stars. Until now, the team did not have enough data to confidently confirm the subtler—but important—effect of higher temperatures on that mass-size relationship, Crumpler said.

The study combines observations from the Sloan Digital Sky Survey, which uses telescopes in Chile and New Mexico, and the European Space Agency’s Gaia mission. Both projects are continuously mapping and tracking millions of stars, galaxies, and other cosmic objects.

“The next frontier could be detecting the extremely subtle differences in the chemical composition of the cores of white dwarfs of different masses,” said Nadia Zakamska, a Johns Hopkins astrophysics professor who directed the research. “We don’t fully understand the maximum mass a star can have to form a white dwarf, as opposed to a neutron star or a black hole. These increasingly high-precision measurements can help us test and refine theories about this and other poorly understood processes in massive star evolution.”

Dark Matter Investigations and Future Prospects

The observations could also help attempts to spot signs of dark matter, such as axions or other hypothetical particles, Crumpler said. By providing a more detailed picture of white dwarf structures, the team could use this data to uncover the signal of a particular model of dark matter that results in an interference pattern in our galaxy. If two white dwarfs are within the same dark matter interference patch, then dark matter would change the structure of these stars in the same way, Crumpler said.

Even though dark matter has gravity, it does not emit light or energy that telescopes can see. Scientists know it makes up most of the matter in space because its gravity affects stars, galaxies, and other cosmic objects in ways similar to how the sun affects our planet’s orbit.

“We’ve banged our heads against the wall trying to figure out what dark matter is, but I’d say we have jack diddly squat,” Crumpler said. “We know a whole lot of what dark matter is not, and we have constraints on what it can and can’t do, but we still don’t know what it is. That’s why understanding simpler astrophysical objects like white dwarf stars is so important, because they give hope of discovering what dark matter might be.”

Reference: “Detection of the Temperature Dependence of the White Dwarf Mass–Radius Relation with Gravitational Redshifts” by Nicole R. Crumpler, Vedant Chandra, Nadia L. Zakamska, Gautham Adamane Pallathadka, Stefan Arseneau, Nicola Gentile Fusillo, J. J. Hermes, Carles Badenes, Priyanka Chakraborty, Boris T. Gänsicke and Stephen P. Schmidt, 18 December 2024, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad8ddc

Other authors include Vedant Chandra and Priyanka Chakraborty of Center for Astrophysics | Harvard & Smithsonian; Gautham Adamane Pallathadka, Stefan Arseneau, and Stephen P. Schmidt of Johns Hopkins University; Nicola Gentile Fusillo of Università degli Studi di Trieste; J.J. Hermes of Boston University; Carles Badenes of University of Pittsburgh; and Boris T. Gänsicke of University of Warwick

This research was supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 679DGE2139757, a Peirce Fellowship from Harvard University, the Johns Hopkins President’s Frontier Award, a seed grant from the JHU Institute for Data Intensive Engineering and Science, the Johns Hopkins Provost’s Undergraduate Research Award, the Alfred P. Sloan Foundation, and the Heising-Simons Foundation.

Sloan Digital Sky Survey telescopes are located at Apache Point Observatory, funded by the Astrophysical Research Consortium and operated by New Mexico State University, and at Las Campanas Observatory, operated by the Carnegie Institution for Science. Funding for the European Space Agency’s Gaia Data Processing Analysis Consortium has been provided by national institutions participating in the Gaia Multilateral Agreement.

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