Using a catalog of 26,041 white dwarfs observed by the Sloan Digital Sky Survey, astronomers have confirmed a long-predicted effect in these ancient ultradense stars.
Concept art of two white dwarfs with the same mass but different temperatures; the hotter star (left) is slightly puffier, while the cooler star (right) is more compact. Image credit: Roberto Molar Candanosa / Johns Hopkins University.
Stars that are not massive enough to turn into neutron stars or black holes at the end of their stellar evolution expel their outer layers, leaving behind their cores as compact remnants known as white dwarfs.
All stars that have initial masses ranging from 0.07 to 8 solar masses, which is around 97% of all stars, end their lives as white dwarfs.
“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,” said Dr. Nicole Crumpler, an astrophysicist at Johns Hopkins University.
“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.”
The new 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 astronomers isolated gravitational redshift to measure how higher temperatures influence the volume of their gaseous outer layers.
The team’s 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, we did not have enough data to confidently confirm the subtler — but important — effect of higher temperatures on that mass-size relationship,” Dr. Crumpler said.
“The next frontier could be detecting the extremely subtle differences in the chemical composition of the cores of white dwarfs of different masses,” said Dr. Nadia Zakamska, an astrophysicist at Johns Hopkins University.
“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.”
“The observations could also help attempts to spot signs of dark matter, such as axions or other hypothetical particles,” Dr. Crumpler said.
“By providing a more detailed picture of white dwarf structures, we could use these 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.”
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,” Dr. 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.”
The study appears in the Astrophysical Journal.
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Nicole R. Crumpler et al. 2024. Detection of the Temperature Dependence of the White Dwarf Mass-Radius Relation with Gravitational Redshifts. ApJ 977, 237; doi: 10.3847/1538-4357/ad8ddc
