Dead stars reveal secrets of extreme gravity and dark matter

Concept art of two white dwarf stars with the same mass but different temperatures. The hotter star (left) is slightly puffier, while the cooler star (right) is more compact. Credit: Roberto Molar Candanosa/Johns Hopkins University.

Scientists have confirmed a long-predicted but elusive effect in dying stars called white dwarfs: hotter white dwarfs are slightly puffier than cooler ones, even when they have the same mass.

This finding, based on studying over 26,000 white dwarfs, could help us better understand extreme gravity and potentially uncover the mysteries of dark matter.

White dwarfs are the leftover cores of stars that have burned through their nuclear fuel, like what will eventually happen to our sun.

These stars are incredibly dense—just a teaspoon of their material weighs more than a ton.

Their gravity is hundreds of times stronger than Earth’s, making them fascinating objects to study.

White dwarfs serve as natural laboratories to test the effects of extreme gravity. They’re also useful for exploring exotic concepts like dark matter and quantum gravity.

However, to search for unknown physics, scientists must first fully understand the “normal” physics of these stars.

“If you want to look for dark matter or other strange phenomena, you need to understand the basic physics first,” explains Nicole Crumpler, an astrophysicist at Johns Hopkins University who led the study.

The research, published in The Astrophysical Journal, relied on measurements of how light waves are affected by the intense gravity of white dwarfs.

As light escapes these stars, it loses energy and shifts to a redder color, a process called gravitational redshift. This redshift results from the warping of spacetime caused by the stars’ extreme gravity, as predicted by Einstein’s theory of general relativity.

The team used data from the Sloan Digital Sky Survey and the European Space Agency’s Gaia mission, which are mapping millions of stars and cosmic objects. By grouping white dwarfs based on their size and gravity, scientists could isolate the redshift effect to study how temperature impacts their structure.

The study confirmed that hotter white dwarfs are slightly larger than cooler ones of the same mass because their outer gaseous layers expand at higher temperatures. While this effect had been predicted for years, scientists didn’t have enough data until now to confirm it.

This builds on earlier work by the same team. In 2020, they showed that white dwarfs shrink as they gain mass, due to “electron degeneracy pressure,” a quantum mechanical effect that keeps them stable for billions of years.

Understanding white dwarfs better could also help in the hunt for dark matter, the mysterious substance that makes up most of the universe’s mass but doesn’t emit light or energy.

Dark matter’s presence is known because its gravity affects the motion of stars and galaxies. If two white dwarfs are in the same region of dark matter, scientists believe it could subtly alter their structure in measurable ways.

“We’ve been trying to figure out what dark matter is for so long, but we still don’t have answers,” Crumpler says. “That’s why studying simpler objects like white dwarfs is so important—they give us hope of uncovering dark matter’s secrets.”

The research also sheds light on massive star evolution, helping scientists refine theories about how stars become white dwarfs, neutron stars, or black holes.

“This is the next frontier,” says Johns Hopkins astrophysics professor Nadia Zakamska. “We’re aiming to detect the subtle differences in white dwarf cores and refine our understanding of massive stars.”

This study is an exciting step forward in understanding the universe, from the strange behavior of white dwarfs to the invisible forces shaping our galaxy.