Dead stars reveal the hidden physics of the universe

A new study of more than 26,000 white dwarf stars has finally confirmed a subtle, long-predicted phenomenon: hotter white dwarfs are slightly larger than cooler ones, even when they share the same mass. 

The research, led by experts at Johns Hopkins University, verifies a key aspect of how these incredibly dense stellar remnants behave under extreme gravity. 

Dead stars and the search for dark matter 

The study, published in The Astrophysical Journal, lays important groundwork for using white dwarfs as astrophysical laboratories to probe fundamental physics and search for elusive dark matter.

“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 Nicole Crumpler, a Johns Hopkins 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.”

Dead stars with tremendous density 

White dwarfs are the remnants of once-sunlike stars that have burned through their nuclear fuel. These stripped-down cores are so compact that a teaspoonful of their matter weighs more than a ton. 

Such tremendous density produces gravitational pulls hundreds of times stronger than Earth’s, profoundly affecting the physics of light and matter in their vicinity.

The researchers relied on how light escaping these dead stars becomes stretched – “redshifted” – as it fights against the intense gravitational field, a phenomenon predicted by Einstein’s general relativity. 

By examining the redshift and understanding the stars’ gravity, the team could disentangle the various effects on the white dwarfs’ size.

How temperature influences dead stars

The new result builds on a 2020 study from the same group that confirmed white dwarfs shrink as they gain mass due to “electron degeneracy pressure,” a quantum mechanical effect stabilizing their cores. 

Until now, however, scientists lacked sufficient data to confidently verify the more subtle temperature effect.

In this study, the researchers used data from the Sloan Digital Sky Survey and the European Space Agency’s Gaia mission, both of which continuously map millions of cosmic objects. 

By carefully averaging measurements of the white dwarfs’ motions and grouping the stars according to their mass and gravity, the team isolated how temperature influences the stars’ volumes. 

The analysis shows that hotter white dwarfs “puff up” slightly compared to cooler ones, confirming the long-predicted effect.

Insights into massive star evolution

The implications extend beyond confirming existing theories. White dwarfs can help researchers understand processes in massive star evolution, including what determines whether a star ends its life as a white dwarf, neutron star, or black hole.

“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.”

A step toward dark matter detection

Crumpler and her team also view this better understanding of white dwarf physics as a stepping-stone toward detecting dark matter, the invisible substance believed to comprise most of the universe’s matter. 

If certain models of dark matter affect stars’ structures in predictable ways, comparing white dwarfs in different regions of the galaxy might reveal subtle patterns.

“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,” said Crumpler. 

“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.”

A deeper understanding of the universe

White dwarfs’ extreme conditions provide a unique testbed for exploring both everyday and exotic physics. 

By confirming this subtle temperature-size relationship, the new study strengthens the foundation on which scientists can stand as they chase the most elusive mysteries of the universe – be it the nature of dark matter, quantum gravity, or the lifecycle of stars.

With ever-improving data, researchers are one step closer to unlocking the full potential of these ultra-dense cosmic remnants, ensuring that what may seem ordinary at first glance can help illuminate the extraordinary truths hidden in the cosmos.

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