White dwarfs are giving up more of their secrets, with the discovery that the hotter they are, the puffier their outer layers are. While this finding may sound immaterial, understanding the structure of white dwarfs could ultimately prove key in discovering what mysterious dark matter is made from.
White dwarfs are the core remains of sun-like stars that have used up all their usable nuclear fuel. In five billion years' time, our sun will turn into a white dwarf after its red giant phase. The sun's outer layers will be cast away into deep space, revealing its pearlescent core. White dwarfs can pack the mass of a star into a volume the size of Earth, meaning they are extremely dense — a tablespoon of white dwarf material can weigh tons. Their interiors push physics to the extreme, but theory can make predictions about white dwarfs depending upon their mass and temperature.
White dwarfs are born hot, often in the region of about 180,000 degrees Fahrenheit (100,000 degrees Celsius), although some have been found even hotter. That they're so hot is not surprising — they are the extinguished core of a star, after all, and have undergone gravitational contraction when they stopped producing energy. They then begin the slow process of cooling down over time.
The minimum size of a white dwarf is controlled by something called electron degeneracy pressure. Inside a white dwarf, electrons can only be crushed together so much before quantum mechanical effects prevent them from being compacted any further. (Neutron stars, which have more mass, are able to override electron degeneracy pressure, forcing electrons and protons to merge to form neutrons, and hence neutron stars are governed by neutron degeneracy pressure.)
Related: White dwarfs: Facts about the dense stellar remnants
So that determines a white dwarf's minimum size, while their maximum size depends on their mass (the more massive, the larger they are) and their temperature. Theory predicts that the hotter a white dwarf is, the more bloated its outer layers should be.
Now, for the first time, astronomers have shown that this theory is correct. Researchers led by Nadia Zakamska of Johns Hopkins University measured the gravitational redshift of the light coming from more than 26,000 white dwarfs scattered about our Milky Way galaxy, as observed by the Sloan Digital Sky Survey and the European Space Agency's Gaia spacecraft. Gravitational redshift is an effect resulting from the mass of the white dwarf warping the space around it, in accordance with Albert Einstein's general theory of relativity, which results in the wavelength of the white dwarf's light becoming stretched.
More compact white dwarfs have strong gravitational redshift because their gravity is stronger than that of more voluminous white dwarfs. Zakamska's team found that the observed gravitational redshifts did indeed match predictions that hotter white dwarfs will be puffier, even if they have the same mass as cooler white dwarfs.
So, no surprises there — but the findings are possibly more important for what they could ultimately reveal to us. That's because astronomers can use our understanding of white dwarfs as a baseline to search for more exotic phenomena such as 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 of Johns Hopkins University in a statement. "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."
For decades, many astronomers have been placing their bets on dark matter being a kind of hypothetical particle called a WIMP: a weakly interacting massive particle. However, the failure to detect WIMPS has led to another candidate rising to prominence: axions. Another breed of hypothetical particle, axions are predicted to exist by quantum chromodynamics, which is our best quantum theory of the strong force that binds quarks together to form protons, neutrons and ultimately atomic nuclei.
In a galaxy suffused in a halo of WIMP dark matter, the WIMPs would congregate near the center of the galaxy and smoothly thin out toward the edge of the galaxy. Not so with axions; quantum interference patterns would result in the distribution of axions in a galaxy's dark matter halo to come in peaks and troughs, each extending thousands of light-years.
Related: What is dark matter?
So what does this have to do with white dwarfs? If two (or more) white dwarfs are located in one of the axion peaks, the additional dark matter could alter their interior structure in subtle ways that would become apparent as unexpected variations in temperature, mass or gravitational redshift that we could only recognize because of how well we understand white dwarfs.
"That's why understanding simpler astrophysical objects like white dwarfs is so important, because they give hope of discovering what dark matter might be," said Crumpler.
We're not there yet, though — there's still more that has to be learned about white dwarfs.
"The next frontier could be detecting the extremely subtle differences in the chemical composition of the cores of white dwarfs of different masses," said Zakamska.
Understanding white dwarfs therefore doesn't just provide a window into our sun's future when it becomes a white dwarf in about five billion years time. They also potentially act as a portal into the realms of general relativity, quantum physics and dark matter.
The new findings were published on Dec. 18 in The Astrophysical Journal.
