by White dwarfs are the stars that will be left behind when “stars like the sun fade,” smashing into space in cooling ash.
Recent observations have suggested that some of these stellar bodies may actually take longer to cool than previously thought. This means that white dwarfs may have a way to generate energy after their “deaths,” defying the classic picture of them being inert dead stars. As a result, some white dwarfs could be billions of years older than currently thought.
Analyzing data from the Gaia space mission in 2019, scientists discovered a population of white dwarfs that appear to have stopped cooling for billions of years. Now, a team of researchers led by Antoine Bédard from the University of Warwick and Simon Blouin from the University of Victoria think they know the mechanism behind this baffling discovery.
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Fountain of starry youth
White dwarfs are born when stars with the same mass as the sun exhaust the fuel supply necessary for nuclear fusion at their cores. This supply is made up of the lightest element in the universe: Hydrogen. When fusion, the conversion of hydrogen to helium, stops in the stellar core it also cuts off the energy that has pushed out and protected the star from collapsing under its own gravity for often billions of years.
Pushed by gravity, the outer layers of the star where nuclear fusion is still present are torn away. These layers “puff out” to tens, or even hundreds, of the star’s original radius during the so-called red giant phase of the star’s life. Eventually, these surrounding layers dissipate, leaving the cooled, exhausted core star behind as a white dwarf.
For the sun, this transformation will begin in about 5 billion years; the red giant phase of our star will see it rise to the radius of Mars. During this time, the sun will be up until it swallows the inner planets, including the Earth. 97% of the stars in the Milky Way will go through the same basic process of becoming a white dwarf.
Scientists previously considered white dwarfs with depleted nuclear fuel to represent the final, almost inert stages of small stars. It is theorized that plasma that was once broiling and turbulent stellar material is frozen within their core, as heat is no longer produced. This would result in a white dwarf solidifying from the inside out over billions of years.
Bédard and his colleagues think that, in the case of some white dwarfs, the dense plasma inside does not freeze from the inside to the outside. They have found the heavenly fountain of youth, so to speak, which lies beneath the shell of some of these stars.
Inside these aging white dwarfs, solid crystals form as the dead star cools – crystals that can float on a denser liquid.
As this solid material floats upward, the team says it displaces liquid material moving downward. The transport of heavier material in the cores of these white dwarfs releases gravitational energy. This energy can be converted into heat, which stops the cooling process for billions of years.
“This explanation matches all the observational properties of the unusual white dwarf population,” Bédard said in a statement. “This is the first time this transport mechanism has been observed in any type of star, which is exciting: it’s not every day that we discover a completely new astrophysical phenomenon!”
One of the big questions raised by the discovery of white dwarfs that stop their cooling processes is what makes them different from their “dead” counterparts, per se. White dwarfs are just as cool as expected.
“The difference is probably due to the composition of the star,” explained Blouin. “Some white dwarf stars are formed by the merger of two different stars. When these stars collide to form the white dwarf, it changes the composition of the star in a way that allows floating crystals to form.”
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Currently, when scientists look at a white dwarf, they assume that the cooler it is, the more ancient it is. However, the slow cooling of these strange white dwarfs may mean that they have temperatures that make them appear much younger than they actually are.
Thus, the discovery of these age-defying white dwarfs and the mechanism they use as a “source of youth” could redefine the way astronomers date stars in general.
“The transport mechanism we have discovered means that some white dwarfs have been shining as brightly as ‘normal’ stars for billions of years,” said Bédard. “This helps aging and our use of white dwarfs to reconstruct the formation of our galaxy.”
The team’s research was published on March 6 in the journal Nature.
