Stars that have stopped cooling for more than 8 billion years could affect galactic dating
Earth’s hydrogen-burning sun is currently growing brighter through nuclear fusion, but like all the celestial sparks that dot the sky, it will run out of fuel at some point and eventually turn into a white dwarf. Dew.
Simon Blouin, an astronomer at the University of Victoria, said in an interview that stars can be thought of as being similar to campfires because, like the embers of a flame, their light becomes weaker as they cool over time.
White dwarfs have long been considered “dead stars” because they slowly solidify into solid bodies after billions of years of cooling.
But a new study co-authored by Blouin in the journal Nature reveals a population of white dwarfs that has stopped cooling for more than 8 billion years.
Their discovery stems from the 2019 discovery that some white dwarfs do not cool as expected, calling into question the “dead star” consensus. Returning to the campfire analogy, Blouin said it’s like leaving a campfire and coming back a year later to find the embers still hot.
“That means there are white dwarfs that stay the same brightness and the same temperature for billions of years,” he said, adding that scientists believe the brightness of the star could be reduced because there is no fuel left to burn. He added that he was confused as to what was being maintained.
A typical white dwarf cools until crystals form, becoming solid as they freeze from the inside out.
In the group of outlier white dwarfs studied by Bruin and his co-authors, heavy elements were ejected inside the star as crystals formed. As a result, the crystals became lighter and floated to the surface of the white dwarf’s liquid, like ice rushing onto a glass of water.
“This means that you can’t simply freeze the star from the inside out, because the crystals that form in the core don’t stay there and are buoyant,” Blouin said. The movement of the rising crystals as the liquid rushes down toward the core creates energy that powers the star and keeps it glowing.
Apart from challenging the standard definition of white dwarfs as dead, this study also has implications for galactic archeology.
White dwarfs, scientists have been able to tell, are a useful indicator of the age of the Milky Way: the cooler a star, the older it is.
Dating groups of stars by measuring the temperature of white dwarfs ultimately helped astronomers trying to reconstruct the formation of galaxies. Studies showing that some white dwarfs remain at a constant temperature for billions of years make it more difficult to use them as indicators of age. It will also affect previous discoveries that may have underestimated the ages of some stars.
Although things are still unclear, researchers believe the newly discovered behavior may have been facilitated by the white dwarf colliding with another star and merging with a larger object.
“We think there are thermonuclear reactions going on that can change the composition of the stars, and that’s why these stars behave differently,” Blouin said.
More than 97 percent of the Milky Way galaxy eventually becomes a white dwarf, but whether the collision of two white dwarfs could cause a cooling hiatus phenomenon is still an open question.
Researchers were able to detect the anomaly thanks to the European Space Agency’s Gaia satellite. The satellite increased the number of known white dwarfs by ten, providing scientists with even more data.
The identification of white dwarfs, which goes against the scientific consensus, was possible thanks to a temperature “traffic jam,” as Blouin puts it. By observing stars on a scale from hot to cold, the researchers found that the central region is packed with star clusters that are cooling slowly.
Models show that normal white dwarfs also undergo a cooling hiatus of about 1 billion years, compared to the outlier’s 8 billion year hiatus. The researchers now believe that the cooling phenomenon they discovered could cause most white dwarfs to age by about a billion years, and are trying to learn more about how old the stars are.
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