Artist's impression of two white dwarfs in the process of merging. Depending on the combined mass, the system may explode in a thermonuclear supernova, or coalesce into a single heavy white dwarf, as with WDJ0551+4135.
Photo: University of Warwick/Mark GarlickThe collision of two dying stars, known as white dwarfs, around 1.3 billion years ago has produced a star heavier than the sun with a rare carbon-rich atmosphere, a development that—if confirmed—could help astronomers better understand the cosmic events that made life on earth possible.
The massive star is one of a handful of merged white dwarfs to be identified so far, and the first to be discovered by the composition of gases in its atmosphere, according to a study published Monday in the journal Nature Astronomy.
“There is no way we could think of a normal star evolving by itself that would give you the results we’ve seen,” said lead author Mark Hollands, a postdoctoral research fellow at the University of Warwick in Coventry, England. “And studying white dwarfs that don’t explode can tell you about the ones that do.”
The research team led by University of Warwick astronomers first observed the star with the European Space Agency’s Gaia Telescope in October 2018 after noticing its faint glow compared with surrounding white dwarfs. Researchers have since been studying properties of the star, which was named WDJ0551+4135.
Using spectroscopy—a method used to break down light emitted by stars to determine their composition, temperature and density—the team discovered the Milky Way star has an atmosphere made up of hydrogen and carbon, as opposed to hydrogen and helium. “It seemed completely bizarre,” Dr. Hollands said.
Only when the researchers learn more about the star’s core composition can their merger hypothesis be proved, he said.
The sun stays bright by burning hydrogen and helium at its core, but five billion years from now, scientists predict it will become a white dwarf: a remnant core of a star that has exhausted its fuel and shed its outer layers, shrinking to a size similar to Earth.
White dwarf stars are typically about 0.6 times the mass of the sun, but this star is nearly twice the average mass for white dwarfs. Despite being heavier than the sun, the star is two-thirds the size of Earth.
Researchers also speculate the star may have been formed from a merger because of its age and temperature. The star was found traveling faster than 99% of nearby white dwarfs that appear to have the same cooling age. The faster a star travels, Dr. Hollands said, the older and colder it is because it has spent more time robbing energy from its neighbors and burning its available fuel. This suggests the star is older than it looks, he added.
Most stars live in pairs, dancing around each other until one day, after billions of years, gravity brings them close enough to collide. If the product acquires enough mass, it can explode. Astronomers believe stars with a mass 1.4 times greater than the sun can explode in powerful events called supernovae.
Although this star—with 1.14 times the mass of the sun—didn’t explode, scientists can study it to understand the conditions that lead to supernovae and the frequency at which they occur.
“If we can find more examples of white dwarf merger products, we can open another venue of trying to solve the origin of [these events],” said Dan Maoz, an astrophysics professor at Tel Aviv University in Israel who wasn’t involved in the study. “Suspicions have been growing about what’s behind these supernovae, and we think it’s the mergers of white dwarfs.”
Last week, astronomers identified the largest explosion ever recorded from a black hole. The eruption—five times greater than the previous record holder—spewed out colossal amounts of stellar material.
This material, also released by supernovae from white dwarf stars, contains all the elements in the periodic table. “Supernovae are nature’s way of cooking up these elements,” Dr. Maoz said.
His work involves mapping the double white dwarf systems in the Milky Way to estimate if these merger events can explain the frequency of supernovae, which occur about once every 200 years, he said.
“When we see these systems that will merge in the future or already merged, we’re seeing a part of this element synthesis process,” Dr. Maoz said. “If we want to understand how we came to be, it’s important to understand these explosions.”
“We really are stardust,” he added.
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2020-03-02 16:00:00Z
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