Sirius B, right, is a tiny white dwarf star – a corpse of a star the size of our sun. NASA’s Hubble helped uncover how these stars age.
NASA / ESA / G. Bacon (STScI)
NASA’s Hubble Space Telescope just discovered that some dying stars or white dwarfs are on an antiaging regimen effective enough to prevent an intergalactic Sephora. The secret of the star bodies is a hydrogen coating that slows their post-mortem cooling so much that current estimates of their age could differ by up to 1 billion years.
“In the past, some models were calculated for slowly cooling white dwarfs, [but] This is the very first time this effect has been observed, “said Francesco Ferraro, an astrophysicist at the University of Bologna who coordinated the star studies published Monday in the journal Nature Astronomy.
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He added, “Our discovery suggests caution when using white dwarf cooling sequences as a clock.”
One day our glorious golden sun will die like all other stars in the universe. It will retreat from covering us during our morning coffee, and much like the rest of our cup of joe that is often forgotten on the dining table, the remnants of the old star will slowly cool down. It becomes a white dwarf.
White dwarfs are the last evolutionary stage of low-mass stars – like the sun – and are sometimes referred to as the “bare” core of what used to be blazing objects. Before a star enters the realm of the white dwarfs, it energizes itself by fusing hydrogen with the somewhat heavier element helium. As soon as there is no more hydrogen left, it fuses the helium into even heavier elements.
During the secondary fusion, the outer layers of matter of the stars are released. This is what Hubble regularly captures in its fantastic, colorful images of magnificent nebulae – stars shedding their outer layers.
By comparing two huge clusters of stars, M13 (left) and M3 (right), astrophysicists were able to understand how white dwarfs cool off.
ESA / Hubble & NASA, G. Piotto et al.
This leaves behind an exhausted, energyless, “naked” star corpse – a white dwarf.
“Without any source of energy, a star can only cool down and gradually weaken its luminosity,” said Ferraro. “This is exactly the accepted model for white dwarfs.”
Analysis of Hubble images from the telescope’s Wide Field Camera 3 by Ferraro and his team revealed that some white dwarfs are surrounded by a thin residual layer of hydrogen that provides a final burst of energy. Contrary to popular belief, not all white dwarfs get dark and cool or age at the same rate.
“This discovery changes the definition of white dwarfs that we are currently teaching our students,” he said. “Stable thermonuclear burning can still occur on the surface of a white dwarf.”
The researchers reached their novel conclusion by comparing two similar galactic globular clusters, or areas crowded with tons of stars, M3 and M13. In fact, they are so similar that Ferraro calls them twins.
“As with humans,” he noted, “twins are similar but not identical and may have some specific differences in their stellar population.”
It turns out that M13’s white dwarf population of 467 white dwarfs is much larger than that of M3’s 326 white dwarfs. The strong contrast in the population shows that the cooling rates of the stars within the “twin” clusters are not the same. Some of the white dwarfs of M13 that are covered in hydrogen are cooling down more slowly.
“The difference was even more significant because M3 has more stars than M13,” said Ferraro.
He says that some dying stars may retain a hydrogen-rich outer shell because they skip a step that involves mixing the elements as they degrade. Typically this step burns the last pieces of hydrogen.
Although the team’s results suggest that several age estimates of white dwarfs could be completely wrong, Ferraro argues that the stellar systems we use to determine the age of the universe are likely to be safe from error. This is because astronomers use other indicators when making such conclusions.
“Now we are investigating white dwarfs in other ancient star clusters – similar to M13 – to provide additional evidence for this phenomenon,” said Ferraro, saying he expected to be surprised by the dying stars because “they are among the am most “compact objects in the universe: one white dwarf teaspoon weighs as much as 10 elephants.”