A new study states that white dwarfs, the stellar remains of long-dead stars, “look younger than they really are.”
Thanks to data from the Hubble Space Telescope, astronomers have discovered the first evidence that white dwarfs can slow the rate of aging by burning hydrogen on the surface.
Experts compared cool white dwarfs in two massive star collections: the M3 and M13 globular clusters.
About 70 percent of all white M13 dwarfs have an outer hydrogen wrap, which allows them to burn for longer and therefore cool more slowly.
White dwarfs are the incredibly dense remnants of stars the size of the sun after depleting nuclear fuel, reduced to about the size of Earth.
Approximately 98% of all stars in the universe will end up being white dwarfs, including our own Sun.
Experts compared cool white dwarfs in two massive star collections: the M3 and M13 globular clusters. Globular clusters are dense balls of a million ancient stars, all bound by gravity. This image shows a wide field view of M3. The researchers reveal that “essentially zero” of all M13 white dwarfs are slow-burning
According to the European Space Agency, the study challenges the prevailing view of white dwarfs as inert, slowly cooling stars.
“We have found the first observational evidence that white dwarfs can still suffer from stable thermonuclear activity,” said study author Jianxing Chen of the University of Bologna and the Italian National Institute of Astrophysics.
“It was quite a surprise, as he disagrees with what is usually believed.”
White dwarfs are common objects in the cosmos. They are the stars that cool slowly and have left out their outer layers during the later stages of their life.
Studying these cooling stages helps astronomers understand not only white dwarfs, but also their earlier stages.
The researchers examined the M3 and M13 clusters, as they share many physical properties such as age and “metallicity,” elements other than hydrogen and helium.
But the populations of stars that will eventually give rise to white dwarfs are different in the two clusters.
In particular, the overall color of stars at an evolutionary stage known as the horizontal branch is bluer in M13, indicating a population of hotter stars.
This makes M3 and M13 together a “perfect natural lab” to test how different populations of white dwarfs cool.
“The excellent quality of our Hubble observations provided us with a complete view of the stellar populations of the two globular clusters,” Chen said. “This allowed us to really contrast the evolution of the stars in M3 and M13.”
Using Hubble’s wide-field 3 camera, the team observed M3 and M13 at near-ultraviolet wavelengths, allowing them to compare more than 700 white dwarfs from the two clusters.
To investigate the physics that underpins the evolution of the white dwarf, astronomers compared cooling white dwarfs into two massive star collections: the M3 and M13 globular clusters. These two clusters share many physical properties such as age and metallization, but the populations of stars that will eventually give rise to white dwarfs are different. This makes M3 and M13 together a perfect natural laboratory to test how different populations of white dwarfs cool.
This image shows a wide field view of M13, a globular cluster. According to experts, about 70% of M13 white dwarfs are slow-burning
They found that M3 contains standard white dwarfs that simply cool stellar nuclei.
M13, on the other hand, contains two populations of white dwarfs: standard white dwarfs and those that have managed to hold on to an outer hydrogen wrapper, allowing them to burn for longer.
By comparing their results with computer simulations of stellar evolution to M13, the team found that approximately 70% of M13 white dwarfs burn hydrogen on their surfaces and slow the cooling rate.
M3, meanwhile, has “essentially zero” slow-burning white dwarfs, according to the study’s author, Professor Francesco Ferraro, also at the University of Bologna and the National Institute of Astrophysics in Italy.
“At the moment, no star system with 100% slow white dwarfs is known,” he told MailOnline.
This discovery could have consequences for how astronomers measure the age of stars in the Milky Way.
The evolution of white dwarfs had been previously modeled as a predictable cooling process, according to the team.
This relatively simple relationship between age and temperature has led astronomers to use the cooling rate of the white dwarf as a natural clock to determine the age of star clusters, particularly globular and open clusters.
The NASA image shows the Hubble Space Telescope floating on the background of space
However, hydrogen-burning white dwarfs can cause these age estimates to be inaccurate for up to 1 billion years, unless other methods of stellar system aging are used.
“Our work suggests caution in adopting the cooling sequence of the white dwarf as a clock,” Professor Ferraro said.
“Of course, this can affect the age of any stellar system whose age is based exclusively on the cooling sequence of the white dwarf.
“Fortunately, white dwarfs are not the most widely used clocks to measure the age of stellar systems.”
Our Sun will become a red giant in about five billion years before it finally shrinks into a compact white dwarf.
When that happens, Professor Ferraro said our Sun will be a normal white dwarf, not a slow cream.
Slow-burning white dwarfs are essentially generated by parent stars of low mass and little metallization, he said, so there is “no chance of them having slow aging.”
The research team is now investigating other M13-like clusters “to further limit the conditions that lead stars to maintain the thin hydrogen envelope that allows them to age slowly,” according to Professor Ferraro.
“Our discovery challenges the definition of white dwarfs as we consider a new perspective on how stars age,” he said.
The study was published in Nature Astronomy.