We thought that one important issue in astronomy was almost entirely certain. When a star of a certain mass stops nuclear fusion, it dies and becomes a white dwarf that emits only residual heat. Gradually it cools and darkens, leaving in its place a cold, dead crystal called a black dwarf.
We thought the universe was not yet old enough to complete such a process anywhere, so it was impossible to prove this theory. Apparently we were probably wrong: astronomers have found evidence of a white dwarf star that still burns hydrogen stably, but only on the surface and not in the nucleus.
This suggests that these stellar fossil nuclei may be aging and dying more slowly than we thought, or that they are sustaining life by burning hydrogen-rich outer layers.
“We found the first observational evidence that white dwarfs can still conduct stable thermonuclear fusion. “It’s pretty amazing because it goes against what has been said so far,” said Jiansing Chen, an astronomer at the Italian National Institute of Astrophysics.
The white dwarf is a low-mass, late evolutionary stage of a star up to eight times the mass of the sun. When these stars reach the main sequence stage and can no longer synthesize hydrogen in the nucleus, the outer layers of matter are removed.
The remaining nucleus no longer has the pressure generated by the synthesis, directed outwards and collapses into an ultraviolet object. It is a white dwarf and its maximum mass is about 1.4 solar masses.
White gnomes are extremely hot. The universe is about 13.8 billion years old; According to modeling, it takes much longer for this white dwarf to cool completely. However, people are very interested in this process. Astronomers predict that about 97 percent of the stars in the universe, including our own sun, will end their lives this way. Knowledge of the evolution of white dwarfs will also help us predict the end of the world.
Astronomers have calculated how long it takes for a white dwarf to cool, which means that if we know its mass and temperature, we can also calculate its age. This can even be a good tool for estimating the age of star clusters in which white dwarf populations are found.
We can also observe many clusters containing white dwarfs at different evolutionary stages and then compare them. This is exactly what Chen and his team did, using the Hubble Space Telescope; They studied white dwarfs in two star clusters – M3 and M13.
These two clusters are interesting in that the stars in them have the same metal content – that is, elements heavier than lithium. Such elements were not abundant in the universe until several generations of stars appeared and died before the elements synthesized in their own nuclei were released into space. This means that based on the number of these elements it is possible to determine the approximate age of the stars.
In M3 and M13, the stars are in an evolutionary stage called the horizontal branch. This stage occurs when a solar-mass star runs out of hydrogen fuel in its core. It then passes through the red giant phase, in which it removes the outer layers and already begins to synthesize helium.
The same metal content of the stars in the two clusters means that their stars must be about the same age, but some of the stars in M13 are hotter than in M3, indicating that they may have cooled differently.
After near-ultraviolet observations, the researchers took detailed data from more than 700 white dwarfs across two clusters to determine what might be causing such a difference.
The white dwarfs of the M3 were fairly even, just as expected to cool the ultraviolet nuclei. However, the white dwarfs in Grova M13 split in two: normal and those with a hydrogen-rich membrane.
Computer simulations of stellar evolution have shown that these stars, which make up 70 percent of the stars in M13, burn hydrogen in its outer shell. This keeps them warm for a longer time, which is why they “age” more than those white dwarf stars, whose roles shine only with residual heat.
The finding is huge because it means that white dwarfs can be estimated to be billions of years old if they belong to the hydrogen synthesizing species.
Here I will tell you that in 2019, astronomers discovered another species of white dwarf that burns at abnormally high temperatures, so hot that it would take 8 billion years for cooling to take place. This fact is still unexplained, but if we believe the new discovery, it indicates that we probably do not know this stage of stellar evolution as fully as we thought.
“Our discovery calls into question the definition of a white dwarf, as we face a whole new way of star aging. “We are currently studying other star clusters like M13 to learn more about the environmental conditions that keep stars from a thin outer layer of hydrogen, which in turn causes them to age more slowly,” said Francesco Ferraro, an astronomer at the Italian National Institute of Astrophysics.