We thought we had solved everything. When a star of a certain mass leaves nuclear fusion, dies and becomes a white dwarf, all that is left to keep it shining is residual heat. It will eventually cool to darkness, leaving a cold, dead crystal known as a black dwarf.
We do not believe that the Universe is old enough for such a process to be fully completed, so we have not been able to confirm it. Now it seems we would have been wrong: astronomers have found evidence of a white dwarf star on which hydrogen continues to burn stably, albeit on the surface and not in the core.
This suggests that these remaining stellar nuclei could age and die even more slowly than we thought, delaying their eventual disappearance by burning their hydrogen-rich outer envelopes.
“We have found the first observational evidence that white dwarfs can still undergo stable thermonuclear activity,” said astronomer Jianxing Chen of the Alma Mater Studiorum Università di Bologna and the Italian National Institute of Astrophysics.
“It was quite a surprise, as he disagrees with what is usually believed.”
White dwarfs are the late evolutionary stage of low-mass stars, about eight times the mass of the Sun. When these stars end their lifetime from the main sequence and are no longer able to fuse hydrogen into their nuclei, they expel their outer material.
The remaining core, no longer supported by external melting pressure, collapses into an ultradense object. It is the white dwarf and has a maximum mass of about 1.4 times the mass of the Sun.
These white dwarfs are extremely hot. The Universe is about 13.8 billion years old; according to the modeling, a white dwarf takes a long time to cool completely. But we are deeply interested in this process. Astronomers predict that approximately 97% of all stars in the Universe, including the Sun, will end their lives this way. Knowing how white dwarfs evolve can help predict the end of the Universe.
Astronomers have calculated how fast white dwarf stars should cool, which means that if we know the mass and temperature of a white dwarf, we should know how old it is. It can be used as a tool to estimate the ages of star clusters in which populations of white dwarfs can be found.
We can also observe multiple clusters to see the different evolutionary stages of white dwarfs and compare them. This is what Chen and his team were doing, using the Hubble Space Telescope’s wide-field camera 3 to study white dwarfs in two star clusters, called M3 and M13.
These two clusters are interesting, because the stars they contain have a similar metallicity, that is, the abundance of elements heavier than helium. These elements were not abundant in the Universe before some generations of stars had come and gone, merging elements into their nuclei and releasing them into the Universe as the stars died and were ejected into space. This means that the abundance of these elements can be used to determine approximately how old the stars are.
The stars of M3 and M13 are in an evolutionary stage known as the horizontal branch. This is just after a solar mass star has run out of hydrogen to melt into its core. It has passed the stage of red giant, in which it moves away from the outer envelope and now fuses helium.
Similar metallizations of the stars in the two clusters mean that the stars should be about the same age, but some of the stars in M13 burn more than those in M3, suggesting that they could cool differently.
Taking near-ultraviolet observations, the researchers looked more closely at more than 700 white dwarfs from the two clusters to see what difference this could make.
The white dwarfs of M3 were quite similar, exactly what we would expect to see in the ultra-dense stellar nuclei. The white dwarfs of M13, however, had two flavors: normal and those that had retained a hydrogen-rich wrapper.
Computer simulations of stellar evolution showed that these stars, which comprise approximately 70 percent of M13 stars, burn hydrogen in their outer shells. This keeps them warmer for longer, which means they age at a different rate than white dwarf stars that glow only with residual heat.
This is a big problem, as it means that our age estimates for white dwarfs could go down to a billion years, if they are of that variety that burns hydrogen.
And, to add another key to the works, in 2019 astronomers found a different type of white dwarf that burned abnormally, enough to delay cooling 8 billion years. This has yet to be explained, but in combination with the new discoveries, it suggests that we may not fully understand this stage of stellar life as well as we thought.
“Our discovery challenges the definition of white dwarfs as we consider a new perspective on how stars age,” said astronomer Francesco Ferraro of the Alma Mater Studiorum Università di Bologna and the National Institute of Astrophysics of Italy.
“We are now investigating other M13-like clusters to further restrict the conditions that lead stars to maintain the thin hydrogen envelope that allows them to age slowly.”
The research has been published in Nature Astronomy.