2024-04-07 19:52:30
Three University of Victoria scientists have discovered why some white dwarf stars suddenly stop cooling. This pause offers them a reprieve of around 10 billion years.
In 2019, using data from the European Space Agency’s Gaia space telescope, a first group of researchers discovered that there was a population of white dwarf stars that stopped cooling for billions of years.
Today, these searches (New window) (in English), published by Simon Blouin and his team in the scientific journal Naturesolve this mystery and explain why.
When a star runs out of fuel, it cools. Solid crystals form in its core and quietly crystallize it from the inside until it is completely solidified, explains Simon Blouin, co-author of the research and an astrophysicist affiliated with the University of Victoria.
However, the composition of certain crystals makes them less dense than the fluid in which they are formed. The crystals will therefore float on the surface of the star, like ice cubes in a glass of water.
As they float, the crystals move heavier elements toward the center of the star, releasing gravitational energy, and it is this source of energy that allows some stars to stop cooling.
We are really talking regarding a second stellar life. These stars, even though they no longer have fuel to burn, are as bright as stars that burn fuel.
A quote from Simon Blouin, co-author of the research and astrophysicist affiliated with the University of Victoria
How do white dwarf stars form?
Before becoming white dwarfs, these stars are first suns, like the one that lights up our days.
The Sun, like most stars, burns hydrogen: this is what allows it to shine.
In billions of years, its core will run out of it and it will burn the hydrogen present in its outer layers. It will become much larger and will then be what we call a red giant, explains Simon Blouin.
The Sun’s ray will reach the Earth’s orbit and therefore engulf all the planets within this distance.
A quote from Simon Blouin, co-author of the research and astrophysicist affiliated with the University of Victoria
Over time, there will be gigantic solar winds and the red giant (the Sun) will lose a lot of mass, eventually becoming a white dwarf the size of Earth.
About 97% of stars end their lives as white dwarfs. The others become black holes or neutron stars.
Reassessing our cosmic clock
The study of white dwarfs is one of the best-known ways to determine the age of the universe.
According to Robert Lamontagne, an astrophysicist affiliated with the University of Montreal, it is enough to find the coldest white dwarf, and therefore the least luminous.
As we know the cooling rate of these stars, we only need to add the normal lifespan of this type of star. We can therefore determine that the universe must be at least as old as the time required for the coldest white dwarf to have reached this temperature, he explains.
The research of Simon Blouin and his colleagues, however, raises an interesting question.
If this 10 billion year pause occurs for only a fraction of white dwarfs, it won’t change our estimate of the age of the universe, but if this delay applies to all white dwarfs, as the the scientist and his team currently suspect, it might change the age of the universe.
However, Robert Lamontagne and Simon Blouin agree that this discovery does not call into question the other independent methods and series of measurements used to determine the age of the universe.
Answers, but even more questions
As for what comes next, Simon Blouin already has a long list of questions he wants to try to answer.
In the short term, he wants to determine whether this pause in cooling applies to all white dwarfs or not.
To do this, he and his colleagues will study pairs of white dwarfs, i.e. dwarfs in orbit around each other.
The stars in these binary systems were born together, so, in theory, they should be the same age.
The idea is to find systems with one white dwarf that is solidified and one that is not, then compare their ages. If we find a difference between the two, it is an indicator that something is missing in the current models, explains Simon Blouin.
There is nothing more fascinating than a mystery to a scientist.
A quote from Robert Lamontagne, astrophysicist affiliated with the University of Montreal
For Robert Lamontagne, these discoveries are excellent news. They show that we still have a lot to learn. Despite the fact that we have been studying the universe for a very long time, there are still a lot of things that we don’t understand, rejoices the astrophysicist.
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