2024-03-22 12:31:25
Approximately 97% of the stars in the Milky Way Galaxy will eventually becomewhite dwarf” leaves behind a celestial body. A white dwarf is a “dead star” whose nuclear fusion reaction has stopped, and it gradually releases its heat and cools down. However, in recent years, this explanation does not apply, and white dwarf stars have been discovered one after another that appear to have stopped cooling for billions of years. Since the heat of a white dwarf is used as an indicator to estimate its age, it is important that there is a process by which it does not age.
A research team led by Antoine Bédard from the University of Warwick, Simon Blouin from the University of Victoria, and Sihao Cheng from the Institute for Advanced Study in Princeton modeled the interior of a white dwarf star. the result,Gravitational energy produced by the levitation of a small solidified massThe researchers discovered that the white dwarf generates enough heat to keep its surface temperature constant for more than 8 billion years. This result is likely to have an impact on various studies using the age of white dwarfs as an indicator.
[▲Figure 1: An artist’s impression of a white dwarf with a solidified core. Immediately after formation, a white dwarf is thought to be entirely in a “liquid” state, and as it cools, it becomes “solid” starting from the center (Credit: Travis Metcalfe & Ruth Bazinet, Harvard-Smithsonian Center for Astrophysics )]
■“White dwarf” is not so “dead star”
Stars like the Sun release energy through nuclear fusion reactions that occur in their cores, while also preventing them from collapsing due to gravity. However, because the source element will eventually be depleted, the nuclear fusion reaction will eventually stop and gravity will crush it. The final fate depends on the mass of the star, but it is said that stars with a mass less than 8 times that of the Sun will leave behind a dense core whose 1 cm dice weighs more than 1 ton. This is”white dwarf“is.
As the name suggests, a white dwarf shines white, but this is a remnant of the star’s central core, and is, so to speak, residual heat. White dwarfs are “dead stars” that do not undergo nuclear fusion, so their residual heat gradually escapes into space and cools down.Since the surface temperature and brightness of a white dwarf are relative, the brightness of a white dwarf reflects the number of years since its formation, or in other words, the age of the white dwarf.It has been predicted that
However, as research into white dwarfs progressed, it became clear that it was not that simple. As observational data on white dwarfs increases, more and more cases are being discovered where the age estimated from the brightness of the white dwarf and the age estimated using other methods (*1) are significantly different. . Such white dwarfs make up about 6% of the massive group.
Furthermore, the interior of a white dwarf is expected to be “liquid” when it first forms, and then cool and solidify into a “solid” state. Although the terms liquid and solid here are metaphorical expressions (*2), it is expected that heat (latent heat) will be generated due to phase transition, just as in the liquids and solids that are familiar to us.
In this way, it has become clear that white dwarfs are not as “dead stars” as previously thought, but the heating process that has been elucidated so far only takes about 1 billion years to cool down. All you can do is slow it down. Another unknown process is needed to explain the billions of years of slippage.
*1…For example, the age of a white dwarf estimated from the age of stars in the same star cluster. This estimation is possible because the stars in the cluster are considered to have formed at approximately the same time.
*2…The materials that make up white dwarfs are expected to have electron orbits that are extremely shortened (degenerate) compared to normal atoms. When we use the terms solid and liquid to describe the state inside a white dwarf, we use the words solid when degenerate atoms form a crystalline structure, and liquid when they are random.
■Elucidating the heating process that lasts for more than 8 billion years
Bédard, Blouin, and Cheng Si’s research team built a model of the interior of a white dwarf star to try to solve this mystery. The purpose of this research is to search for cooling-stopping processes that remain stable for billions of years. In 2019, Cheng Si proposed a cooling shutdown process lasting billions of years using another process, but this is considered an unrealistic condition (*3).
[▲Figure 2: Conventional white dwarf models have assumed that there is a large solid mass inside (left side). On the other hand, this study showed that some white dwarfs form smaller solid clumps, and the difference in density causes the solid to rise and the liquid to sink (right) (Credit: Bédard, et al.) ]
For a more realistic theory, the research team examined the solidification process inside white dwarfs in more detail. White dwarfs are thought to solidify from the inside, but until now it was thought that a large mass remained sunk in the center. But when the researchers looked at the process in more detail, they found that it could produce multiple clumps that were much smaller than previously thought.
This solid is small and has a low density compared to a liquid, so it is thought that it will float in the same way that ice does to water. Then, the relatively dense part of the liquid sinks, converting gravitational energy into heat.In this simulation, this processGenerates enough heat to maintain surface temperature for over 8 billion yearsI showed that.
*3…In a 2019 study, it was estimated that the gravitational energy from Neon-22’s sedimentation was the heat source, but to satisfy this theory, the amount estimated to be contained in the massive white dwarf star would have to be 5. We need twice as many Neon 22s.
■Influence on astronomical research using white dwarfs as indicators
In any case, the cooling-stop process occurs only in a small fraction of white dwarfs. Although it is unclear why the heating process of solid matter caused by levitation shown here occurs only in some white dwarfs, the research team believes that the formation process of white dwarfs is the key. Heavy white dwarfs are not formed directly from the core of a star, but are thought to be formed through collisions between stars or white dwarfs. The collision violently disturbs the interior of the white dwarf, which may lead to the formation of small clumps. This hypothesis is consistent with the fact that white dwarfs, which appear to have stopped cooling for billions of years, make up a minority of about 6% of massive white dwarfs.
Estimating the age of a white dwarf is used for various indicators, such as the age of nearby stars. However, since the cooling shutdown process lasts over 8 billion years, which is much longer than the age of the universe, which is 13.8 billion years, it is possible that some of the white dwarfs whose ages are estimated to be older may be older than they actually are. there is. This study may have a major impact on astronomical research based on the age of white dwarfs.
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Written by Riri Aya
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