White dwarfs are the final evolutionary state of stars that aren't massive enough to become neutron stars or black holes. It's what happens when most stars consume all their fuel. The vast majority of stars in the universe become white dwarfs.
Essentially, a white dwarf is the dense, hot core left behind after a star has exhausted its nuclear fuel and shed its outer layers. They are incredibly dense, packing roughly the mass of the Sun into the size of the Earth. To put it differently, a mere teaspoon of white dwarf material would weigh a whopping 10 million tons. This extreme density is due to the pressure of gravity that compresses the matter together extremely closely.
But although white dwarfs play a crucial role in the cosmos, there are many things about them we don't know yet, and they're also pretty difficult to study. Here's why.
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White dwarfs in the cosmos
"We have the entire sky full of stars. Most of the stars we see are currently burning hydrogen in their cores and that's how they generate energy," says Boris Gänsicke, a professor in the Department of Physics at the University of Warwick whose research focuses on white dwarfs. "But eventually, they will burn all the hydrogen that they have. Then they will burn helium, and then, most stars will stop. At that point, the star's core will be composed of carbon and oxygen."
"At that point, they can't generate energy anymore so the core just shrinks due to its own gravity while the other layers disperse in space. The burned out core, which is mainly carbon and oxygen, that's what the white dwarf is."
We met up with Gänsicke at the 2023 European Astronomical Society meeting and discussed what makes white dwarfs so interesting -- and so important.
Stars are usually in an equilibrium between the gravity that wants to pull them together and collapse them under their own weight, and the pressure that pushes them outwards. Usually, this pressure is heat. But when the star stops producing energy, as is the case of a white dwarf, the heat stops. When this happens, the star is only supported by a quantum process called degeneracy.
Small and dense
White dwarfs can be very small. They can be about the size of the Earth but have roughly the mass of the Sun -- so they have a very high density.
Due to this, astronomers have a hard time spotting them.
"Because they're so small, and therefore very dim, they're very hard to find. That's why, in the past, they were first found by accident and people didn't know what they were," explains Gänsicke.
The first white dwarf was discovered in 1783 by William Herschel (although it was formally described only in 1914). The closest white dwarf to Earth, Sirius B, was next to be discovered, in the 19th century. But up until very recently, we only knew of a few white dwarfs. This is where a new generation of telescopes and observatories came in.
Finding white dwarfs
White dwarfs are hot initially, but because they have no heat source, they start to cool down. When they do cool down, they also start to change color.
"If you sort white dwarfs by their color and how bright they are, you get what's called the white dwarf cooling sequence. Because white dwarfs don't burn anymore, they're very hot initially but they keep cooling with time. As they cool, they become more and more red."
Researchers suspected that there are three branches for white dwarf evolution. Astronomers were aware of the white dwarf cooling sequence before, but they had few data points. Then, Gaia came along.
Gaia is a space observatory of the European Space Agency (ESA), launched in 2013. Gaia produced an unprecedented, rich, catalog of stars in the universe. Suddenly, astronomers had access to much more data about stars -- including white dwarfs.
"With Gaia, we suddenly had tens of thousands of datapoints," says Gänsicke. With data from Gaia, astronomers showed that the cooling sequences branch off into three main categories. "We started to figure out what causes that splitting, and we realized that one of those branches has to do with the white dwarfs crystallizing."
As the white dwarfs cool down, the water and oxygen start to form a solid, similar to how ice becomes a solid as water cools down. The cores of the white dwarfs eventually become solid.
Usually, white dwarfs are composed of carbon and oxygen. But if the mass of the star is higher, around 8-10 solar masses, the temperature will be high enough to also fuse carbon, in which case the result will be an oxygen-neon-magnesium white dwarf. But most of them consist of carbon and oxygen.
This makes white dwarfs quite unique in their composition and structure.
"As the white dwarfs cool, the carbon and oxygen in the core forms a solid. Just like how water, when it cools, turns into ice and becomes solid. The cores of the white dwarfs eventually become solid. In that process, some heat is released that leads to the split off of one of the three branches. So that was one really exciting thing that came out pretty much on the day of the Gaia data release.
The surface of a white dwarf is still gas, but the core will start to crystallize, and this crystallization will slowly expand. You can almost (but not quite) think of white dwarfs as diamonds in the sky: half their mass is carbon and that crystallizes, and diamonds are crystallized carbon.
But it gets even more interesting. Like traditional archaeologists, who study human history by looking at artifacts and structures that can be observed today, astronomers can trace the history and formation of stars by looking at white dwarfs.
White dwarfs are cosmic time capsules. They hold the history of their parent star and offer a glimpse into the future of our own Sun. By studying these celestial objects, scientists can learn about the life cycle of stars, the evolution of galaxies, and even the fate of the universe.
"Because white dwarfs are the remnants of stars, we're looking at stars that have ceased to exist as normal stars. If you think about the sun, the sun was born about 5 billion years ago, and in another 5 billion years it will become a white dwarf," Gänsicke mentions.
"If we look at white dwarfs today, we get information on what they were like in the past. From the temperature of a white dwarf and its mass, we can work out its age, and we can work out the mass of the star that formed the white dwarf. So that means we can use white dwarfs to work out what stars have existed in the past. With that, we can investigate what's called the star formation history."
So in a sense, white dwarfs are like archaeological findings, cosmic laboratories where researchers can find information about how things were in the past -- and how they're likely to be in the future.
Of course, we don't really know how many white dwarfs there are around. Because many of them are small and faint, the galaxy and the unvierse could be riddled with white dwarfs we've yet to discover. This is where new observatories, like the Vera C. Rubin observatory, could help us get an even better understanding of white dwarfs and consequently, of how stars evolve.
White dwarf FAQ
Of course, we don't really know how many white dwarfs there are around. Because many of them are small and faint, the galaxy and the universe could be riddled with white dwarfs we've yet to discover. This is where new observatories, like the Vera C. Rubin observatory, could help us get an even better understanding of white dwarfs and consequently, of how stars evolve.
Here are just a few of the things we've learned about white dwarfs thanks to astronomy research.
A white dwarf is a small, dense star that is the remaining core of a star like our Sun after it has exhausted its nuclear fuel.
White dwarfs are formed from the remnants of main-sequence stars that have exhausted their nuclear fuel. They form after the star sheds its outer layers and the remaining hot core becomes a white dwarf.
The core of a white dwarf is typically composed of carbon and oxygen. The outer "atmosphere" of a white dwarf is very thin and is usually composed mostly of hydrogen or helium.
Unlike main-sequence stars, white dwarfs no longer undergo nuclear fusion. Instead, they shine due to residual heat from their previous life as a main-sequence star.
Once formed, a white dwarf will exist for billions to even trillions of years. Over this time, it will slowly cool down.
Yes, our Sun will estimated to become a white dwarf, but not for another 5 billion years or so. After exhausting its nuclear fuel, the Sun will expand into a red giant, then shed its outer layers, leaving behind a hot core that will cool over billions of years to form a white dwarf.
The white dwarf that was once our Sun will be incredibly dense, about the size of Earth but with a mass comparable to the Sun. It will initially be very hot and white, but will cool and fade over billions of years.
A black dwarf is a theoretical stellar remnant that is predicted to be created from a white dwarf after it has cooled sufficiently such that it no longer emits significant heat or light. However, the time frame for a white dwarf to cool into a black dwarf is calculated to be longer than the current age of the universe.
We don't know yet. Current models suggest that there are some ten billion white dwarfs in our galaxy alone. The number of white dwarfs in the universe could be uncomprehensibely large.