Introducing Janus, the exotic 'two-faced' white dwarf star
What's all the fuss about a new white dwarf star? What is a white dwarf star anyway?
Our solar system was formed about 4.6 billion years ago. That includes the sun and planets. Ours is an average sun, just a typical one called a yellow dwarf star made of hydrogen and helium held together by gravity and brewing with nuclear fusion. It's about 100 times the diameter of the Earth, but much more massive, of course. It won't always be what you see. About 97% of the stars in our galaxy will eventually become white dwarfs.
Stars don't stay the same; they change over time as they burn the gaseous fuel inside them. The "burning" means the fusion of hydrogen into helium in the inner one-third of the sun's core.
Our sun will soon cool down as the hydrogen runs out, and the helium at the center will contract even further. It will heat up and reignite fusion of the other two-thirds of the hydrogen which surrounds it. This will send out even more energy than before, and the star expands enormously. It'll be cooler, too, which means the color changes to red, and by then it will be called a red giant, about 400-500 times the original size.
Gravity will eventually take hold, literally, and compress everything back again. Depending on the original mass, a star like ours will end up a hot white dwarf ("dwarf" because it'll be much smaller than before) after the red giant phase of its life. It will no longer fuse helium, but it will shine bright white because it will have the same mass of the original sun compressed into the size of the Earth! The helium will have fused into other elements like carbon and oxygen, and eventually iron at the core. It will cool down and shine only from the remaining heat, like a dying ember in a fireplace.
All that will take another 5-7 billion years, so we have time. But we can look at white dwarf stars that have already formed. Recently, scientists have discovered a white dwarf star that is very odd. Its compressed helium and residual hydrogen aren't mixed together evenly. They aren't even in the two layers like the diagram above shows. Instead, they seem to be aggregated into separate halves of the white dwarf. As it rotates, the cooler helium makes it look darker. The existence of these two "faces" have given a reason to name it Janus, after the Roman god of two faces.
How can a white dwarf form like this? Nobody really knows yet. You'd expect an even mixing of hydrogen and helium, or one at the center surrounded by the other like an egg shell. Not so for Janus! This weird half-hydrogen, half-helium two-faced star might be a phase in the development that will look like the diagram above. Scientists suggested that "if the magnetic field is stronger on one side than the other", then it might separate the hydrogen from the helium as they have seen.
Finally, a white dwarf is expected to reach the final stage of its evolution: a black dwarf. This is when the white dwarf has fused all of the elements it possibly can with the temperatures it still has. It would hard to detect it because it would be very dim and cold; its influence on other objects due to its gravity would be the main way to know it's there. But, scientists have estimated that black dwarfs need longer than the current age of the universe (13.7 billion years) to reach that state, so it's going to take a while before we locate any.
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