Jano: a white dwarf star with two faces

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The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Pulsating research topics, astronomical adventures and scientific news about the Universe analyzed in depth.

one has been discovered exceptional white dwarf star which presents two faces: the brighter hemisphere is dominated by hydrogen and the other by helium.

White dwarfs are the corpses that result when small stars, of a type similar to our Sun, die. Indeed, when a solar-type star exhausts its nuclear energy, it collapses under the effect of its own gravity, but the mass is not enough to form a black hole, an extremely compact object is thus formed, the size of the Earth, but containing a mass comparable to that of the Sun. Of course, our own Sun will also inexorably become a white dwarf; but for this it takes a long time: about 5,000 million years.

The gravity on the surface of these white dwarfs is 100,000 times greater than on the Earth’s surface and this causes a segregation in the chemical composition: the heavier atoms (such as iron) sink inwards, while the lighter ones ( such as those of hydrogen and helium) remain in the superficial region.

Normally, hydrogen forms a shallower shell than helium. But, sometimes, there are convection phenomena that alter this arrangement. For example, shortly after the catastrophic collapse that, in the midst of a great glow, gives rise to the white dwarf with a temperature of about 100,000 degrees, the star begins to cool down and convection in the helium shell causes this star to cool down. mix with hydrogen. Eventually, the helium remains on the surface for a long time as the star continues to get colder and colder.

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