Extreme-helium star

These stars are supergiants, less massive than dwarf stars but many times larger and hotter. They are remarkable because they contain almost no hydrogen, the most abudant chemical element in the Universe and the most basic component of all stars. Instead, they are dominated by helium, with significant amounts of carbon, nitrogen and oxygen and traces of all other stable elements.

The origin of extreme-helium stars cannot be traced back to protostellar clouds of helium gas, because no such clouds ever existed in the Milky Way. Moreover, nuclear reactions in stars convert hydrogen to helium, but confine the helium to their hot cores, where it cannot be detected spectrally.

Origin

A star must lose vast amounts of hydrogen gas before its helium can escape to the surface. No known mechanism inside the star can drive off the overlying layers to expose the helium; rather, it appears extreme helium stars form from the merger of two white dwarfs.

White dwarfs contain very little hydrogen. Some are rich in helium and others in carbon and oxygen. A pair of white dwarfs can result from the normal evolution of standard stars. In some cases, one star in the binary could evolve as a helium-rich white dwarf, and the other as a carbon-oxygen white dwarf. Over billions of years of orbiting each other, the two stars lose energy and move steadily closer.

Eventually, the helium white dwarf is consumed by the more massive carbon-oxygen white dwarf; the helium-core is torn apart and forms a thick disk around the carbon-oxygen dwarf. In a process that takes only a few minutes, the disk is gravitationally pulled into the carbon-oxygen white dwarf, and the resulting single star swells up to become a helium-rich supergiant.