In the crowded center of the galaxy, two stars orbit so close together that one strips the outer layers off the other, like wind blowing off a coat. What remains is a hot helium core, a star without a hydrogen envelope. Astronomers are familiar with light and very massive stars of this type: the former are called hot subdwarfs, the latter Wolf–Rayet stars. But the intermediate class, from 2 to 8 solar masses, has remained nearly invisible.
The authors of the new study built a computer model of binary systems and traced the fate of intermediate-mass helium stars (helium stars, IMHeS) at different galactic metallicities: that is, the abundance of elements heavier than hydrogen. It turns out that the main 'sculptors' of this population are metallicity and the efficiency of common-envelope ejection. In the Milky Way, there should be several thousand such stars, and in the Magellanic Clouds, a few hundred more. Nearly all of them — more than 90%—live in pairs: most often with an ordinary main-sequence star, but not infrequently with a white dwarf, neutron star, or even a black hole.
The result is like a census of an invisible city: knowing how many such stars exist and where they hide, observers can aim their telescopes and finally see those that have only been guessed at through indirect signs. The gap between subdwarfs and Wolf–Rayet giants will be closed not only in theory but also in real images of the night sky.
Helium stars without hydrogen are so hot that their temperatures can exceed 100,000 degrees, like placing a tiny piece of the Sun's corona in your living room.
In Peter Watts's novel Blindsight, the alien ship turns out to be a clump of reprogrammed stellar material — bare helium stars could be just the right feedstock for such biotechnology.