Two spinning tops whirl so close they nearly touch. One is scorching blue, the other dim white. Together they complete a full orbit in twenty hours. This is the pair Feige 64: a blue star with a mass of just a third of the Sun's, and a white dwarf, the cooled core of a former star, one and a half times heavier than our Sun.
The blue star seems too hot for its mass. Its temperature is fifteen and a half thousand degrees, like the hottest filament in a lamp. And it shines forty times brighter than the Sun. An ordinary star of that mass would have faded long ago, but this one keeps burning. The secret is that it burns not hydrogen in its core, but helium in a thin shell around it.
How did this happen? Once there were two stars in the system. The more massive one swelled and transferred its material to its neighbor. That one, in turn, also swelled and enveloped both stars in a common gas envelope. Friction inside this envelope caused the stars to spiral closer together, while the envelope dispersed. In the end, a bare hot star and a white dwarf remained.
This pair is direct evidence that blue horizontal branch stars can be born not in isolation, but in binary systems. Previously, this was only speculated. Now we see how one star 'undresses' another and makes it shine brighter than its mass would dictate.
The white dwarf in this pair is heavier than the Sun, but the size of Earth. A teaspoon of its material would weigh as much as several elephants.
In Stanislaw Lem's novel 'Solaris', an ocean planet orbits a double star — a red and a blue sun. Here, a blue star and a white dwarf whirl so close that from a planet, if one were there, the sky would look like two blinding disks.