This can be a white dwarf , a neutron star or a black hole. A black hole has a larger mass, perhaps 10 solar masses. The companion star is usually a normal star such as our sun, with a low mass of around 0.1 to 1.0 solar masses. The two stars orbit in such close proximity that material is stripped from the normal star by the gravitational force of the compact object. Compact binary containing a rapidly spinning white dwarf Binary systems form in much the same way as single stars, condensing under the gravitational force out of vast clouds of gas and dust.
Orbital separation is much greater than shown.
Here we consider binaries where the orbital separation is wide, similar to the distance between the Earth and Sun. At first, the stars evolve independently of each other. The more massive star runs out of hydrogen fuel in its core first and becomes a red giant. The red giant swells to such a size that it engulfs the smaller, less evolved companion star. The dense, small core of the giant star and the companion star orbit each other inside a common envelope .