How can an ancient white dwarf orbit a star only 100 million years old?
In binary star systems, the stars often are close enough that they can exchange mass. Because the masses of the stars are generally not exactly equal when born, the more massive star will reach the red giant stage first, and will shed mass into space through powerful stellar winds. This mass can be captured by a companion star and over time it can 'reset' the clock of the companion star, which will now behave as a massive star. The red giant then evolves into a white dwarf star, but its companion is now a star that is so massive that it may run through its remaining life in only a few hundred million years. It is estimated there are some one million close-binary star systems in our galaxy in which there is transfer of mass from one star to the other. About 1,000 of these were previously known. Sirius is an example of just such a system where an A-type star with an age of about 500 million years is orbited by a white dwarf star, which is usually the corps of a star that has taken billions of years to become a red giant star before leaving a white dwarf remnant behind.
