I maybe wrong, but arn't white dwarfs whats left when a star about the size of our sun reaches the end of it's life. Also don't the leave a planetary nebula? Sirius B is a white dwarf, where is it's planetary nebula? To become a white dwarf a star can not be much bigger than the sun if it is then it becomes a supernova and leaves either a nutron star or a black hole. So for Sirius B to have become a white dwarf it should be twice as old as our sun, which is about half way through it's life. How is it that Sirius A is younger than our sun, while Sirius B is older than the sun.