I take this to mean that the small white dwarf was once a star more massive than the Sun, while its companion may contain no more mass than the Sun. However, the companion has already ejected its outer layers into space, suggesting that it, too, was definitely more massive than the Sun when it was on the main sequence.Ordinary planetary nebulae have one star at their centre, bipolar nebulae have two, in a binary star system. Astronomers have found that the two stars in this pair each have around the same mass as the Sun, ranging from 0.6 to 1.0 solar masses for the smaller star, and from 1.0 to 1.4 solar masses for its larger companion. The larger star is approaching the end of its days and has already ejected its outer layers of gas into space, whereas its partner is further evolved, and is a small white dwarf.
This leads me to wonder how many white dwarfs there are in the nearby universe (say, in our galaxy) that started out as stars with no more mass than our Sun. Are white dwarfs born from solar-mass stars still a rarity in our circa 14 billion year old universe?