A white dwarf forms only after the star has evolved from the main sequence and then passed through a red-giant stage. This occurred when Sirius B was less than half its current age, around 120 million years ago. The original star had an estimated 5 M☉[12] and was a B-type star (roughly B4–5)[100][101] when it still was on the main sequence. While it passed through the red giant stage, Sirius B may have enriched the metallicity of its companion