"There is another type of exploding supernova that also seeds the Galaxy with elements. This is the type Iasupernova. This explosion involves a binary system in which a white dwarf star and an intermediate-mass star (a red giant) orbit each other. The two stars are so close to each other that the white dwarf gradually pulls a considerable amount of material from the outer envelope of the expanding red giant. At a certain point the white dwarf will acquire so much mass that it collapses under its own weight and produces an explosion that blasts the bulk of its material into the interstellar medium—mostly in the form of iron, but also some sulfur, silicon and calcium. Such explosions contributed about 70 percent of the iron we see today in the Galaxy....three processes hold an important key to understanding the evolution of the Milky Way precisely because they occur on very different timescales. By measuring specific abundance ratios in stars from different parts of the Galaxy, astronomers can discover how fast the metal enrichment proceeded and the timescale over which the region was formed."