The passage describes how the sun will change over time as it uses up its fuel. It explains that in about 5 billion years, the core of the sun will shrink and become hotter, causing the outer layers to expand. At this point, the sun will become a red giant star, and the earth's atmosphere will become too hot to support life. Eventually, the sun will shrink further to become a white dwarf, and then over billions more years will cool completely to become a black dwarf.