TOEFLリスニング

🎧 リスニング原文

Narrator: Listen to part of a lecture in an astronomy class. Professor: Now, we often think of stars as permanent fixtures in the sky, but they actually have life cycles. A star's fate depends primarily on its initial mass. For a star like our Sun, which is of low to medium mass, it will eventually exhaust the hydrogen in its core. When that happens, the core contracts and heats up, while the outer layers expand, turning the star into a red giant. This stage is brief in cosmic terms, lasting only a few hundred million years. Eventually, the outer layers are ejected, forming a beautiful planetary nebula, and the remaining core becomes a white dwarf—a dense, Earth-sized object that slowly cools over billions of years. But for stars much more massive than the Sun, the process is different. They burn fuel rapidly and live shorter lives. After a series of nuclear reactions, they may end in a supernova explosion, leaving behind either a neutron star or, if the core is massive enough, a black hole. So, the mass of a star is the single most important factor in determining its ultimate fate.