Star birth is, as the physicist Heinz R. Pagels (1939–1988) wrote in 1985, a “veiled and secret event.” Today, it’s well known that star formation takes place deep inside interstellar clouds of gas and dust in stellar crèches that were once impossible for us to detect. Only after the process is complete does the light from the newborn star manage to leak out and announce to the universe that a new star has been born. It’s a process that takes place in every galaxy across the cosmos, and one that has been going on since shortly after the universe was created some 13.8 billion years ago. With the advent of infrared-enabled instruments, astronomers have been able to peek into the clouds and learn more about this once-hidden process.
It Starts in the Dark
Star birth begins in a region of interstellar space filled with gas and dust called a molecular cloud. This process might ignite in a dark nebula, a cloud that is so dense that light can’t pass through it. Something happens to disturb the thick, slowly moving globules of gas and dust. Perhaps a nearby supernova sends shock waves through the cloud, or another star passes nearby. The action spins the cloud and compresses it. Molecules of gas and the dust particles are crushed together, and that action causes friction heating. More and more gas and dust is pushed into this hot core, which grows more massive very quickly. As it does, its gravitational pull tugs more material in, compressing what’s already in the interior. When temperatures and pressures get high enough, conditions are right for the process of nuclear fusion to begin in the core of this protostellar object. Molecules of hydrogen begin smacking together to form helium. That process releases energy in the form of heat and light, and that’s what powers stars. The birth of the star is marked by the moment when nuclear fusion begins. After that, the newborn star continues to heat up; in the early phase of its life, it has gas jets streaming away from its polar regions. These help dissipate the tremendous heat built up as the star forms. If the stellar newborn has enough material remaining around it, it’s possible that planets can form there.
By the standards of a human lifetime, stars seem to last forever. Even the shortest-lived ones—the massive, hot OB stars—live for a million or so years. On the other hand, dense stellar objects called white dwarfs spend tens of billions of years dwindling down to become cold cinders called black dwarfs. As they go through their lives, stars fuse elements in their cores in a process called nuclear fusion. That’s what the Sun is doing right now. It’s on the main sequence, a phase where stars spend their time fusing hydrogen in their cores. When they stop fusing hydrogen, they leave the main sequence, and that’s when things get interesting.
Stars Like the Sun
When the core runs out of hydrogen fuel, it will contract under the weight of gravity. However, some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up. This heats the upper layers, causing them to expand. As the outer layers expand, the radius of the star will increase and it will become a red giant. The radius of the red giant sun will be just beyond Earth’s orbit. At some point after this, the core will become hot enough to cause the helium to fuse into carbon. When the helium fuel runs out, the core will expand and cool. The upper layers will expand and eject material that will collect around the dying star to form a planetary nebula. Finally, the core will cool into a white dwarf and then eventually into a black dwarf. This entire process will take a few billion years.
Stars More Massive Than the Sun
When the core runs out of hydrogen, these stars fuse helium into carbon just like the sun. However, after the helium is gone, their mass is enough to fuse carbon into heavier elements such as oxygen, neon, silicon, magnesium, sulfur and iron. Once the core has turned to iron, it can burn no longer. The star collapses by its own gravity and the iron core heats up. The core becomes so tightly packed that protons and electrons merge to form neutrons. In less than a second, the iron core, which is about the size of Earth, shrinks to a neutron core with a radius of about 6 miles (10 kilometers). The outer layers of the star fall inward on the neutron core, thereby crushing it further. The core heats to billions of degrees and explodes (supernova), thereby releasing large amounts of energy and material into space. The shock wave from the supernova can initiate star formation in other interstellar clouds. The remains of the core can form a neutron star or a black hole depending upon the mass of the original star.