STAR BIRTH
Revealing the Secrets of Star Birth
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.
A schematic of how a dense cloud of gas and dust collapses to form a star. The core heats up as more material collects there. Wherever temperatures and pressures are high enough, a star will form. It then heats up the surrounding cloud, causing it to glow. Eventually the star eats away the star birth crèche and we see its light.
Star Birth Regions in the Milky Way
Our galaxy contains many places where stars are being born. The most famous is the Orion Nebula (M42). It appears beneath the three belt stars of the constellation Orion, the Hunter, and is about 1,500 light-years away from us. At the heart of the nebula lies a collection of hot young stars. The four brightest are nicknamed “the Trapezium.” These stellar newborns are heating up the surrounding clouds, causing them to glow. Such a glowing cloud is called an emission nebula because it emits light and heat. There are also small, disk-shaped globules of dust in many star birth regions called protoplanetary disks. These are places around young stars where planets can form.
The Orion Nebula is one of the closest and best examples of a star birth region in our galaxy. It lies 1,500 light-years away and contains more than three thousand stars of various ages and sizes, plus many regions where stars are still being formed.
Photo Credit: NASA/ESA/STScI
The nebula surrounding the explosive binary star Eta Carinae is another well-known birth cloud in our galaxy. Nicknamed “the Homunculus,” it lies about 7,500 light-years away in the Southern Hemisphere constellation of Carina. Its clouds glow from the radiation of hot young stars. Their ultraviolet radiation is strong enough to destroy the cloud, which cuts off the supply of material to form other new stars.
Many galaxies are rippling with star birth activity. The Large Magellanic Cloud, a neighbor galaxy of the Milky Way, has a gorgeous star-forming crèche called 30 Doradus, nicknamed “the Tarantula Nebula.” Even from a distance of 160,000 light-years, observers can easily spot clusters of hot young stars that have just exited their nests. They, too, are heating up the surrounding nebula and sculpting the birth cloud as they eat away at the remaining cloud of gas and dust.
The First Stars
The first stars in the universe formed from the hydrogen and helium created in the Big Bang. They were massively huge stars that began coalescing perhaps only a couple of hundred million years after the universe was born. Because their starting masses were so high, these first-borns used up their nuclear fuel very quickly, creating the first heavy elements in a process called stellar nucleosynthesis. When those stars reached the end of their lives, they scattered all their elements into space by means of strong stellar winds and eventually in tremendous supernova explosions. They were the first stellar objects to go through the cosmic process that continues today, spreading elements through interstellar space for the creation of new stars, planets, and in the case of our own planet, the origins of life.