Protostars: New Suns in the Making

protostar
HBC-1 is a protostar, a newborn stellar object that has not yet ignited nuclear fusion in its core. All stars go through this stage during formation. NASA/STScI

Star birth is a process that has been happening in the universe for more than 13 billion years. The first stars formed from giant clouds of hydrogen and grew to become supermassive stars. They eventually exploded as supernovae, and seeded the universe with new elements for new stars. But, before each star could face its ultimate fate, it had to go through a lengthy formation process that included some time as a protostar.

Astronomers know a lot about the process of star formation, although there is certainly always more to learn. That's why they study as many different star birth regions possible using such instruments as the Hubble Space Telescope, the Spitzer Space Telescope, and ground-based observatories outfitted with infrared-sensitive astronomy instruments.  They also use radio telescopes to study the young stellar objects as they're forming. Astronomers have managed to chart nearly every bit of the process from the time clouds of gas and dust start down the path to stardom.

From Gas Cloud to Protostar

Star birth begins when a cloud of gas and dust starts to contract. Perhaps a nearby supernova has exploded and sent a shock wave through the cloud, causing it to start moving. Or, maybe a star wandered by and its gravitational effect began the cloud's slow motions. Whatever happened, eventually parts of the cloud start to get denser and hotter as more material gets "sucked in" by the increasing gravitational pull.

The ever-growing central region is called a dense core. Some clouds are quite large and may have more than one dense core, which leads to stars being born in batches.

In the core, when there's enough material to have self-gravity, and enough outward pressure to keep the area stable, things cook along for quite a while.

More material falls in, temperatures rise, and magnetic fields thread their way through the material. The dense core isn't a star yet, just a slowly warming object.

As more and more material gets swept into the core, it starts to collapse. Eventually, it gets hot enough to start glowing in infrared light. It's still not a star yet — but it does become a low-mass proto-star. This period lasts about a million years or so for a star that will end up being about the size of the Sun when it's born.

At some point, a disk of material forms around the protostar. It's called a circumstellar disk, and usually contains gas and dust and particles of rock and ice grains. It may well be funneling material into the star, but it's also the birthplace of eventual planets.

Protostars exist for a million years or so, gathering in material and growing in size, density, and temperature. Eventually, the temperatures and pressures grow so much that nuclear fusion is ignited in the core. That's when a protostar becomes a star — and leaves stellar infancy behind. Astronomers also call protostars "pre-main-sequence" stars because they have not yet begun fusing hydrogen in their cores. Once they start that process, the infant star becomes a blustery, windy, active toddler of a star, and is well on its way to a long, productive life.

Where Do Astronomers Find Protostars?

There are many places where new stars are being born in our galaxy. Those regions are where astronomers go to hunt the wild protostars. The Orion Nebula stellar nursery is a good place to search for them. It's a giant molecular cloud about 1,500 light-years from Earth and already has a number of newborn stars embedded within it. However, it also has clouded little egg-shaped regions called "protoplanetary disks" that are likely harboring protostars within them. In a few thousands of years, those protostars will burst into life as stars, eat away the clouds of gas and dust surrounding them, and shine out across the light-years.

Astronomers find starbirth regions in other galaxies, as well. No doubt those regions, such as the R136 starbirth area in the Tarantula Nebula in the Large Magellanic Cloud (a companion galaxy to the Milky Way), also are studded with protostars.

Even farther away, astronomers have spotted starbirth crêches in the Andromeda Galaxy. Wherever astronomers look, they find this essential star-building process going on inside most galaxies, as far as the eye can see. As long as there's a cloud of hydrogen gas (and maybe some dust), there's plenty of opportunity and material to build new stars — from dense cores through protostars all the way to blazing suns like our own.

This understanding of how stars form gives astronomers a lot of insight into how our own star formed, some 4.5 billion years ago. Like all the others, it began as a coalescing cloud of gas and dust, contracted to become a protostar, and then eventually began nuclear fusion. The rest, as they say, is solar system history!