April 24, 2024

How the Universe creates stars from cosmic clouds

Graphic timeline representing the evolution of the universe from the primitive universe to the modern universe.

Key Takeaways

  • The birth and life cycle of stars begins in vast, dense molecular clouds, where gravity overcomes pressure, leading to the formation of protostars.

  • These protostars evolve into mature stars through nuclear fusion processes in their cores, with some eventually forming planetary systems in the surrounding accretion disks.

  • This cosmic narrative not only illuminates the origins of stars and planets, but also contextualizes the emergence of life itself in the broader universe.

Just like humans, star birth is a complicated matter. Starting with vast interstellar clouds that collapse under their own titanic gravity, the story of how stars like the Sun are born involves great paroxysms of light and sound (yes, there is sound in space). But when the dust settles (literally), what’s left is a nuclear-powered stellar furnace that will burn brightly for billions of years, surrounded by a family of planets that use the same energy to do everything from powering the climate to create life. This story of star formation is one that astronomers have only recently pieced together through hard-won observations made by the world’s most powerful telescopes. It’s a narrative that not only explains the star-filled night sky, but also places Earth and its inhabitants in their proper cosmic context.

The interstellar medium

To understand the history of star formation, we need to set the cosmic picture. In the current cosmological epoch, much of the Universe’s “normal matter” (excluding “dark” matter) resides in galaxies like the Milky Way (although there is still much matter residing in the vast regions between galaxies as well). Galaxies like ours comprise stars and interstellar gas (also called the interstellar medium, or ISM). The Milky Way, for example, contains more than 100 billion stars and about 10 billion times the Sun’s worth of interstellar material. It is the interstellar material that matters for the formation of new stars and the ISM can take many forms.

Some interstellar gas is evenly distributed in sparse clouds of atomic hydrogen at “hot” temperatures of 8,000 Kelvin. Another form of interstellar material is ultra-hot “corona gas,” millions of degrees in temperature. This material is composed of highly ionized atoms (that is, atoms stripped of some electrons) left over from supernovae, which are apocalyptic explosions of stars much more massive than the Sun. However, neither hot atomic nor hot coronal types of interstellar gas are directly involved in the birth of a star. Both forms of ISM are, as we will see, too thinly spread and at too high a temperature to allow the star formation process to occur in their domains.

It is interstellar gas in the form of molecular clouds which are the focus for star formation. This is because these are the densest and coldest regions of the ISM. With temperatures just tens of degrees above freezing, the gas in these clouds can combine chemically to form things like carbon monoxide, ammonia and molecular hydrogen (hence the “molecular” moniker). Once the molecules are formed, the density in the clouds is high enough to protect them from being split apart by the background stellar UV radiation that fills interstellar space.

So why are low temperatures and high densities critical for star formation? The answer is the Universe’s “eternal war” between gravity and pressure. Clouds are supported by their own weight – their own “self-gravity” – by thermal pressure. The pressure pushes outward, keeping the cloud “inflated,” just as a balloon is inflated by the thermal pressure of trapped air pushing against its rubber skin. To convert interstellar material into a star, you need to gather enough mass so that the sum of its own gravitational pull overcomes the thermal pressure of the cloud and triggers a gravitational collapse. The key physics point is that cold gas has less thermal pressure than hot gas. Equally important, a dense cloud has more self-gravity than a sparse cloud of equal size (more material per cubic meter equals more inward gravitational pull). Put it all together and cold, dense molecular clouds are the only interstellar environment where gravitational collapse can occur. This is why molecular clouds serve as stellar nurseries for the Milky Way.

Gravitational collapse and the birth of protostars

Typical molecular clouds can contain material equivalent to more than a million suns. They can also span hundreds of light years. The size of these clouds means that molecular clouds do not collapse as a single entity. If they did, there would be stars weighing a million solar masses. Instead, today’s largest stars are about 100 times the mass of the Sun. So to study the formation of stars like the Sun, we need to focus on smaller regions of the larger molecular cloud. Molecular clouds are not static structures, but are full of turbulence (that is, random spinning movements, like eddies in a fast-moving river). Turbulence creates local high-density conditions – think of them as “clouds” – where self-gravity can overcome thermal pressure and trigger collapse. It is at the moment when gravity begins to “win” that star formation really begins. Molecular gas and dust begin to fall inward toward the center of the cloud.

The gas and dust accelerate as they fall, eventually reaching speeds of more than 35,000 kilometers per hour. The falling material also becomes denser as it converges toward the center of the cloud. Initially, parcels of gas remain cold as they flow inward. Any heat gained from jostling and clumping as the gas descends is emitted as light. These photons can escape from the ever-shrinking cloud into space (and reach our telescopes). Eventually, though, the falling material becomes so dense that it begins to retain most of its own energy. This includes the heat generated when material collides at hypersonic speeds into the dense, fast-growing central object, which can now be called a “protostar.” Powerful shock waves (a form of sound) form on the surface of the protostar as streams of gas reach its surface. Although most of the energy generated from these shocks remains with the protostar, some energy manages to escape, giving astronomers a key diagnosis of the protostar’s emergence.

The journey from protostar to star

A protostar, however, is not yet a star and the difference lies in the conditions of the core. A star like the Sun is powered by thermonuclear reactions. In the core of a mature star, like the Sun, densities and temperatures are so high that hydrogen nuclei repeatedly collide and fuse into helium, the next heaviest element on the periodic table. It takes four hydrogen nuclei (each of which is just one proton) to form a single helium nucleus (two protons and two neutrons). In the thermonuclear reactions that govern the transformation of hydrogen into helium, a small amount of the hydrogen’s mass is converted into energy (thanks to Einstein’s E = mctwo). Stars, like the clouds from which they are born, use thermal pressure to fight their own crushing gravity. It is the energy released in nuclear reactions that provides this pressure.

When a protostar begins to form at the center of a swirling molecular cloud, it does not have core temperatures and densities high enough to trigger the kind of nuclear reactions in the Sun. Ironically, however, protostars can be much brighter than the Sun. Its luminosity is temporary, borrowed from the energy of the initial fall of the clouds (those shock waves) and a slower contraction of the mass of the protostars. To truly become a mature star, protostars need to gain more mass as they fall. They must also wait for contraction to drive core density upward and raise core temperatures above the critical 10 million degrees Kelvin, where steady helium-producing thermonuclear reactions can begin.

If gravitational decay was all there was to star formation, astronomers would have enough to deal with. Star birth, however, involves another key element besides gravity and thermal pressure. Angular momentum, which you can think of as rotatenot only does it change the basic story of star formation — it’s also the reason you and I are here on a planet to tell the story.

The turbulence of molecular clouds means that every little cloud that’s ready to gravitationally collapse is already spinning in one direction or another (think of all the spinning eddies when you vigorously shake a bathtub of water). Angular momentum is a measure of the cloud’s rotation, and the fundamental laws of physics tell us that it is conserved. This means that when a cloud changes size as it collapses, it cannot lose (or gain) any initial rotational momentum. The conservation of angular momentum has important consequences for our star formation history. Everyone who has watched Olympic figure skating has seen the principle in action: When a skater spins with arms outstretched and then pulls the arms in, his spin rate increases. This is the conservation of angular momentum. If you’re spinning and you get smaller, you’ll also have to spin faster. What should be noted, however, is how much faster the skater turns after just changing their size by an arm’s length. When a spinning cloud collapses into a protostar, it shrinks by a factor of 1 million or more. This is more than enough to reshape the architecture of collapse.

When a spinning cloud collapses, all of the gas does not fall directly into the protostar. Instead, the axis of rotation defines an “equator” for the cloud (the equator would be perpendicular to the axis of rotation). Rotating material falling from the equatorial regions of clouds cannot fall directly onto the star. Instead, it ends up moving around the axis of rotation so quickly that it goes into orbit around the protostar rather than falling over it. In this way, a disk of gas and dust forms around the protostar. This structure, called append disk, will serve as a reservoir of material that slowly spirals through the disk and eventually reaches the protostar. Although originally a theoretical construct (required by conservation of angular momentum), astronomers using the ALMA radio telescope array now have stunning high-resolution images of these disks in star-forming regions around the galaxy.

The appearance of these disks is directly related to the appearance of astronomers like me, who study them. Accretion disks are where planets form. As the dust and gas orbit the protostar, a long series of interactions begin. Dust grains collide to form larger dust grains, which then collide to form pebbles, which then form rocks, and then boulders, and so on, all the way to the planets. Obviously, the process of building the planet involves more than this.

The story of the planet’s formation is complicated, fascinating, and still being worked out. But the important point for our story about star formation is that although it starts with a light-year-long cloud of interstellar gas, it ends with planets. And on some of these planets, life will form, which is why the story of star formation really ends not just with the planets, but with you and me.

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