A Star is Born (Lives of Stars, Part I)

By Dylan Lee, Stellar Astronomy

The stars in our night sky seem everlasting. Modern astronomers chart the motions of these stars, but observations of these intriguing points of light date back to those of the Mesopotamians, from before 3000 BCE. But stars, like everything else, come and go. They have turbulent births and violent deaths, and in the millions of years of their lifetime, they undergo changes in temperature, size, brightness, and color. Let's ignite this series on the life of a star as we explore the formation of these giant orbs of heat and light.

Origins of a Star

The Crab Nebula, photographed by the Hubble Space Telescope

Despite the commonly used term "empty space," the universe is far from empty. It is filled with vast clouds of dust and gas molecules. The scientific name for one of these giant clouds is known as a nebula (plural: nebulae) and is composed almost completely of hydrogen, the lightest and most abundant element in the universe. These vast nebulae are extremely cold, only about 10 to 20 Kelvin (-263° C to -253° C). They are also extremely massive, often hundreds of times the mass of the Sun. These conditions are perfect for forming a star. To understand why, we must see the birth of a star as a struggle between inward gravitational attraction and outward gas pressure.

Let's begin with gravitational attraction. At its foundation, everything in the universe is made of fundamental building blocks, known as matter. The measurement of the amount of matter in an object is known as mass. And all objects with mass (therefore all objects composed of matter) interact with gravity. The result of these interactions is that objects with mass are attracted to each other, hence the term gravitational attraction. Furthermore, if an object has more mass, it will experience a stronger gravitational attraction.

Now, to understand gas pressure, we can use an analogy of people in a room. The gas molecules are represented by the people, and the space that they occupy is represented by the room. Let's imagine a given number of people, say 50, in a small room. The people would feel extremely cramped and stressed since they don't have much room to move around. This is equivalent to having high pressure. On the other hand, if we place the same 50 people in a huge auditorium, they would have abundant personal space. This would be the equivalent of having low pressure. Now let's imagine that these people have perhaps eaten a lot of sugar and are very energetic (this is equivalent of increasing the energy of a gas by increasing its temperature). They might now want to move and run around. Suddenly, the auditorium doesn't seem so large anymore. Increasing the people's energy increased the "pressure" in the auditorium. Likewise, increasing the energy or temperature of a gas will also increase its pressure.

Recall the conditions of our nebula. It is extremely massive with an extremely low temperature. Because its mass is so large, certain areas of the nebula will interact gravitationally with other areas… the nebula is essentially gravitationally attracted to itself! Therefore, the gravitational forces actually act inward, pulling the cloud in on itself. On the other hand, the hydrogen gas molecules attempt to use their pressure to disperse outward throughout space so that they aren’t all condensed together (gas molecules, like people, don't like overly crowded places). However, the gas pressure is tiny, and their effort is therefore futile. This is because gas molecules have a huge volume to "roam around" and, because of the nebula’s low temperatures, very little energy to do so (think of the people in our auditorium now becoming very lazy). With such strong gravitational forces and such weak resistance from the gas pressure, gravity's victory is inevitable. A huge cloud of hydrogen in the nebula condenses and collapses under the pull, forming a dense core, or center.

A diagram of a protostar: a cloud of hydrogen gas with a dense, hot core. The protostar will eventually become a true star.

This collapse has an important side effect, however. The volume of the hydrogen cloud is now much smaller, which increases the gas pressure. Simultaneously, the temperature at the core also increases. To picture this, imagine our auditorium from before shrinking. This results in the people inside becoming aggravated. They subsequently begin to run around, panicked, often running into each other. This is equivalent of the gas molecules jostling around in the cloud, increasing its temperature. This cloud of hydrogen with a dense, hot core is known as a protostar, and is ultimately what will become a young star.

Burning Bright... Or Not

The protostar continues to collapse, as the force of gravity is still greater than the outward gas pressure. Eventually, the pressure and temperature at the core of the protostar reach critical values at which nuclear fusion of hydrogen can begin in the core. Nuclear fusion is the process by which atoms of a lighter element combine to form atoms of a heavier element. The start of nuclear fusion in the core of the protostar marks its transformation into a true star. In a star, the hydrogen atoms combine to form helium, the second lightest element in the universe. More importantly, however, this process releases an enormous amount of energy.

Hydrostatic Equilibrium: a balance between inward gravitational attraction and outward gas pressure

What does this mean for the star? The energy that is released by nuclear fusion is emitted from the star in the form of heat and light, which is why stars are visible in the night sky even though they are billions or trillions of miles away (this is a testament to just how much energy is released by the fusion process). Besides illuminating the star, however, this energy also increases the gas pressure in the star’s core. Now, the outward pressure of the gas can perfectly balance the inward gravitational attraction from the star's mass. This delicate balance between gravity and pressure is called hydrostatic equilibrium. The hydro- prefix comes from the fact that the gaseous hydrogen core of the star is a fluid (liquids or gases are both fluids), and the -static suffix reveals that the pressure from the fluid is not caused by outward motion (in other words, the auditorium from before is not moving; only the people inside it are). Finally, the term equilibrium implies a balance between various quantities or forces that produce a stable state. For a star, this is a balance between gravitational attraction and the pressure produced by nuclear fusion, turning our turbulent protostar into a powerful, stable star that provides its surroundings with heat and light.

Sometimes, though, things can go wrong. Recall that the strength of gravitational forces in the cloud depends on its mass (a larger mass results in stronger gravitational forces and a smaller mass results in weaker gravitational forces). This means that in some lower-mass clouds, the gravitational forces do not compress the protostar far enough to reach the critical temperature and pressure required to begin fusing hydrogen to form helium, and thus no energy is released. (The approximate mass for this to happen is less than 1/12 of the mass of the Sun.) The resulting brown dwarf, named for its dull color and small size, does not emit heat or light as a true star does, but ends up as a dark, dense ball of gas.

An artist's depiction of a brown dwarf: a small, dull object which didn't reach the critical temperature and pressure to being nuclear fusion

As the conditions for star formation above reveal, the birth of a star requires just the right conditions. Part of a vast nebula with a huge amount of mass and extraordinarily low temperatures must first begin to compress. Then, the outward pressure inside the star must perfectly balance the inward gravitational attraction of the star. Not only this, the compression of the protostar must create enough heat and pressure to begin nuclear fusion. If it doesn't, the emergent brown dwarf will end up drifting through the universe, unseen. But if all the conditions are met, a glorious, dazzling star will be born, with the potential to supply a planet like Earth with just the right resources to sustain life.

It's almost miraculous, isn't it?

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Fraknoi, Andrew, et al. Astronomy. Houston, Texas, OpenStax, Rice University, 2017.

Nagaraja, Mamta Patel. "Stars", Universe, NASA Science, December 2018, https://science.nasa.gov/astrophysics/focus-areas/how-do-stars-form-and-evolve.

Protostar diagram created in Google Drawings.

Photos are linked to sources.

Cover photo: Eagle Nebula taken by the Hubble Space Telescope: http://hubblesite.org/image/3862