Going Inside a Star to See How It Works

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The closest star to the Sun, Proxima Centauri is marked with a red circle, close to the bright stars Alpha Centauri A and B. Courtesy Skatebiker/Wikimedia Commons.

The stars have always intrigued people, probably from the moment our earliest ancestor stepped outside and looked up at the night sky. We still go out at night, when we can, and look up, wondering about those twinkly objects. Scientifically, they are the basis of the science of astronomy, which is the study of stars (and their galaxies). Stars play prominent roles in science fiction movies and TV shows and video games as backdrops for adventure tales. So, what are these twinkling points of light that seem to be arranged in patterns across the night sky?  

A star chart showing the Big Dipper
Stars are more than simply objects in the sky. They teach us about the workings of the universe, from the earliest stars to the current ones. People have long used star charts like this one to find their way around the sky at night. Stars are also useful navigational aids for sailors as well as stargazers. Carolyn Collins Petersen

Stars in the Galaxy

There are thousands of stars visible to us from Earth, particularly if we do our observing in a really dark sky viewing area). However, in the Milky Way alone, there are hundreds of millions of them, not all visible to people on Earth. The Millky Way is not only home to all those stars, it contains "stellar nurseries" where newborn stars are being hatched in clouds of gas and dust.

All stars are very, very far away, except for the Sun. The rest are outside of our solar system. The closest one to us is called Proxima Centauri, and it lies 4.2 light-years away. 

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A Hubble Space Telescope view of Proxima Centauri. NASA/ESA/STScI

Most stargazers who have observed for a while start to notice that some stars are brighter than others. Many also seem to have a faint color. Some look blue, others white, and still others faint yellow or reddish hues. There are many different types of stars in the universe. 

The double star Albireo in Cygnus.
Notice the two slightly different colors of the stars that make up Albireo, the double star in the nose of Cygnus the Swan. They can be easily seen through binoculars or a small telescope.  Courtesy N.B., via Wikimedia Commons, Attribution-Share Alike 4.0 license.

The Sun is a Star

We bask in the light of a star — the Sun. It's different from the planets, which are very small in comparison to the Sun, and are usually made of rock (such as Earth and Mars) or cool gases (such as Jupiter and Saturn). By understanding how the Sun works, astronomers can gain a deeper insight into how all stars work. Conversely, if they study many other stars throughout their lives, it's possible to figure out the future of our own star, too. 

Layers of the Sun
The layered structure of the Sun and its outer surface and atmosphere gives astronomers insights into how other stars are structured. NASA 

How Stars Work

Like all other stars in the universe, the Sun is a huge, bright sphere of hot, glowing gas held together by its own gravity. It lives in the Milky Way Galaxy, along with approximately 400 billion other stars. They all work by the same basic principle: they fuse atoms in their cores to make heat and light. It's how a star works.

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A cutaway of the interior of the Sun. Most stars have similar types of zones, including the cores where nuclear fusion takes place. NASA/MSFC

For the Sun, this means that atoms of hydrogen are slammed together under high heat and pressure. The result is a helium atom. That process of fusion releases heat and light. This process is called "stellar nucleosynthesis", and is the source of many of the elements in the universe heavier than hydrogen and helium. So, from stars like the Sun, the future universe will get such elements as carbon, which it will make as it ages. Very "heavy" elements, such as gold or iron, are made in more massive stars when they die, or even the catastrophic collisions of neutron stars.

How does a star do this "stellar nucleosynthesis" and not blow itself apart in the process? The answer: hydrostatic equilibrium. That means gravity of the star's mass (which pulls the gases inward) is balanced by the outward pressure of the heat and light—the radiation pressure—created by the nuclear fusion taking place in the core.

This fusion is a natural process and takes a tremendous amount of energy to initiate enough fusion reactions to balance the force of gravity in a star. A star's core needs to reach temperatures in excess of about 10 million Kelvin to start fusing hydrogen. Our Sun, for instance, has a core temperature of around 15 million Kelvin.

A star that consumes hydrogen to form helium is called a "main-sequence" star for all the time it is a hydrogen-fusing object. When it uses up all its fuel, the core contracts because the outward radiation pressure is no longer enough to balance the gravitational force. The core temperature rises (because it's being compressed) and that gives it enough "oomph" to start fusing helium atoms begin into carbon. At that point, the star becomes a red giant. Later, as it runs out of fuel and energy, the star contracts in on itself, and becomes a white dwarf.

How Stars Die

The next phase in the star's evolution depends on its mass because that dictates how it will end. A low-mass star, like our Sun, has a different fate from stars with higher masses. It will blow off its outer layers, creating a planetary nebula with a white dwarf in the middle. Astronomers have studied many other stars that have undergone this process, which gives them greater insight into how the Sun will end its life a few billion years from now.

A planetary nebula in Aquila.
Could our Sun end its life looking like the planetary nebula NGC 678? Astronomers suspect that it may well do so. ESO 

High-mass stars, however, are different from the Sun in many ways. They live short lives and leave behind gorgeous remains. When they will explode as supernovae, they blast their elements to space. The best example of a supernova is the Crab Nebula, in Taurus. The core of the original star is left behind as the rest of its material is blasted to space. Eventually, the core could compress to become a neutron star or a black hole.

The Crab Nebula
Hubble Space Telescope's view of the Crab Nebula supernova remnant. NASA/ESA/STScI

Stars Connect Us with the Cosmos

Stars exist in billions of galaxies across the universe. They are an important part of the evolution of the cosmos. They were the first objects to form more than 13 billion years ago, and they comprised the earliest galaxies. When they died, they transformed the early cosmos. That's because all those elements they form in their cores get returned to space when stars die. And, those elements ultimately combine to form new stars, planets, and even life! That's why astronomers often say that we are made of "star stuff".