What is a Star?

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.

What's UP There?

We've seen them in movies and TV shows and video games as backdrops for science fiction action and adventure tales. Maybe you've seen them when you step outside on a dark night. What are they? Why, they're the stars, of course, and we owe our existence to them.  

Stars look like bright points of light that twinkle. There are thousands of them in your field of view (more if you're in a really dark sky viewing area), and millions beyond our view.

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. 

As you gaze for awhile, you 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 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).

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.

For the Sun, this means that atoms of hydrogen are slammed together under high heat and pressure and the result is a helium atom. The act of smashing them together releases heat and light. This process is called "stellar nucleosynthesis", and is  the source of all the elements in the universe heavier than hydrogen and helium.

That means that everything you see—and even you, yourself—is made of atoms of material made inside a star. 

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 around 15 million Kelvin.

A star that consumes hydrogen to form helium is called a "main-sequence" star. When it uses up all its hydrogen, 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 helium begins to fuse into carbon. The star becomes a red giant. 

How Stars Die

The next phase in the star's evolution depends on its mass. 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. 

High-mass stars, however, will explode in a supernova, blasting 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 either a neutron star or a black hole.

Stars Connect Us with the Cosmos

Stars are found in billions of galaxies across the universe. They are an important part of the evolution of the cosmos. That's because all those elements they form in their cores get returned to the cosmos 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". 

Edited by Carolyn Collins Petersen.