What Is a Star and What Are They Made Of?

Have you ever asked yourself “What is a star?” or “What are stars made of”? Most people know what a star is, but few can tell you of the top of their head what a star is actually made of. Although it is a common misconception that stars are “giant balls of gas”, they’re anything but. In fact, these large, luminous, incandescent bodies are made of extremely hot, ionized gas called plasma.

What is Plasma?

Many of us were taught that there exists just three states of matter: solid, liquid and gas. However, when energy is added to the third state, excited gas atoms become a whole new state entirely. Instead of bouncing around like gas atoms, these excited particles begin to behave according to fluid dynamics. This high energy state is called plasma, the fourth state of matter. Therefore, the real answer to “What is a star?” is not a ball of gas, but a ball of plasma held in place by strong magnetic fields.

In fact, you may be more familiar with plasma than you think. Have you ever noticed that when you look at a flourescent light a neon sign before it’s turned on, the glass is (sometimes) clear? Yet, when you turn on the light, the sign begins to glow? When the light is turned on, you energize the gas atoms held within the tube. This excites the gas atoms into the plasma phase allowing the light to glow.

So what are stars made of that lets the star glow?

What Are Stars Made Of?

Almost all stars, including our own Sun, are primarily composed of the two lightest elements on the periodic table, hydrogen (73%) and helium (25%), while the remaining 2% consist of other heavier elements. It makes sense that stars would contain significant amounts of hydrogen as it is the most abundant element in the entire Universe. Logically, helium is the second most abundant as it is created from hydrogen fusion. Interestingly, this pattern does not continue for the heavier elements with oxygen being the third most abundant.

In all, the answer to “What are stars made of?” can simply be answered by: hydrogen, helium, and a small percent of everything else.

What Are Stars Made of and How Do We Know?  

Astronomers can determine the composition of stars by analyzing the light they emit through emission spectroscopy. This technique, invented by chemists, lets astronomers determine what elements are contained within a star. When electrons in an atom are excited, like the ones within a star, the excited electrons emit photons of light. Astronomers are able to view this light through spectrometers that separate the light into specific wavelengths. This splits the light into its component colours, showing the observer exactly what wavelengths of light the star is emitting.

Every element in the periodic table emits a specific colors or wavelengths of light – like a chemical fingerprint. Using this technique, astronomers have concluded that the Sun, and many other stars, are made of hydrogen, helium, and a very small fraction of other chemical elements.  

Stars in Our Galaxy

When you go outside at night and look up at the stars, it may seem as though you are gazing at stars far into the distant Universe. Surprisingly, the stars you’re actually seeing are just in our own galaxy, the Milky Way. Stars outside of our galaxy cannot be seen by the naked eye from Earth. In fact, even with the most powerful telescopes most of the stars in our Universe are much too distant and faint to be seen.  

If you’re fortunate enough to be under a dark enough sky, you’ll be able to see the faint band of the Milky Way above. During the winter, this band is especially dim. This is because you are gazing towards the outer arm of the galaxy. During the summer, the Milky Way looks especially bright after letting your eyes adjust. At this time, you are looking towards the center of our galaxy where many, many nearby stars live.

When Did Stars Start Forming?

Stars have been forming since the beginning of the Universe. Around 13.7 billion years ago, shortly after the Big Bang, the Universe was a hot dense mess. Conditions were so extreme that nuclear fusion caused hydrogen atoms to fuse together and form helium.  The Universe then began to expand and cool down at a rapid rate, allowing hydrogen and helium to create heavier and heavier elements.

At this point, stars began to form. These stars, however, were enormous with huge masses that limited their lifespan. It is suspected that these stars only lived within a million years of forming and likely ended their lives as supernovae. These cataclysmic events are responsible for creating the heaviest elements we have today, such as oxygen, carbon, gold and uranium.

What Is a Star’s Fate?

The death of a star depends on how massive a star is. However, the Sun and other stars contain so much matter, that it takes billions of years for the hydrogen in the core to be completely fused into helium. When this happens, the star contracts under the weight of its own gravity while fusion in the upper layers continues to expand. This expansion evolves the star into a red giant. The remaining helium in the core fuels the star by forming carbon. When this fuel is expended, the core expands and cools into a white dwarf.

A star with similar mass to our Sun will have a lifespan of around 10 billion years. Not to worry, our Sun is still fairly young at 4.5 billion years old, giving it about 5 billion years of life left.