What Are Stars Made Of?

Few would argue that we have the Sun to thank for making Earth a hospitable place to live. We can see its blinding light and feel its warmth on us on a clear day. It may be harder to wrap our minds around the fact that every star acts much the same way. What are stars made of that they are able to produce such impressive amounts of energy?

What Do Stars Consist Of

What Is a Star?

It’s easy to look up at the heavens on a clear night and pick out stars among the sea of darkness that surrounds us. What differentiates a star from any other object in our galaxy and beyond?

Simply put, a star is a large, luminous sphere massive enough to ignite the fusion of elements within its core. It does so through the strong gravitational forces putting pressure on its core.


What Elements Make Up a Star?

What Elements Make Up a Star
Fusion of helium in the core of low mass stars. (Image credit: RicHard-59 on Wikimedia Commons CC BY-SA 3.0)

To generate nuclear fusion, stars need to mash two elements together to form a new element.

For every star, these two elements are hydrogen and helium. In fact, approximately 98% of these celestial objects are made up of these two elements, with hydrogen being the vast majority. The remaining 2% are heavier elements such as carbon, nitrogen, oxygen, and iron.

Surprisingly, none of these elements are unique to stars. The elements that provide the heat and light we need to survive aren’t even hard to find here on Earth!

The Layers of a Star

All that hydrogen and helium has to end up somewhere. Stars have a total of six different layers that each function in a unique way.

Artist's Concept of the Layers of the Sun
Artist’s concept of the layers of the sun. (Image credit: NASA)


At the very center of a star is the core. This incredibly dense layer creates the gravity needed for the fusion reactions that give life to the star. These reactions cause the core to be the hottest part of the star and generate energy outward.

Radiative Zone

After a fusion reaction takes place, energy is expelled from the core and moves outwards. In the radiative zone, this energy radiates the same way a fluorescent light bulb emits heat. It travels this way hundreds of thousands to millions of miles (or kilometers) to the convection zone. 

Convection Zone

Once energy reaches the convection zone, it learns a new way to travel. Radiation is no longer efficient so far from the core, so energy is transferred similarly to how heat moves from a pot to boiling water. Convection becomes the norm until the energy reaches the star’s surface.


After clearing the surface, energy travels through the first layer of a star’s atmosphere, the photosphere. This very thin layer sees a lot of energy converted to visible light.


The remaining energy travels through the chromosphere before escaping into space. Scientists theorize that this layer may help conduct heat through the star and out to the corona. The chromosphere does not emit enough light to be visible except when the rest of the star is hidden from view, as in a solar eclipse.


A star’s corona, or crown, is the outermost layer. Although invisible to the naked eye, this ring of plasma extends up to millions of miles into space. It is also significantly hotter than the other layers of the atmosphere.

A Star’s Life Cycle

Life Cycle of a Star
From Nebula to Star to Black Hole, created with InspirationLite. (Image credit: Jacobbykhovsky on Wikimedia Commons, CC BY-SA 4.0)

During the long life of a star, it continuously fuses hydrogen atoms into helium. As massive as these giant bodies are, this process can take upwards of ten billion years to burn through all its hydrogen. Once it runs out, a star enters its final millions of years, relying on helium for fuel.

In the case of our Sun, it’s about halfway through its life cycle. Fortunately, we should still be able to rely on our star for another five billion years. I don’t know about you, but I don’t plan on being around when that happens.

How Does a Star Form in the First Place?

Stars come to life from areas within galaxies called nebulae that have an abundance of hydrogen gas. These clouds, much larger than any star, are known to form pockets of mass where gravity begins to take effect.

The gravitational pull generated at the core of this growing ball of hydrogen builds in mass and pressure. The effect superheats the core to 10 million Kelvin, the point where nuclear fusion can begin.

The Energy That Is Light

Of the potentially 200 billion stars in the Milky Way alone, no two of them are exactly the same. One of the optimal ways to see these differences is through the light each star emits.

As mentioned earlier, a portion of a star’s energy becomes light the moment it reaches the photosphere layer. The color of this visible light is dependent on its temperature as it leaves the star’s surface.

The hottest stars shine a vibrant shade of blue. Cooler stars appear as white, yellow, or orange, with the coolest stars in the known universe a deep red.

In this image taken by the NASAESA Hubble Space Telescope, two stars shine through the center of a ring of cascading dust.
In this image taken by the NASAESA Hubble Space Telescope, two stars shine through the center of a ring of cascading dust. (Image credit: NASA)

What Keeps Them Shining So Bright?

Each time a collision between two atoms happens inside a star, a nuclear reaction occurs. Our Sun, a medium-sized star, converts four million tons of gas into heat and light every second.

These constant and consistent reactions will likely be so for several billion more years. With this seemingly limitless fusion taking place, stars like our Sun are able to provide a persistent source of both forms of energy. Unless a star ceases to exist, you can bet on the fact that you’ll be able to see it at the same brightness night after night.

Why Do Stars Twinkle?

As light travels from distant stars to Earth, this light moves unimpeded in a straight line until it reaches our atmosphere. These thin beams contact all the various gas particles within, bouncing around before finally reaching your eye. These frantic movements give starlight a beautiful twinkle.

This twinkle can pose a problem for astronomers who want to see clear images from the far reaches of space. Since the entire Earth is covered in layers of atmosphere, there’s no avoiding this issue while planetside. To remedy this problem, we’ve successfully placed the Hubble and now the James Webb telescopes in orbit above our atmosphere.

Why stars twinkle? On this StarTalk explainer video, Neil deGrasse Tyson and comic co-host Chuck Nice are going to give you the answers.

Final Thoughts

It’s incredible to consider that nothing more than hydrogen, helium, and a whole lot of pressure provides the warmth and light we need to survive here on Earth. These two elements exist in differing forms through six stellar layers, each functioning to expel energy into the confines of space.

Fortunately for us, this energy becomes the innumerable points of light that we’re able to capture with our cameras on a majestic clear night.

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.