What Are Stars Made Of? – The Explosive Stellar Beauties

Stars have long captivated our imaginations with their brilliance and enigmatic nature. As children, we wish upon them, knowing that stars can somehow make our dreams come true. But have you ever wondered what are stars made of? 

At their core, stars are giant fusion reactors that transform hydrogen into helium through the process of nuclear fusion. This fusion process releases incredible energy, powering the star’s luminosity.

Let’s dive into the elemental composition of stars, unraveling the secrets of their cosmic chemistry and shedding light on the awe-inspiring ingredients that make up these celestial wonders.

A Star’s Physical Properties

The physical properties of a star can vary depending on its size, age, and stage of evolution. However, here are some fundamental physical properties of stars.


Mass is one of the most essential properties of a star. It determines the star’s temperature, luminosity, and lifespan. Stellar mass typical measurements are in units of solar masses (M☉), where one solar mass equals the mass of the Sun.

Diameter and Radius

The diameter or radius of a star refers to its size. Stellar diameters can range from a fraction of the Sun’s diameter to hundreds of times greater than the Sun’s diameter. The hugest stars, such as red supergiants, can have diameters over one thousand times that of the Sun. Whoa! 

What are stars made of?
Image: The Jellyfish galaxy JW39 from Hubble, ESA, and NASA.


Measuring a star’s surface heat helps determine its temperature, typically represented using the Kelvin scale. Stars can have a wide range of temperatures, from relatively cool stars like red dwarfs with temperatures around 3,000 Kelvin to scorching stars like blue giants with temperatures over 30,000 Kelvin.

Kelvin is used to quantify temperatures outside of Earth. On Earth, we use Fahrenheit or Celcius as a means to quantify temperature. But the temperatures simply become too “big” once we’re away from Earth, so we use Kelvin as the standard.

Spectral Type

The spectral type of a star is a classification system that categorizes stars based on their spectral characteristics, determined by their surface temperature. The most common spectral classes are O, B, A, F, G, K, and M, with O-type stars being the hottest and M-type stars being the coolest.


The star’s total amount of energy emitted per unit of time is luminosity. It is related to both the star’s temperature and its size. Astronomers measure luminosity in units of solar luminosities (L☉), where one solar luminosity equals the luminosity of our Sun.


Stars are made of hydrogen and helium but contain trace amounts of other elements. The composition of a star affects its behavior, including its energy production through nuclear fusion.

But stars are not just composed of hydrogen and helium. Instead, they contain a fascinating mixture of various elements, including carbon, nitrogen, oxygen, and heavier elements like iron and gold. These elements forge within stars through a series of complex processes, including stellar nucleosynthesis and explosive supernova events.


The age of a star refers to the time since it formed. Young stars are typically more active and have higher surface temperatures, while older stars are often cooler with less activity. Star age estimates come from factors such as a star’s location in a star cluster or its stage of evolution.

A Deeper Dive Into What Are Stars Made Of

Stars are certainly the building blocks of the universe. They create and distribute the heavy elements found in nearby planetary systems. Oxygen, nitrogen, and carbon come from stars, so understanding what are stars made of and how they’re born, live, and die teaches astronomers much about our universe.

How Stars Are Born

Dark matter comprises about 90% of most galaxies. Another seven percent contains cosmic dust and gases. And only about three percent of the universe consists of stars. The Elephant’s Trunk Nebula, pictured below, is a stellar nursery where stars are born.

How Stars Are Born
Image: NASA/JPL-Caltech/W. Reach

Stars are born within the stellar nurseries of the dust clouds. Here’s what happens to bring a star to life.

  • Turbulence within the clouds creates clumps with a large enough mass that the dust and gas collapse under the gravitational attraction.
  • The center material starts heating up.
  • The hot core at the collapsing cloud’s center becomes a protostar and then a star.
  • As the star forms, the spinning and collapsing dust and gas clouds can break into two or three portions, creating star groups.
  • The remaining dust might form into comets, asteroids, or planets. Or it might stay in dust form.

The Life of A Main Sequence Star

The Sun took about 50 million years to mature, and now it will stay on its main sequence for about ten billion years.

Hydrogen’s nuclear fusion fuels the star and forms helium within its core. That energy flows outwards from the star to make it shine. And it also provides necessary pressure so the star doesn’t collapse under its own weight.

How Stars Die

Larger stars tend to have shorter lives but still live for billions of years. Nuclear reactions stop once a star has fused all the hydrogen at its center. And then, without the energy and pressure, the star’s core starts heating and collapsing in on itself.

Hydrogen fusion outside the core continues. But the superheated center of the star forces it outward, causing the outer layers to expand and cool until the star transforms into a red giant.

The hot core creates helium-consuming nuclear reactions when this process happens in massive stars. And that produces heavier elements up to iron. Then, as the star becomes more unstable, it varies between burning furiously and dying down. And the irregular pulses throw off the outer layers, creating clouds of dust and gas.

Those clouds mix with surrounding interstellar dust and gas clouds. The heavy element and chemical compounds from the star’s death get recycled and form the building blocks for new stellar nurseries. As stars live and die, they produce most of the universe’s elements.

Image: Chandra/Harvard

Star Size and Luminosity

The most abundant stars in the universe are the small red dwarfs. They swell into red giants as they die before their core shrinks into a white dwarf.

Massive stars like the Sun swell into yellow or red supergiants before they eject their outer layers. Then they collapse inward before exploding as supernovas. Finally, any remaining cores shrink into neutron stars.

However, the most massive of all stars, hypergiants, run out of fuel quickly, explode into supernovas, and then form dense black holes where no light escapes their gravity. These hypergiant stars aren’t very common, as only a few exist in the Milky Way galaxy.

In addition, bright blue stars are the most massive and the hottest. Red dwarfs have the least mass and luminosity. They have cooler temperatures and are red in color, like their name. The Sun lies somewhere in between and is yellow. So you can see that a star’s mass affects its temperature, color, and brightness.

Image: Globular star cluster NGC 2419 from Hubble/ESA/NASA.

Wrap-Up: What Are Stars Made of?

Stars are essentially enormous hot gas balls that glow. Hydrogen atoms in a star’s core smash together, forming helium. The occurring nuclear fusion releases massive energy amounts that make the star shine. The hot gas pushes out from the star’s center to oppose gravity’s inward pull and keep the star from collapsing. The balance between gravity’s force and the nuclear gases pressing outward hold the star together at a steady temperature.

As stars live and die, they create most of the elements in the universe, like carbon and oxygen. These elements make up comets, asteroids, and planets. So stars provide the building blocks for us to live on Earth.