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Star Birth, Main Sequence and Fusion


  • Learn how stars are born, beginning with a protostar. 
  • Learn about stars in later stages of life, including main sequence stars, brown dwarfs, red giants, and black holes.


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Different Types Of Stars

When we think of stars, we usually think of our sun or the stars we see in the night sky, but our universe is made up of many different types of stars. The bright lights we see in the sky may not actually be stars at all. They might be planets, meteors, or even other galaxies. Some of the different stars in our galaxy include main sequencered giantswhite dwarfs, and brown dwarfs. But before we talk about the different types of stars, let's start by talking about how stars are born.


The life cycle of a star
Star Lifecycle


How Stars Are Born

This diagram shows the life cycle of a star. A star starts in a stellar nebula. Then, depending on size, it takes one of two paths. Average stars become red giants, turn into planetary nebulae, and end their lives as white dwarfs. Massive stars become red supergiants, undergo a supernova explosion, and become either a neutron star or a black hole.

Stars start their life as little pieces of dust in huge clouds of dust and gas that exist in most galaxies. These clouds are calm, and nothing happens in them for ages until something sets them off. The thing that disturbs them could be just a little turbulence, pressure from an explosion in space, or even a collision with another cloud. It doesn't matter what sets them off, but all of the sudden, the dust particles begin to collide with one another.

When they collide, they stick together, then more collide, then more. These particles collide and grow until they are big enough to produce their own gravity. Now they don't even have to collide - they can pull in other pieces of matter with their gravitational pull.

Eventually, this clump of particles gets so big it begins to collapse under its own weight. Over millions of years of this collapse, the center of the clump gets very hot. This huge clump is now called a protostar. Aprotostar is a developing star that isn't hot enough to do nuclear fusion.

Star Adulthood

Diagram of nuclear fusion
Nuclear Fusion Diagram 2

It takes time, but the protostar eventually becomes hot enough, around 7 million kelvin, for nuclear fusion to occur in its core. At this point, the protostar is considered a star. Nuclear fusion is when hydrogen atoms fuse and produce helium atoms. This reaction releases energy. This energy pushes outward on the star. So while the weight of the star pushes in towards the core, energy produced by the nuclear fusion in the core pushes outward.

Sometimes, though, protostars don't get big enough or hot enough for hydrogen fusion to start in their core. If this happens, the protostar is called a brown dwarf. A brown dwarf is a protostar that never grew big enough to do fusion in its core.

Brown dwarfs are heavier than our gas giant planets but not big enough to be a star. The brown dwarf cools down over billions of years until it is the temperature of the rest of the universe and fades into the background.

For stars that do achieve fusion, they continue to grow bigger for many years. This, in turn, increases its size and the heat in its core. Finally, the pressure pushing outward equals the pressure pushing inward, and the star stabilizes.

The star is now a main sequence star and will remain in this state until it burns through all of its fuel. Main sequence is a way astronomers have of classifying stars. They plot the color of a star versus its brightness, and what appears is a band of stars. Stars that lie on this band are known as main sequence stars.


Plotting star temperature and luminosity
Main Sequence Stars


This picture shows how scientists plot a star's luminosity versus its temperature. You can see how most stars live along what scientists call the main sequence and how white dwarfs lie below the main sequence because they are not as luminous for their temperatures. Giants and supergiants lie above the main sequence because they are more luminous for their temperatures.

Star Old Age

How long a star lives depends on its mass and how quickly it consumes hydrogen. Larger, brighter stars burn out faster than smaller, cooler ones. A star like our sun takes about 50 million years to reach its main sequence and then stays there for 10 billion or so years.

After living a long time, the star slowly runs out of hydrogen. The outer layers of the star expand, and although they are losing temperature, their luminosity increases, and they are brighter and bigger. These are now the red giants and red supergiants. A red giant is a large, cool star in the later stages of its life. A red supergiant is a massive, cool star in the later stages of its life.

The giants have turned their hydrogen to helium, but now the core is dense enough that the helium is turning into carbon. This provides an additional source of energy for a while. When all of the helium in the core has fused to carbon, the delicate balance of pressure from gravity pushing in and pressure from energy pushing out is now gone.

The End Of A Star's Life

After the red giant and red supergiants have turned all their helium to carbon, they become either a planetary nebula or undergo a supernova explosion.

Images of a planetary nebula and a supernova
Planetary Nebula and Supernova

A red giant will become a planetary nebula. Here, the outer layers of the red giant are forced away until only the core of the star remains. Theplanetary nebula is the glowing outer layers of a dying star. After the outer layers are all gone, only the glowing core of the dying star remains and is called a white dwarf.

The red supergiants end their life a little more spectacularly. They undergo a supernovaexplosion, the explosion near the end of the life of a massive-size star. After the explosion, the star on this path ends its life as either a neutron star, a star of extremely high density composed entirely of neutrons, or a black hole, an object that is so dense not even light can escape it.

Lesson Summary

Stars start off as specks in clouds of interstellar dust. Something disturbs the cloud, and the dust particles start clumping together. The clumps grow big enough to have their own gravity. This gravity pushes inward on the star, heating up the core. Once the core is hot enough, nuclear fusion starts and a star is born. After billions of years, the stars run out of fuel and become red giants before turning into white dwarfs, neutron stars, or black holes.



Main Sequence stars have fusion occurring in their cores. Fusion is when atoms combine to form larger elements. This video explains the basics:

The video below shows a demonstration to help you understand fusion.