Supergiant Star: Definition & Facts

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Instructor: Mary Ellen Ellis

Mary Ellen is a science and education writer with a background in chemistry. She holds an M.S. in analytical chemistry and has worked as a high school science teacher.

Supergiant stars are the biggest and brightest stars in the universe. Learn more about the definition of a supergiant star, explore star classification, read some facts about the life of a supergiant, and study examples of supergiant stars that we can see. Updated: 11/02/2021

Definition of a Supergiant

The name of this type of star is pretty descriptive. It's not just a giant, it's a supergiant. These are the biggest, most massive, and most luminous stars in the universe. Masses of supergiant stars range from between eight and twelve times the mass of the sun in our solar system. The luminosity of supergiant stars, or the amount of energy they give off, can be between 10,000 and one million times that of our sun. The radius of a supergiant may be anywhere between 30 to 1,000 times that of the sun.

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  • 0:01 Definition of a Supergiant
  • 0:34 Star Classification
  • 1:26 The Life of a Supergiant
  • 3:49 Supergiant Stars We Can See
  • 4:43 Lesson Summary
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Star Classification

Astronomers have classified and named different types of stars based on their properties. These include the size of the star, the kind of energy it emits (often a specific color of visible light), how bright it is, and the temperature of the star. There are different classification schemes, but the most traditional one gives descriptive names to star types like, red dwarf, white dwarf, giant, and supergiant.

The classification categories of stars also generally correlate with the lifespan of a star. For instance, a white dwarf is a star at the end of its life cycle. We can't always classify stars by life stages, though, because not all stars age and die in the same manner. While some stars will end as a white dwarf, others will end in a spectacular supernova, which is basically the explosion of a star, while others will shrink into a black hole.

The Life of a Supergiant

All stars begin a protostar. This is a cloud of gas that, over time, will collapse and condense under the force of gravity. As this happens, the cloud gets smaller and begins to glow with energy from the heat of the energy of gravitational collapse. This process can take up to 100,000 years. Eventually the pressure and heat of the center of the protostar becomes great enough for nuclear fusion to begin, and the protostar becomes a star.

Nuclear fusion is a reaction that fuses hydrogen atoms together to make helium. This reaction gives off a tremendous amount of energy and is the source of the light and other forms of energy that stars emit. Once a protostar begins this reaction, it has become a main sequence star. Most of the stars in the universe, including our sun, are main sequence stars. There is a lot of variation in the main sequences. The biggest, hottest, and most luminous main sequence stars become supergiants.

A star becomes a supergiant when it runs out of hydrogen to burn in its core. Main sequence stars are in a state of equilibrium in terms of size. The fusion reaction in the core causes the material of the star to expand outward, but the force of gravity resists that expansion. The result is a star with a stable size, that is, until the reaction stops.

Stars that are destined to become supergiants burn through their hydrogen quickly and then fuse helium and even bigger elements. This continues until they have created a core of iron. The speed and intensity of the fusion reaction creates a tremendous amount of energy. This allows the star to expand and overcome the force of gravity and explains how these stars get to be so large compared to others.

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