Red Supergiant: Definition, Facts & Life Cycle

Red Supergiant: Definition, Facts & Life Cycle
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  • 0:01 Definition of a Supergiant
  • 1:00 Gravity and Nuclear…
  • 2:05 Day in the Life of a…
  • 2:54 Death of Supergiants
  • 3:38 Lesson Summary
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Lesson Transcript
Instructor: Katie Chamberlain

Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

This lesson defines red supergiants and provides some interesting facts and life cycle information. Check your knowledge with a quiz that follows the lesson.

What Does It Take to Be a Supergiant?

A red supergiant is an aging giant star that has consumed its core's supply of hydrogen fuel. Helium has accumulated in the core, and hydrogen is now undergoing nuclear fusion in the outer shells. These shells then expand, and the now cooler star takes on a red color. They are the largest known stars.

As stars age, they go through various phases of their lives. These phases are so diverse that it is hard to believe that they are all just the same star (much like an infant, a teenager, and an elderly human seem quite different). At various points in a star's life, different things will happen depending on the size of the star. Only large stars with a mass of about ten solar units will go on to become red supergiants. The famous star, Betelgeuse, is a red supergiant star in the constellation Orion.

Gravity and Nuclear Fusion in Stars

The Hollywood lifestyle might look luxurious, but it is difficult to be a star. There are a number of forces at work at all times just to keep them twinkling: gravity is pushing inwards, and the energy released from nuclear fusion is pushing outwards. All stars begin as main sequence stars, fusing hydrogen into helium within their core. As this hydrogen depletes, however, so does the energy pushing outwards. Gravity is then the dominant force, and the outer layers contract inwards.

This contraction increases the pressure inside the star, and it causes hydrogen to fuse in the outer shell layers (where it has not yet been depleted). This new source of energy counteracts gravity, and the layers expand outwards creating a supergiant star. Since these large outer layers burn cooler than the core (3,500-4,500 K), the star appears red. VX Sagittarii is an example of a lovely red supergiant star.

A Day in the Life of a Supergiant

Apart from increasing in size and changing color, red supergiant stars kick off the fusion in their cores again. The helium that accumulated during their main sequence phase begins to fuse into carbon. When the pressure gets high enough, the carbon will fuse into oxygen, and these changes will continue as heavier and heavier elements are fused.

As the different types of fusion occur, the red supergiant will swell and contract, thus making it variable in size. In general, however, red supergiants have radii approximately 1,500 times larger than the radius of our sun. The largest of the supergiants are called hypergiants. The largest such hypergiant is VY Canis Majoris, which has a radius 1800 times larger than our sun's.

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