What is a Giant Star?

by Carson

Do you know that some stars swell to the point that they span tens or sometimes hundreds of times their original diameter? Today, we’ll talk about the formation, evolution, and mechanism of a giant star. Let’s find out.

How Do Giant Stars Form?

Although giant stars are a different kind of star from average main-sequence stars, it isn’t actually formed right away. Instead, stars go through the main-sequence phase for a long time, perhaps billions of years, before entering the giant phase, which lasts much shorter than the main-sequence phase.

In main-sequence stars, hydrogen is fused to helium at the star’s core, causing heat and energy to be emitted. That stream of particles counters the gravity of the star crushing the star into one piece, causing it to stay in a fixed spherical shape. However, that resource has to run out eventually. The more massive the star is, the more compressed its core becomes, and the faster it fuses hydrogen, to the point where the star’s lifespan shortens as its mass increases. When the hydrogen runs out, the giant phase begins.

Surprisingly, this phase of the star does not start with the actual enlargement of the star. Instead, it first shrinks. As there is now no hydrogen fusion going on, there will be no particles from the star, and gravity will cause the star to collapse. However, as the core gets denser, heavier elements start fusing. In this case, the first element to fuse after hydrogen is helium, which is the product of hydrogen fusion happening all the time in the main-sequence phase.

The fusion of helium is stronger than the original hydrogen fusion that kept the star in balance, causing the star to swell in size, blowing its outer layers away while keeping the core small. This is how giant stars become so large.


As heavier elements fuse, it’s so powerful that the outer layers are being pulled apart. However, the resources there burn so quickly that it only lasts the star a short time. When the resource runs out, the star contracts and then expands again as it finds a new element to fuse in its core. For low-mass stars incapable of a supernova, the core doesn’t get dense enough to fuse carbon. Therefore, the collapse continues, and the star spits out its outer layers to expose its small, Earth-sized core, known as a white dwarf.

However, the journey continues for high-mass stars, in which the core exceeds the Chandrasekhar limit of about 1.4 Solar masses, as one element is fused after the other is used up. The nuclear fusion gets more and more powerful, pushing the outer layers further out, making the star brighter, and it loses mass more quickly. In this case, the star is now a supergiant. They are some of the most luminous stars in the Universe as they emit lots of energy. That’s how stars like Betelgeuse and Antares get so bright in the night sky. In the final stages of supergiants, they may even become some of the largest stars in the Universe.

The Demise of A Giant Star

We talked about the demise of a low-mass star in the last section, as they cannot become a supergiant and become a faint, small white dwarf. But what about high-mass stars? Do they continue the journey of nuclear fusion forever? The answer is no. After using a few intermediate elements, iron eventually builds up in the star’s core. A star cannot fuse iron, so once the resources run out, there will be no nuclear fusion happening at the star.

Instead, the massive pull of gravity causes the star to collapse in on itself. This happens so quickly that the core falls in a matter of seconds. The outer layers then explode in a tremendous show of supernova as the core becomes a newly formed neutron star or black hole, depending on the star’s mass.


Today, we learned that a giant star is a star in its late stages with inflated outer layers. They can span tens, if not hundreds, of times the Sun’s diameter or sometimes even be over a thousand times wider than our star. We also learned how a giant star ends its life, settling down as a white dwarf, neutron star, or black hole. Remember that red dwarfs, the smallest stars in the cosmos, do not go through a giant phase. If you want to learn about these intriguing, bright stars, please visit the webpages in the references below.


  1. Fraser Cain. (2009, February 10). Giant Stars. Retrieved October 15, 2022, from https://www.universetoday.com/25134/giant-stars/
  2. (2015, May 7). Background: Life Cycles of Stars. Retrieved October 15, 2022, from https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-lifecycles.html
  3. (2022, January 21). Red giant stars: Facts, definition & the future of the sun. Retrieved October 15, 2022, from https://www.space.com/22471-red-giant-stars.html
  4. (2015, April 10). How quickly does a supernova happen? Retrieved October 15, 2022, from https://phys.org/news/2015-04-quickly-supernova.html

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