What Makes a Star a Red Supergiant?

The constellation Orion and the red supergiant Betelgeuse.
The constellation Orion holds the red supergiant star Betelgeuse (the red star in the upper left part of the constellation. It is due to explode as a supernova -- the end point of massive stars. Rogelio Bernal Andreo, CC By-SA.30

Red supergiants are among the largest stars in the sky. They don't start out that way, but as different kinds of stars age, they undergo changes that make them big...and red. It's all part of star life and star death. 

Defining Red Supergiants 

When astronomers look at the largest stars (by volume) in the universe, they see a great many red supergiants. However, these behemoths are not necessarily—and almost never are—the largest stars by mass.

It turns out they're a late stage of a star's existence and they don't always fade away quietly. 

Creating a Red Supergiant

How do red supergiants form? To understand what they are, it's important to know how stars change over time. Stars go through specific steps throughout their lives. The changes they experience are called "stellar evolution". It starts with star formation and youthful star-hood. After they are born in a cloud of gas and dust, and then ignite hydrogen fusion in their cores, stars usually live on something astronomers call the "main sequence". During this period, they are in hydrostatic equilibrium. That means the nuclear fusion in their cores (where they fuse hydrogen to create helium) provides enough energy and pressure to keep the weight of their outer layers from collapsing inwards.

When Massive Stars Become Red Supergiants

A high-mass star (many times more massive than the Sun) goes through a similar, but a slightly different process.

It changes more drastically than its sun-like siblings and becomes a red supergiant. Because of its higher mass, when the core collapses after the hydrogen burning phase the rapidly increased temperature leads to the fusion of helium very quickly. The rate of helium fusion goes into overdrive, and that destabilizes the star.

A huge amount of energy pushes the outer layers of the star outwards and it turns into a red supergiant. At this stage, the gravitational force of the star is once again balanced by the immense outward radiation pressure caused by the intense helium fusion taking place in the core.

The star that transforms into a red supergiant does so at a cost. It loses a large percentage of its mass out to space. As a result, while red supergiants are counted as the largest stars in the universe, they are not the most massive because they lose mass as they age, even as they expand outward.

Properties of Red Supergiants

Red supergiants look red because of their low surface temperatures. They range from about 3,500 - 4,500 Kelvin. According to Wien's law, the color at which a star radiates most strongly is directly related to its surface temperature. So, while their cores are extremely hot, the energy spreads out over the interior and surface of the star and the more surface area there is, the faster it can cool. A good example of a red supergiant is the star Betelgeuse, in the constellation Orion.

Most stars of this type are between 200 and 800 times the radius of our Sun. The very largest stars in our galaxy, all red supergiants, are about 1,500 times the size of our home star.

 Because of their immense size and mass, these stars require an incredible amount of energy to sustain them and prevent gravitational collapse. As a result, they burn through their nuclear fuel very quickly and most live only a few tens of millions of years (their age depends on their actual mass).

Other Types of Supergiants

While red supergiants are the largest types of stars, there are other types of supergiant stars. In fact, it is common for high mass stars, once their fusion process passes beyond hydrogen, that they oscillate back and forth between different forms of supergiants. Specifically becoming yellow supergiants on their way to becoming blue supergiants and back again.

Hypergiants

The most massive of supergiant stars are known as hypergiants. However, these stars have a very loose definition, they are usually just red (or sometimes blue) supergiant stars that are the highest order: the most massive and the largest.

The Death of a Red Supergiant Star

A very high-mass star will oscillate between different supergiant stages as it fuses heavier and heavier elements in its core. Eventually, it will exhaust all its nuclear fuel that runs the star. When that happens, gravity wins. At that point, the core is primarily iron (which takes more energy to fuse than the star has) and the core can no longer sustain outward radiation pressure, and it begins to collapse.

The subsequent cascade of events leads, eventually to a Type II supernova event. Left behind will be the core of the star, having been compressed due to the immense gravitational pressure into a neutron star; or in the cases of the most massive of stars, a black hole is created.

How Solar-type Stars Evolve

People always want to know if the Sun will become a red supergiant. For stars about the size of the Sun (or smaller), the answer is no. They do go through a red giant phase, though, and it looks pretty familiar. When they begin to run out of hydrogen fuel their cores begin to collapse. That raises the core temperature quite a bit, which means there's more energy generated to escape the core. That process pushes the outer part of the star outward, forming a red giant. At that point, a star is said to have moved off the main sequence. 

The star chugs along with the core getting hotter and hotter, and eventually, it begins to fuse helium into carbon and oxygen. During all this time, the star loses mass. It puffs off layers of its outer atmosphere into clouds that surround the star.

Eventually, what's left of the star shrinks to become a slowly cooling white dwarf. The cloud of material around it is a called a "planetary nebula", and it gradually dissipates. This is a far more gentle "death" than massive stars discussed above experience when they explode as supernovae. 

Edited by Carolyn Collins Petersen.

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Millis, John P., Ph.D. "What Makes a Star a Red Supergiant?" ThoughtCo, May. 1, 2018, thoughtco.com/red-supergiant-stars-3073597. Millis, John P., Ph.D. (2018, May 1). What Makes a Star a Red Supergiant? Retrieved from https://www.thoughtco.com/red-supergiant-stars-3073597 Millis, John P., Ph.D. "What Makes a Star a Red Supergiant?" ThoughtCo. https://www.thoughtco.com/red-supergiant-stars-3073597 (accessed June 18, 2018).