What is a Red Giant Star?
A red giant is a star in the later stages of its stellar life. It is said that stars only spend 1% of their lives in this stage. Our Sun is now in the main sequence. It generates energy by converting hydrogen into helium.
In the main sequence, the core of a star is in a state of hydrostatic equilibrium. This balance is established because of nuclear fusion reactions in its core. It will stay in this stage for about 10 billion years before it becomes a red giant star. As its name stays, a red giant is a K or M-type star.
Red giant stars have different ways of generating their energy. There are three groups of them in the giant branch: the red-giant branch (RGB), the horizontal branch (HB), and the asymptotic-giant-branch (AGB).
Red giants are commonly on the RGB where they fuse hydrogen into helium through the shell that surrounds the helium core. In the HB, helium is fused in the cores of red giants through the triple-alpha process. Cool and luminous stars populate the AGB. Their cores are made up of carbon and oxygen as well as helium-burning and hydrogen-burning shells.
Red giant stars are some of the biggest and most luminous stars in our night sky. Their life cycle is also an interesting one. Before their lights fade out, they will eject glowing shells of gas called planetary nebulae.
Some notable examples of red giants are Arcturus, Aldebaran, Gacrux, Pollux, and Mira.
Characteristics of a Red Giant
Spectral class: K or M type (including carbon stars and S-type)
Mass: About 0.8 to 8 solar masses
Surface Temperature: Lower than 5,000 K
Lifespan: A few thousand to one billion years
Red giant stars are in the K and M spectral class. They have a yellow-orange to red appearance, an indication of a cooler surface temperature.
Other stars that belong in this type are carbon stars and S-type stars. They differ in the amount of carbon and oxygen in their atmospheres. A carbon star has more carbon while an S star has about an equal amount of oxygen and carbon in its atmosphere.
After the main sequence, stars with a mass of approximately 0.8 to 8 solar masses will become red giants. They are greater in size which is why they are more luminous than our Sun. In the Hertzsprung–Russell diagram (HR diagram), the roman numerals II and III correspond to the luminosity class of bright giants and giants respectively.
The luminosity of stars in the red giant branch is about three thousand times greater than the solar luminosity. Those in the horizontal branch have about 75 solar luminosities. Stars spend thousands to a billion years in the red giant stage. After that, they will enter a different phase as they become nearer the end of their stellar evolution.
Life Cycle of a Red Giant
The life of every star starts in interstellar clouds. Very young stars called “protostars” gather mass from the gas and dust around them in their natal cloud. This stage lasts approximately 500,000 years.
The young stars start to shrink because of their own gravity. The process goes on until they become hot enough to support hydrogen fusion in their core. The fusion of hydrogen to helium will start when the core is about 10 million K or more. By this time, it is already a star in the main sequence.
The smallest stars in the universe, the red dwarfs, will remain in the main sequence for about 80 to 100 billion years. The stay of Sun-like stars is about 10 billion years. Large and massive stars have shorter lives. They will only be in the main sequence for about 20 million years.
After the main sequence, stars will move to another phase. Their mass will determine the next stage of their stellar life. Average stars, or stars with 8 solar masses or less, will become giants. Massive stars with more than 8 solar masses will become supergiants.
Giants and Supergiants
Supergiants are much bigger and more massive than red giants. The brightest of this type of star is Betelgeuse. It is very noticeable in the constellation of Orion because all its other stars have a blue hue. Only Betelgeuse has a reddish hue.
Red dwarfs have a lower mass than the average stars. They will neither become giants nor supergiants. After the main sequence, they will become blue dwarfs, white dwarfs, and lastly, black dwarfs.
Stars remain in the red giant stage for about thousands to one billion years. When fusion is not possible in its core anymore, it contracts and heats up. After that, it will expel its outer material which results in the formation of a planetary nebula.
The name planetary nebula has nothing to do with planets. The name was derived because of the limitations of early telescopes. Astronomers in the past thought that these objects resemble the shape of planets and describe them that way.
These are spectacular deep-sky objects of ionized gas. Examples below are the Lion Nebula and the Cat’s Eye Nebula.
What remains after the outer layer of a star is blown off is its core. This stellar core remnant which is now a white dwarf becomes very dense. It is also very hot, with a temperature of about 100,000 Kelvin. This cools down after billions of years.
A white dwarf is also called a degenerate dwarf. It does not support fusion anymore. Since it is very dense, its size can be only a little bigger than our planet. However, it can be as massive as the Sun. That is how dense it is!
Features: Inside a Red Giant
A star like our Sun will become a red giant when it runs out of hydrogen fuel to burn. It will move away from the main sequence and will become larger, denser, and redder. By this time, the star’s core contains the converted helium during the main sequence phase. There are other heavier elements as well but they are very minimal.
Shown below are the layers of a star in the main sequence (a) and (b) after the main sequence when hydrogen is already exhausted.
Generating energy through helium fusion in the core requires higher temperatures. A star has no nuclear energy yet because hydrogen fusion has stopped. By this time, the star’s core will begin to shrink and contract because of gravity. This shrinking and inward movement of material will increase the temperature of the core.
The heat produced from the shrinking core of the star will flow towards the cooler shell. Slowly by slowly, the hydrogen layer on the outer shell of the core will heat up. This shell will be heated until it reaches a critical point for hydrogen fusion to occur. It also increases the luminosity of the star.
Why Stars Expand During The Red Giant Phase
The helium core of the star continues to shrink and contract. It provides the energy needed to sustain the fusion on its hydrogen fusion shell. Energy from this shell flows outward into the outer layers of the star and its outer atmosphere becomes inflated. This is the reason why stars expand during the red giant phase.
When the Sun reaches this stage of expansion, it will engulf Mercury and Venus in the solar system, and possibly including the Earth. As the outer layer of the star expands, its surface temperature also gets cooler. This is because the heat will be distributed to a larger area.
The prominent “red” color associated with this type of star indicates this decrease in temperature. But in reality, giant stars look more orange than red in appearance.
It is interesting to note that there are many contradicting things about red giants. The cores of these stars collapse and become hotter. Meanwhile, their outer layers expand and become cooler.
Notable Red Giant Stars
Arcturus is the fourth brightest star that we can see in the night sky. It is also called Alpha Boötis as it is the most prominent star in the constellation of Boötes (the herdsman). This red giant belongs in the spectral class K0III. It is an aging star which is about 7.1 billion years old.
The apparent magnitude of Arcturus is −0.05, making it the northern celestial hemisphere’s brightest star. It is also the brightest K-type giant overall. It is nearly as massive as the Sun but its outer layer has expanded about 25.4 times. Arcturus radiates with the luminosity of 170 Suns. This star is about 36.7 light-years from our Sun.
Aldebaran is Alpha Tauri in the Bayer designation. This red giant is the fourteenth brightest star in our night sky. Its radius is roughly 44 times that of the Sun’s. It is also more luminous by more than 400 times. Though it is bigger, its surface temperature is cooler at 3,900 K.
This star is in the red giant branch as indicated in its spectral type K5+ III. It is slightly variable, with an apparent magnitude between 0.75–0.95. A Jupiter-like exoplanet was discovered around this star. This system is located about 65.3 light-years away from us.
Gacrux is a red giant star with the spectral class M3.5 III. It belongs in the constellation of the Southern Cross where it is the third brightest star. It has an apparent magnitude of +1.64.
This M-type star is about 30% as massive as the Sun. However, its size has expanded to about 120 times the solar radius. It is also more luminous by about 760 times. It lies 88.6 light-years away from us.
Pollux is a K-type star in the constellation of Gemini. It is the closest giant star neighbor of our Sun. Its apparent magnitude is 1.14, making it the brightest star in its constellation.
Pollux was once an A-type star when it was in the main sequence. The hydrogen in its core has already been exhausted and now, it is a red giant star with the spectral type K0 III. It is nearly twice as massive as the Sun.
The radius of this K-type star is larger than our Sun by approximately nine times. An extrasolar planet was discovered orbiting around this giant star. It was given the name Thestias. This planetary system is located 33.78 light-years away from us.
Mira is a red giant star in the constellation of Cetus. It is a binary system with the components designated Mira A and B.
The prominent member of the pair is Mira A. It is in the asymptotic giant branch. It is an important star because it serves as the prototype of variable stars called the Mira variables. When it pulsates, its brightness changes dramatically over a long period of 80 to 1,000 days or more. The brightness of this star changes in every cycle. Its maximum apparent magnitude is 3 and the minimum is 9.
Mira A was the earliest discovered non-supernova variable. This red giant is as massive as the Sun but it is much larger. It has expanded to more than 400 times the solar size. It is also more luminous.
This red giant star is losing mass because of stellar wind. It sheds material that looks like a tail which is 13 light-years long. The estimated distance of this star to us is 200 to 400 light-years.
When the Sun becomes a red giant, the habitable zone also moves outward, giving chances of life to farther regions in the solar system like the moons Enceladus and Europa.
Our planet’s fate is uncertain whether it will be engulfed by the Sun or not when it becomes a red giant. Scientists are still divided about it. Even if it is not swallowed, life would still cease to exist because it will be orbiting closer to the then-aging star.
- What is a red giant star?
- What are the characteristics of a red giant star?
- How is a red giant different from a main-sequence star?
- What happens after the red giant stage of the stellar life cycle?
- What are some of the known red giants in our night sky?