Giant star

A giant star is a special kind of star that is much bigger and brighter than a regular star. It has a larger radius and shines more brightly, even though it might have the same surface temperature as a smaller star. These giant stars appear above the main group of stars, called the main-sequence, on a special chart called the Hertzsprung–Russell diagram.

Giant stars can be up to a few hundred times bigger around than our Sun and can shine more than ten times as brightly. Even bigger and brighter stars than giants are called supergiants and hypergiants. While a very hot and bright star on the main-sequence might sometimes be called a giant, experts still refer to all main-sequence stars as dwarfs, no matter how large they seem.

Formation

A star turns into a giant when it runs out of hydrogen for fusion at its core and leaves the main sequence. What happens next depends mostly on how heavy the star is.

Internal structure of a Sun-like star and a red giant. ESO image.

For stars a bit heavier than 0.25 times the sun, once the core runs out of hydrogen, it shrinks and gets hotter. Hydrogen then starts to burn in a layer around the core. The outer part of the star grows bigger and cooler, but it doesn’t get much brighter. The star becomes a subgiant. The core keeps getting bigger and hotter, but in stars up to about 10-12 times the sun’s weight, it doesn’t get hot enough to start burning helium right away. After a few million years, the core collapses quickly, and the outer layers grow even more. This makes the star much brighter, and it becomes a red-giant branch star. It will burn hydrogen in a layer around the core for much of its life.

Heavier stars, above about 0.4 times the sun’s weight, will eventually get hot enough to burn helium into carbon and oxygen. When this happens, the star can shrink a bit and become less bright before growing bigger again. When the helium runs out, some stars can grow even bigger and brighter, becoming what is called an asymptotic giant branch star. These stars don’t last long before they run out of fuel and turn into a glowing cloud of gas before becoming a white dwarf.

Very heavy stars, more than 12 times the sun’s weight, become bright blue giants and later red supergiants. They burn heavier elements and can end their lives in a big explosion. Stars between 8 and 12 times the sun’s weight have features of both lighter and heavier stars.

Very light stars, less than about 0.25 times the sun’s weight, never become giants. They can keep burning hydrogen for much longer than the age of the Universe and will eventually become helium white dwarfs, but this hasn’t happened yet because the universe isn’t old enough.

Subclasses

There are many different types of giant stars, and astronomers use special names to describe smaller groups of them.

Subgiants

Main article: Subgiant

Subgiants are stars that are similar to giants but have some different features. Some subgiants are just very bright main-sequence stars, while others are stars that are changing and will become giants.

Examples include:

Gamma Geminorum (γ Gem), an A-type subgiant;
Eta Bootis (η Boo), a G-type subgiant;
Delta Scorpii (δ Sco), a B-type subgiant.

Bright giants

Bright giants are stars that are very bright. They are in between regular giants and supergiants. Well-known bright giants include:

Red giants

Main article: Red giant

Red giants are cooler stars that are very large and bright. They can be in different stages of their lives. The most common red giants are called RGB stars.

Examples include:

Pollux, a K-type giant.
Epsilon Ophiuchi, a G-type red giant.
Arcturus (α Boötis), a K-type giant.
Aldebaran, a K-type giant
R Doradus, a M-type giant.
Mira (ο Ceti), an M-type giant.
Chi Cygni, a S-type giant.
TX Piscium, a C-type giant (carbon star).

Yellow giants

Yellow giants are stars that are not as cool as red giants but are still very large. They are less common than red giants and can change in brightness. Some yellow giants are:

Sigma Octantis (σ Octantis), an F-type giant;
Capella Aa (α Aurigae Aa), a G-type giant.
Beta Corvi (β Corvi), a G-type bright giant.

Blue (and sometimes white) giants

Main article: Blue giant

Blue giants are very hot stars. They can be very massive or less massive. Some blue giants include:

Meissa (λ Orionis A), an O-type giant.
Alcyone (η Tauri), a B-type giant, the brightest star in the Pleiades;
Thuban (α Draconis), an A-type giant.