Supergiant

Supergiants are some of the biggest and brightest stars in the universe. They are much larger and shine much more strongly than our Sun. These stars appear at the top of a special chart called the Hertzsprung–Russell diagram, which helps scientists understand how stars behave.

Supergiant stars can be very hot or relatively cool. Their temperatures can range from about 3,400 Kelvin to over 20,000 Kelvin. Despite their great size and brightness, these stars live shorter lives than smaller stars like our Sun because they burn through their energy much faster.

Definition

The term supergiant describes very large and bright stars. These stars were first noticed by a scientist named Ejnar Hertzsprung. He saw that most stars could be grouped into two main areas on a special chart called the Hertzsprung–Russell diagram. Some stars were bigger and brighter than others, and these were called giants. Later, it was found that some of these giants were even larger and brighter, so they were named supergiants.

Supergiants can be blue, yellow, or red, depending on their temperature and color. Blue supergiants are very hot, yellow supergiants are warmer, and red supergiants are the coolest of the three. These stars are very important for scientists who study how stars change and grow over time.

Supergiant stars can also be identified by looking at their light, or spectra. This light shows special patterns that help scientists know how big and bright a star is. These stars have changed a lot from their younger selves and have more heavy elements inside them than smaller stars. Because they are so large, their light shows changes that help scientists classify them into different groups.

Properties

Supergiants are very big and bright stars. They have masses from 8 to 12 times that of the Sun. They shine from about 1,000 to over a million times brighter than the Sun. Their sizes can vary a lot, usually from 30 to 500 times the size of the Sun, and sometimes even bigger.

Because they are so massive, these stars burn through their fuel quickly. They start by burning helium and then move on to heavier elements. Eventually, they explode in bright events called supernovae.

The way these stars shine also depends on their temperature. Cooler red supergiants are larger than hotter blue supergiants, even though they might shine just as brightly. Some of these stars change in brightness over time. Scientists study these changes to learn more about stars.

Evolution

Main article: Stellar evolution

Very big stars, called O type and the largest B type stars, become supergiants. Because they are so heavy, they live for only a short time—between 30 million years and a few hundred thousand years. We usually see them in young parts of galaxies, like in open clusters, the arms of spiral galaxies, and irregular galaxies. They are harder to find in the busy centers of spiral galaxies, and they are rare in elliptical galaxies or globular clusters, which are made mostly of older stars.

When these big stars run out of a special fuel called hydrogen in their middle, they start to grow bigger. Unlike smaller stars, they begin to use a new fuel called helium right away. This means they do not get much brighter. They turn into red supergiants. These stars are big enough to use even heavier fuels, so they do not lose their outer layers like smaller stars do. Instead, they keep using heavier fuels until they collapse. They do not end up as small, cool stars called white dwarfs. Instead, they leave behind a dense leftover called a neutron star or black hole, often after a big explosion called a supernova.

Some of the biggest stars, heavier than about 40 times the mass of our Sun ^☉, do not turn into red supergiants. They change quickly and become blue supergiants or very bright yellow stars before becoming hotter again. The biggest stars, heavier than about 100 times the mass of our Sun ^☉, hardly change from their starting point. They use their fuels so well that they mix everything from the outside to the middle. They keep using hydrogen until it is almost all gone, then quickly change through many hot and bright stages. Scientists think these stars will explode as supernovae, but they are not sure exactly when this will happen. The biggest stars ever seen in the universe may have been even brighter and heavier than any we know today. These early stars, called population III, might have ended their short lives in very big explosions.

Supernova progenitors

Main article: Supernova

Big explosions in space, called Type II supernovae, come from huge red stars known as red supergiants. Another type, called Type Ib/c supernova, comes from hotter stars that have lost much of their outer layers. These giant stars end their lives in a big explosion because they are too heavy.

Scientists used to think only red supergiants could explode, but now we know some explosions come from blue supergiants instead. Research continues to learn more about these stars.

Well-known examples

Supergiants are rare and very bright, so we can see many of them without a telescope. Some of the brightest stars in the sky are supergiants. For example, Rigel in the constellation Orion is a blue-white supergiant. The three stars of Orion's Belt are also blue supergiants. Deneb in Cygnus is another blue supergiant. Antares and VV Cephei A are red supergiants. Betelgeuse is a red supergiant and one of the brightest stars in Orion.