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Red and Blue Giant Stars

Remember when we talked about sun-sized stars? We said that at the end of their lives these stars expand, taking up much more space than before. This is exactly what a giant star is.


As a sun-sized star gets old, it starts to run out of its hydrogen fuel. When the process of burning hydrogen in the star’s core begins to slow down, the core gets more compact and dense. This means all the stuff in the middle of the star gets close together. As the center gets smaller and smaller it starts to heat up again. When it gets hot enough it will start to burn a new fuel called helium.
Once ignited, helium burns much hotter than hydrogen. The extra heat pushes the outer layer of the star out much further than it used to be. This makes the star much larger.
Imagine a hot air balloon. As the air inside the balloon gets hotter, it stretches the balloon out further and further. As the giant star gets hotter, its outside stretches out further and further. When our own sun begins to stretch into a giant star, it will engulf MercuryVenus, and Earth.
Hubble photograph of Giant Star Rebirth

Ironically, when a main-sequence star nears its death, it becomes bigger and forms into a giant star

Did you know?

Many of the stars you see at night are giant stars. This is because like a lighthouse, giant stars glow very brightly. When the Sun becomes a giant star, its light will shine much further into space than it does right now.
The night sky

Because they are so bright, giant stars can be seen from enormously long distances.

Blue Giant Stars

Blue giant stars are large and compact, this causes them to burn their fuel quickly. This makes their temperatures very hot. These stars often run out of fuel in only 10,000 – 100,000 years.
A blue giant is extremely bright, or luminous. Like a lighthouse, they shine across a great distance. Even though blue giant stars are rare, they make up many of the stars we see at night because they shine so brightly.
Blue giant stars die in a spectacular way. They grow larger like the sun-sized stars. However, instead of shrinking and forming a planetary nebula, they explode in a supernova. Supernova explosions can be brighter than an entire galaxy, and can be seen from very far away.
Milky Way stars

Blue giant stars are large, so they last shorter than red giants since they burn fuel faster.

Red Giant Stars

In a few billion years, our Sun will turn into a red giant star and engulf the inner planets of our galaxy. Red giant stars reach sizes of 100 million to 1 billion kilometers in diameter. This is about 1000 times the size of our sun. Red giants are bigger than blue giants. Their energy is spread over a larger area. This causes red giants to be cooler than blue giants.
A sun.

Red giant stars are bigger than blue giants, so they are cooler.

Future of Our Sun

When the sun becomes a red giant in approximately 5 billion years, life on Earth will cease to exist. Even if the red giant does not completely engulf Earth, the extreme heat from the sun will boil up the oceans and cause hot rock to form on the surface of our planet. However, when stars become red giants, they may create other solar systems where life can thrive.

Other Great Resources

Our Sun Vs. Giant Stars: 

Written by: Varsha Rammohan.