In the night sky: Orion
In the night sky: Orion

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2.2.1 Types of star

Not all stars are the same and it is mainly their differences in mass that produce the wide variety of stars we see and many we can’t.

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Red Dwarfs

Red Dwarfs are small stars with temperatures cooler than that of the Sun. They are the most common stars in our galaxy and are less than half of the mass of the Sun. They are positioned on the lower main sequence on the Hertsprung-Russell diagram.

Red Giants

Red Giants are cooler than the Sun, so they have a red–orange tinge to the visible light they emit. They may be over 100 times the size of the Sun and are stars near the end of their life. They come above the main sequence on the Hertsprung-Russell diagram.

Supergiants

Stretching across the upper regions of the Hertsprung-Russell diagram the Supergiants are truly enormous. Rigel is the brightest star in the Orion constellation and is a blue-white supergiant. Supergiants are high mass stars. Near the end of their life, when a supergiant dies, it explodes as a supernova, then shrinks to become a black hole.

White Dwarfs

White Dwarfs are faint but hot stars in the bottom left of the Hertsprung-Russell diagram. They are very small and dense, formed when a main sequence star reaches the end of its life. White dwarf stars gradually cool over time until they no longer emit light.

Brown Dwarfs

The smallest, dimmest and coolest stars are Brown Dwarfs. They appear at the lowest part of the main sequence on the Hertsprung-Russell diagram. They are also known as ‘failed stars’ and are very difficult to detect as they do not have sufficient mass for nuclear fusion to occur.

Explore star types for yourself.

Activity 2.2

Pick a star, this could be a star you’ve heard something about already or a group of stars such as brown dwarfs, and see what else you can find out about it.

For example, where is it on the Hertzsprung-Russell diagram that you looked at in The life of a star, approximately how old is it, roughly how far away is it, what constellation is it part of?

Write a short paragraph about what you found most interesting.

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Next, you’ll look at the reactions going on inside a star, and see how they affect how the star evolves.

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