Are some stars bigger than the Earth?

Question posed by Courtney.

Short answer: Yes!

In fact, most stars are bigger than the Earth. The diagram below compares the size of our sun with the sizes of the other planets in our solar system. The sun is much bigger than the Earth, and it isn't even a particularly large star.

The Earth and Sun to Scale by T. Briggs is licensed under CC BY-SA 4.0

Stars are made when the stuff that they are made of collapses and gets crushed together so hard that it starts burning through a process called nuclear fission. To make stuff collapse in on itself like this, you need a lot of gravity.

To get a lot of gravity, you need a lot of stuff. The Earth is made up of quite a lot of stuff, and therefore has a strong enough gravitational pull to keep you, cars, elephants, dogs and kettles on it. But it isn't made up of enough stuff for its gravity to crush all the stuff that it is made of together hard enough to start nuclear burning.

How much stuff do you need for a star to form?

The smallest stars around, called brown dwarfs because they're quite small and don't give off much light, need about seven hundredths of the mass of our sun to be able to start burning helium in their cores. That might not sound like much, but to put it into perspective that's over twenty-thousand times the mass of the Earth! Even Jupiter only has around 318 times the mass of Earth.

The diagram above (nabbed from elsewhere- click the image to go there) shows the difference in size between our sun and the star OGLE-TR-122b, the smallest star yet discovered, and the similarity in size between that star and Jupiter, the largest planet in our solar system.

Most stars in the universe are brown or red dwarf stars, but they're difficult to see.

You said most stars are bigger than the Earth. Which ones aren't?

When stars use up all of their fuel, their gravitational pull causes them to collapse even further. When they cool down they can become a white dwarf, a neutron star or even a black hole (depending on their mass). All three of these can take up a smaller volume of space than the Earth, but contain much more mass.

Interesting info

• Depending on how massive stars are, different things happen when they run out of fuel: What happens when a star dies?
• A black hole can be formed when a really massive star dies.
• Our star, the sun, is about average as far as stars go: it isn't unusually big, small, bright or dim.

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