How Hot Can a Star Be? (And Other Star Questions)

By Dr. Marc Rayman

Asked by a first grader at Nauman Elementary School in Cedar Park, Texas.

First, let me tell you about something that may be surprising: the Sun is a star, and it is not a very unusual one. It looks to us to be so much brighter than all the other stars only because we are much closer to it. This is just the same as getting very close to a light bulb at home, which would make it so bright it could hurt your eyes. But if you looked at exactly the same light from across the room, it would not be so bright, and if you tried to see it from the other end of your street, that same light would be very dim. If you were at school or even in another city, that light would be too faint to see at all. So if our Sun were as far away as any of the other stars, it would just be one of the many beautiful and faint points of light we enjoy seeing at night. In all the answers below, I'll tell you something about the Sun along with other stars.

How hot can a star be?

A hot, massive Wolf Rayet star.The hottest stars may be almost 100,000 (100 thousand) degrees Fahrenheit. That big number means the stars are far, far hotter than anything you have ever seen or felt here on Earth. Our Sun is about 10,000 (10 thousand) degrees F -- much cooler than the hottest stars, but still super hot! The oven in your house can't come close to this blazing temperature.

But those temperatures are just at the surface. Deep inside, the cores of stars are even hotter. Some stars can be about 200,000,000 (200 million) degrees inside, and the Sun is around 25,000,000 (25 million) degrees at its core. For short times, as stars that are much larger than the Sun are ending their lives in huge explosions, the inside temperature could reach as high as 10,000,000,000 (10 billion) degrees.

How high up is a star?

Our nearest star, the Sun, is 93,000,000 (93 million) miles from Earth. That is a really long way, but all the other stars are much much much much MUCH farther away. I can't give just one distance (well, OK, yes I can, but you have to read more first), as every star is at a different distance, just like you can't say how far away a tree is -- some are near and some are far. But you could say some of the stars you can see at night are around 100,000,000,000,000 (100 trillion) miles. While some stars are a little closer than that, most are farther away, but because the much more distant stars are too far for us to see without telescopes, I think 100 trillion miles is a good answer. That is about 1,000,000 (1 million) times farther away than the Sun -- no wonder other stars don't seem as bright as the Sun!

What is the biggest star?

The largest stars are so big that they may be as much as 2,000,000,000 (2 billion) miles across. If a star that big were in the center of our solar system, where the Sun is now, it would be so big that it would swallow up all the planets from Mercury to Saturn, including Earth! The Sun is about 860,000 (860 thousand) miles across. If your parents drove you in a car at 60 miles/hour (about how fast they drive on the highway), to drive all the way around the Sun would take 5 whole years -- without time to stop for meals, gas, or resting!

All stars are so huge that our entire Earth, with all its oceans, mountains, cities, forests, deserts, and everything else is tiny compared to them. If the largest star were a ball as tall as your school room, Earth would be about the thickness of one of your hairs.

The constellation Orion

By the way, you might be able to go outside on the next clear night and see one of the largest stars. Try to find the lovely constellation Orion, with its distinctive "belt" of 3 stars in the center, and then look above the belt to the nearest bright star. It may appear just slightly red. It has the odd name Betelgeuse (pronounced BAY-tell-juice). It may not be the largest star, but it is certainly one of the largest. We don't know its exact size, but it is around 600,000,000 (600 million) miles across. It could stretch from where the Sun is out past the orbit of Mars, and might even get all the way to Jupiter. (To help you find Orion, you can download a star finder at The Space Place. Orion is visible November through April.)

As long as you are looking at Orion, notice the bright star below the belt, about the same distance from the belt as Betelgeuse but in the opposite direction. It is called Rigel (pronounced like RYE-jell), and it is one of the brightest stars around. It is farther away than most of the stars we see without a telescope, but it still manages to outshine most of them. It is about 40,000 (40 thousand) times brighter than the Sun.

Could you be blinded by looking at a star?

Our own star, the SunIf you were close enough to a star, it would blind you. That's why it is not safe to look at the Sun, the star that is so close to us. But here on Earth, where you and I live, we are so far away from all the other stars that it is safe to look at them. Now if you could travel in a spaceship across the great distances to get close to other stars, you would have to be sure not to look directly at them, or their strong light would blind you.

How does the temperature of a star impact things around it?

The closer you are to a fire, the hotter it feels. And while you might like getting close enough to a fire for it to keep you warm, you have to be careful not to get so close that it burns you. Well, the closer something is to a burning hot star, the hotter the star will seem. So the planets that orbit closest to stars usually are the hottest. Mercury and Venus, which are closer to the Sun than Earth, receive more heat from the Sun and so are warmer than Earth. And Earth, which is closer to the Sun than Mars, is warmer than Mars. If a planet is hot enough, all the water boils away. Think how terrible it would be if Earth were too hot to have water! But if our planet were much farther from the Sun than it is now, all our water might be frozen solid, as hard as rock. Then how would we even be able to drink? And just warming up some ice to melt it for a drink wouldn't be good enough. Our oceans and lakes would be frozen, and it would be far too cold for plants and animals to live. We are lucky to have such a beautiful planet here at just the right distance from the Sun, where the Sun's heat keeps us comfortably warm but not too hot.

I am writing this on a jet flying at night, and I can just make out a few stars through the window. I never tire of seeing them.

These questions have required me to give you many big numbers, and it's easy to be overwhelmed. I'm sure that not only first graders, but most adults as well, will find these numbers to be mind-bogglingly large. I hope you are able to take the time to go outside and look at the stars. You don't have to know how hot or big or high they are to perceive their great beauty. Perhaps though, just knowing that their heat or size or distance are so fantastic will make them appear just a little more beautiful.

Check out Dr. Marc Rayman's answers to more questions!

article last updated October 9, 2018
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