How do you tell how hot or cold something is?  You can touch it with your finger or place your hand near it.  What if the thing is too far too touch, though?  You can use your eyes to see if a mug of hot chocolate on a table across a room has steam rising off the top or if a glass of ice has beads of water on its sides.  What if the thing is really, really far away?  Like the sun?  Well, that's easy.  You feel the heat when you can see the sun is in the sky.  Yes, but just how hot is it?  Also, what sort of hot is it? 

Science tip!  When figuring out how cold something is, never lick a metal pole in winter.  You might not get your tongue back.

You deal with hot and cold every day.  What sort of clothes should you wear outside?  Is that food cooled down enough to eat yet?  It's cold in here.  Will someone please turn on the heat?  Temperature is a measure of how hot or cold something is.  You can figure it out by using a thermometer.  This will tell you how how or cold something is in degrees.  However, there are some temperatures in space that would break your thermometer.  That is, if you could even get it that far into space in the first place.

Just how hot is it?

Stars are hot.  No, I do not mean movies stars, and no, I do not mean hot as in nice to look at.  The stars that float in space have very high temperatures, so high that the way we measure heat on Earth does not work.  It's a good thing our star, the sun, is so far away, because thermometers, gloves, umbrellas, and yes, fingers, would all melt near a star.  Kelvin is a measurement we use to find the temperature of things that are REALLY hot or REALLY cold, like things in space.  It begins at the coldest point we can think of, where nothing moves, not even our building blocks, and it goes up to some thermometer-melting heights.

Antarctica, about as cold as it gets on the Earth's surface.

If your hands are cold but you stick them under your arms to warm them up, does that mean you are cold?  Or is it just the outside of you that's cold?  Surface temperature is how hot or cold something is on its outside.  Even though they are very smart and awesome, scientists cannot see inside of a star.  Everything they can measure comes from the light that beams down to us.  It's kind of like you staring at that steaming mug across the room.  I just hope that mug is not 9,900 degrees Fahrenheit like the outside of our sun.

The surface of the sun is way hot.

Would you touch a light bulb that's on?  I hope not.  Would you touch a light bulb that's off?  Sure, just be careful when you do.  How about a glowing log in the fire?  No.  How about black ashes?  Maybe.  You know that things that are glowing are more likely to be letting off heat than things that are not glowing.  In space, it's not that simple, so we use the light, or radiation, that things give off in space to tell how hot they are.  Black-body radiation is the name for the types of light and heat that is let off by things.  This works best for things that are very, very far away.  We do not have to touch the sun to figure out how much heat it's letting off.  Instead we can just use the different types of light that it lets off.  That's a good thing.  We would lose a lot of scientists that way.

Just right for roasting marshmallows.

How can you tell how hot a star is?  You throw away everything you know about temperature (this means your thermometer) and use degrees Kelvin.  Since you cannot get all the way up in space, you study the light coming off of the sun.  This will should let you know how hot it is on its outside.  After that, you tell everyone just how smart you are.  Maybe some hot stars (yes, good-looking people in movies) will be impressed with how much you know about hot stars.


Physics 4 Kids.  "Three Big Temperature Scales", 2011.  <>

Windows to the Universe.  "Kelvin Temperature Scale"  Windows to the Universe, 2009.  <>

Brightstorm.  "Blackbody Radiation"  Brightstorm, 2012.  <>