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Subjects
The same kind of object, glowing harder Three round glowing bodies sit in a row on a marked rail. The one on the left has a few short rays. The middle one has more rays and they are longer. The one on the right is surrounded by many long rays. Under each body a small pointer sits further along the rail than the one before it, so the row reads as one thing getting hotter from left to right.

Why a Blue Star Is Hotter Than a Red One

About 13 minutes

You already know that a star's color tells you how hot it is, and that blue is the hot end of that scale. It is worth stopping on, because it is backwards from everything else you have ever been told about color.

Every tap in your house has a red mark and a blue mark, and red is the hot one. Fires are drawn in red and orange. Cold weather on a forecast is blue. Ice is blue.

The sky disagrees with all of it. And the sky is right.

Anything hot enough starts to glow

A thing does not have to be a star to glow. It only has to be hot.

Look at the metal inside a toaster while it is working, or the ring on an electric hob. It starts as ordinary dull metal. Then it goes a deep red. Turn it up and the red gets brighter and slides toward orange.

Blacksmiths have used this for as long as there have been blacksmiths, and they have never needed a thermometer. They read the iron. Dull red is one temperature. Cherry red is hotter. Orange is hotter still, and a bar that has gone almost white is the hottest of the lot.

English already knows the rule and gives it away for free. Red hot and white hot are both ordinary phrases, and everybody agrees which one means hotter. You have known this since you learned to talk. Nobody mentioned that the scale carries on past white, into blue.

A glow is never one color

Here is the part that makes the whole thing work, and it is the part the first article did not say.

A hot object does not send out one color. It sends out a whole spread of them at once — a smear that runs from the invisible warmth you can feel on your face, through every color your eyes can see, and out the far side into light so energetic that your eyes cannot pick it up at all.

Heating something does not swap one color for another. It slides that entire spread along.

Where a star's glow sits on the scale of light A long horizontal scale runs across the picture. Light carrying little energy is at the left end and light carrying a lot of energy is at the right end. A narrow window in the middle of the scale is bracketed and labelled as the part human eyes can see, with the red end of that window on the left and the blue end on the right. A wide hump sits on the scale. The hump is the whole spread of light a star gives out, and it can slide left and right along the scale while keeping exactly the same width. A dashed line drops from the top of the hump to show where the glow is brightest. what your eyes can see red end blue end less energy in each bit of light more energy invisible invisible The hump is the whole glow. Its width never changes; only where it sits does.

Drag the handle from a cool star up to a scorching one, and watch the hump move.

Notice what does not change. The hump keeps exactly the same width the whole way across. A star is never sending you one pure color; it is always sending a broad spread. All that moves is where that spread is brightest.

That is why a cool star looks red. Its hump is sitting off in the invisible warmth, and only its right-hand edge pokes into the part your eyes can see — the red end. Heat it up and the hump slides right, until the whole visible window is covered and the star looks white. Keep going and the hump moves past the window entirely, so now only its left-hand edge reaches you, and that edge is the blue end.

This is a picture of the idea rather than a measuring instrument, but the sliding is real and it goes in exactly this direction.

Red, white and blue all fall out of one hump moving one way.

Go and check it on a real sky

You do not have to take anyone's word for this. Orion is the easiest constellation to find in the whole sky, and it happens to contain both ends of the argument.

Betelgeuse and Rigel in the constellation Orion The constellation Orion drawn as seven dots joined by faint lines. Two dots at the top are the shoulders, three dots in a short slanted row across the middle are the belt, and two dots at the bottom are the feet. The upper left shoulder is drawn as a large dot labelled Betelgeuse, a red star at about three thousand five hundred degrees. The lower right foot is drawn as a large dot labelled Rigel, a blue white star at about eleven thousand degrees. The two are in the same small patch of sky. Betelgeuse red about 3,500° Rigel blue-white, about 11,000° the belt Orion
Betelgeuse and Rigel sit in the same small patch of sky. One is about 3,500 degrees at the surface, the other about 11,000. You can see the difference with your eyes alone.

Betelgeuse is one of the shoulders, and it is noticeably orange-red. Rigel, diagonally opposite at a foot, is a cold-looking blue-white. Our own Sun sits between them, at around 5,500 degrees.

Look on a clear night, away from bright lights, and give your eyes a minute to settle. Most stars will look plainly white. These two do not, and once you have caught the difference between them you will start noticing it everywhere.

If you live in the northern half of the world, Orion is a winter evening sight. In the southern half it is a summer one, and it hangs upside down, so Rigel is above and Betelgeuse below. The colors are the same either way.

Two things the color does not tell you

It is easy to over-read a color, so it is worth being clear about the limits.

A hot object gives out a whole spread of light at once, not a single color. So why does a very hot star look blue rather than white?

Next time somebody turns a tap, look at the two marks on it and remember that the universe labels them the other way round. Red is the cool end. Blue is where things get serious.