Colors and Temperatures of Stars

Now, let’s find some stars that have a different color than usual.

Can you find any stars that have some unusual color, different from white, yellow, silver?

If you would like to point it out to the rest of us, raise your hand so that I can give you a light pointer.

Demonstrate pointer. Have several volunteers point out stars of different colors.
DIGITAL EFFECT (optional): Orion and/or Boötes outline
Display the outline for Orion, as a constellation with a distinctive colored star (i.e., Betelgeuse). You may do the same for Aldebaran in Taurus and Arcturus in Boötes, toggling them on and off.

For each volunteer, ask…

What color does that star seem to you? 

Yes, there are stars of many different colors.

Why do you think the stars shine with different colors? 

[Accept a few answers.] 

Most of the stars we see are big balls of hot, glowing gas. But all fires are not the same temperature and similarly, not all stars are the same temperature. For that reason they are different colors. 

Which stars do you think are hotter: the yellow or the red stars? Raise your hand if you think the yellow stars are hotter…raise your hand if you think the red stars are hotter…

Let’s do an experiment to find out: Here I have a regular white light bulb which gives off light from a hot metal wire. I can regulate its temperature.

Turn on white bulb in the center and vary brightness to demonstrate. Make sure that everyone can see the light bulb directly.

VISUAL (alternative movie): Star Temperature
Alternatively, use the “incandescent” movie. Fade on and immediately play a short video clip of an incandescent tubular bulb dimming, as a demonstration of color being a function of temperature. May be used instead of a live demonstration. You may need to move a little so that you can see the wire inside the light bulb. Right now, it is the hottest I can make it.

What color is it?  [White.]

I’ll cool it off a little. [Start dimming slowly.]

What color is it now? [Yellow.]…

and if I cool it off more, what color does it turn? [Orange.]

and just when it’s about to die out [Red, then black.]

Start making it brighter again slowly. 

If we slowly make it hotter again, what colors does it get? 

[Red…orange…yellow…white.] 

Of all these colors, what color are the hottest stars? [White.] 

And the coolest? [Red.]

Which stars are hotter than red but cooler than white? [Orange and yellow.]

Fade off the “incandescent” light (or movie).

There are stars that are even hotter than the white stars.
If I could make the light bulb hotter it would turn bluish.

Have you ever seen a blue fire? 

[On a gas stove, an oxyacetylene (welder’s) torch.]

A gas stove’s flame is blue for a chemical reason,
but a welder’s torch physically glows blue because it is very, very hot.

On the other hand, have you seen the dying cinders in a fireplace?  

[They look very red.]

Similarly with the stars, the blue stars are very hot
and the red stars are the coolest, even though they’re still very hot! 

Optional

Red stars are usually less than 4,000°C (7,000°F), blue stars are usually above 10,000°C (18,000°F), and medium hot stars like our Sun are 5,000–6,000°C (roughly 10,000°F). These are all surface temperatures. Temperatures in the centers of stars are millions of degrees!

Stars, like all fires, also cool off with time.

What will happen to the color of a blue star as it cools off?  

[It will turn from blue to bluish-white, to white, to yellow, to orange, to red, and finally to black.]

What color is the closest star to us?

[Yellow.]

What’s the name of that star?

[The Sun.]

The Sun is an average yellow star. Do you think the Sun will always be yellow?

[No. It will cool off and become orange, then red.]

Billions of years from now, our Sun will become a red star and things might look very different on Earth.
Let’s take a look at what things might look like when that happens.