Stars are hot.

Really hot. Hot enough to have energy to spare for their planets. If our star wasn’t hot, we couldn’t live on Earth. And our star isn’t even particularly hot for a star. It’s a middle-aged star of low mass, so it’s relatively cool compared to other stars.

You might also notice that stars aren’t all the same color. There are redder stars and bluer stars and more whitish stars.

We know stars are hot. They’re also bright. And they’re different colors. But how does that all translate to radiation—and how can we see it?

There is one fundamental fact about reality you should know before we consider stars in depth. If something is hot, it means its particles are moving around a lot.

What does that even mean?

Consider a solid, liquid, and gas of the same material—say, water.

How do you get a solid to melt and then evaporate?

Well…let’s consider what would happen if we placed an ice cube in a pan and turned on the stove.

First, it melts. Why? We added heat.

If we turn the stove up to water’s boiling point, it will evaporate away—because we added even more heat.

The reason for this lies at the atomic level. See all those little circles up above? Those represent individual atoms, the building blocks of the universe.

Atoms are always moving. It’s a fundamental fact of reality.

If you add more energy to a substance, atoms will start to move faster. In order to melt a substance, you have to add enough energy that the atoms break from one another and flow freely—as a liquid.

Add enough energy, and they’ll break away from each other entirely and whiz about in gaseous form. All because adding energy increases the speed at which atoms move.

Keep in mind that we’re talking about something called thermal energy here. This simply describes how fast the atoms are moving. We measure it in Kelvins.

Kelvins are like degrees Celsius—just take a Celsius temperature, add 273°, and you have the temperature in Kelvins. The Kelvin scale begins at absolute zero, which is a theoretical state at which there is no motion of atoms.

So…so far we’re talking about atomic motion. How the heck does that reach our telescopes?

Well. Let’s take a step back, and imagine ourselves in a crowed room.

And when I say crowded, I mean very crowded. Extremely crowded. The kind of crowded that you’ll find with atoms in the dense gases of stars. Oh, and did I forget to mention that everyone in the room is zipping around really fast?

When I say really fast…yes, I mean extremely fast. So fast that collisions happen all the time. There’s no way to control it. The people in this room are moving about so fast that they are constantly running into each other.

Because we’re actually talking about atoms here, and not people, we have to imagine what happens when atoms collide.

Atoms are not solid objects. They are a nucleus of protons and neutrons surrounded by a cloud of whizzing electrons. When atoms collide, they don’t collide so much as their electrons collide.

So what happens when electrons collide?

Same thing that happens when anything collides. Like cars. Consider this: how do you get the front of a car to completely crumple up?

I’d rather not think about how this car got this way. But I know how it happened scientifically. It took a huge amount of force to bend the metal, and that took energy. Which means that whatever collision happened, it must have generated energy.

The same thing happens between electrons when atoms collide. Energy is generated. And  this energy is enough to tug them away from their nucleus a bit, to a higher orbit—also known as energy level.

Electrons can’t sustain an orbit at a higher energy level. They’ll always drop back to their ground state within moments. But when they do, they emit the same amount of energy that just made them move.

They emit this energy in the form of a photon—a bundle of electromagnetic waves.

Hold on a second…what are electromagnetic waves again?

They’re basically all the radiation we collect in different kinds of telescopes. Everything we can ever know about the universe with our current technology comes from the electromagnetic radiation we can observe.

Sound familiar?

Electrons emit bundles of these when they collide. Electrons collide constantly in dense, hot conditions. These conditions are found in stars. So stars emit radiation at all the different wavelengths because their electrons collide.

When radiation is emitted by a heated object, such as a star, it’s called blackbody radiation.

Why? It comes from a word in German. This word refers to an object that works as a perfect emitter and a perfect absorber. Such an object would appear black at room temperature. But if it were much hotter, it would start to glow.

…Which basically describes how a star behaves. A star is opaque and, if it were much, much cooler…well, if it weren’t a star and were instead at room temperature, it would be as black as the space around it.

But…it’s not. It’s a star, so it’s not at room temperature. It’s tremendously hot. So it emits radiation—specifically, it emits blackbody radiation.

Here’s the thing, though. A star emits blackbody radiation at all wavelengths. So how come all stars don’t appear white to our eyes, a combination of all the colors of visible light?

Because…well, because of this graph.

Each curve on this graph represents data from a different heated object. It shows you that although the object emits all wavelengths of radiation, it emits some much more than it emits others.

Basically, wherever the graph peaks, the wavelength at that point is the one the object emits the most—the wavelength of maximum intensity.

The rainbow column represents the wavelengths of visible light. Beyond purple is ultraviolet, and beyond red is infrared. The bottom two curves peak in the infrared, so they will be most easily seen with infrared telescopes.

However…those two objects also emit all the colors of visible light. So those colors will blend together into white, and those objects will be visible to the human eye as very dim, but white.

The objects at 5000, 6000, and 7000 Kelvins all emit visible light the most. Because the wavelength of maximum intensity is a visible light wavelength, those objects will appear as those specific colors to the human eye.

For example, the 5000K object will appear yellow. The 6000K object will appear bluish-green. The 7000K object will appear much more purple.

Now you can see why different stars appear as different colors—because their wavelengths of maximum intensity are different colors of visible light.

Next up, let’s get back to our discussion of stellar spectra from a few posts ago…