Hey, Phil Plait here and this is Crash Course
Astronomy. In last week’s episode, I mentioned that nearly all the information we have about
the Universe comes in the form of light. But how does that light get made? What can it
tell us about these astronomical objects? And honestly, what is light? Here’s a hint. Light is a wave. It took centuries of thought
and experiments to figure that out, and to also figure out that, at its most basic, light
is a form of energy. It travels in waves, similar to waves of water in the ocean. Except
with light, the things doing the waving are electric and magnetic fields. Literally—light
is a self-contained little bundle of these two fields, intertwined. That’s why we call
light electromagnetic radiation. The details of this are very complex, but we can make
some pretty good overall observations about light just from thinking of it as a wave. If you’re floating in the ocean, you’ll
move up as a wave passes you, then back down, then back up again when the next wave rolls
by. The distance between these crests in the wave is called the wavelength. Since light
is a wave, it has a wavelength as well, and this may be its single most important feature. That’s
because the energy of light is tied to its wavelength. Light with a shorter wavelength has more energy,
and light with a longer wavelength has less energy. And our eyes have a really convenient
way of detecting these different energies: color! What you think of as the color violet is actually
light hitting your eye that has a short wavelength. Red light has a longer wavelength, about twice
the distance between crests as violet light. All the colors in between—orange, yellow,
green, blue—have intermediate wavelengths. This spread of colors, wavelengths, is called
a spectrum. Over millions of years, our eyes have evolved
to detect the kind of light the Sun emits most strongly. Well, that makes sense; that
makes it easier for us to see! We call this kind of light visible light. But that’s just the narrowest sampling of
all the different wavelengths light can have. If light has a slightly shorter wavelength
than what our eyes can see, it’s invisible to us, but it’s still real. We call that
ultraviolet light. Light with shorter wavelengths than that fall into the X-ray part of the
spectrum, and light waves with the shortest wavelengths of all are called gamma rays. At the other end, light with slightly longer
wavelengths than the reddest color we can see is called infrared light. Light waves
longer than that are called microwaves, and those with the longest wavelengths of all
are called radio waves. These different groups don’t really have hard and fast definitions;
just think of them as general guidelines. But together, we call all of these different
kinds of light the electromagnetic or EM spectrum. And remember, energy goes up when the wavelength
gets shorter. So ultraviolet light has a higher energy than violet, X-rays have a higher energy
than that, and gamma rays have the highest energy of all. Infrared light has lower energy
than red light, microwaves lower than that, and radio waves have the lowest energy. When you look at the whole EM spectrum, you’ll
probably notice that we really do only see a teeny little sliver of it. Most of the Universe
is invisible to our eyes! That’s why we build different kinds of telescopes -- to
detect the kind of light our eyes can’t detect. They let us see a lot of stuff
that otherwise we’d never notice. So you might be asking: how is light made?
Well, one of the most basic properties of matter is that when you heat it up it gains
energy, and then it tries to get rid of that energy. Since light is energy, one way to
get rid of energy is to emit light. Another important property of matter is that
the kind of light an object emits depends on its temperature. An object that’s hotter
will emit light with a higher energy, that is, a shorter wavelength. Cooler objects give
off light with a longer wavelength. You may have seen this in action. Heat up
an iron bar and it starts to glow red, then orange, then yellow as it gets hotter. The color, the wavelength,
of light emitted changes as the bar heats up. Astronomers use a shorthand for this. We say
that light with a shorter wavelength is “bluer”, and light with a longer wavelength is “redder”.
Don’t take this literally! We don’t really mean more blue or more red, just that the
wavelengths are decreasing or increasing. So in this lingo, ultraviolet light is bluer
than blue, and X-rays are bluer than ultraviolet. So objects that are more energetic, that have a
higher temperature, are bluer than cooler, redder objects. This rule of thumb works really
well for dense objects like iron bars and stars. Even humans! You emit light, but it’s in the
far infrared, well beyond what our eyes can see. There are less dense objects in space, too,
like gas clouds, and the way they emit light is different. To understand that, we have to zoom in on
them. Way, way in, and look at their individual atoms. And to understand that, we need to take a
brief diversion into atomic structure. Atoms are the building blocks of matter. In general,
atoms are made up of three subatomic particles: Protons, neutrons, and electrons. Protons
have a positive electric charge, electrons a negative charge, and neutrons are neutral.
Protons and neutrons are much more massive than electrons, and occupy the centers of
atoms, in what’s called the nucleus. Electrons whiz around the nucleus, their negative charge
attracted by the protons’ positive charge. The type of atom depends on how many protons
it has in the nucleus. Hydrogen has one proton, helium two, lithium three, and so on up the
periodic table of elements. It’s common to think of the electron as
orbiting the nucleus like a planet orbits the Sun, but that’s not really the case.
The real situation is fiendishly complex and involves pretty hairy quantum mechanics, but
in the end, the electron is only allowed to occupy very specific volumes of space around the
nucleus, and those depend on the electron’s energy. Think of these like stairs on a staircase,
where the landing is the nucleus. When you walk up the stairs, you have to use energy
to go up. And when you do, you have to go up a whole step at a time; if you don’t
have the energy to get to the next step, you can’t move. You can be on the first step,
or the second step, but you can’t be on the first-and-a-halfths step. There isn’t
one! Electrons are the same way. They whiz around
the nucleus with a very discrete amount of energy. If you give them an additional precise
amount of energy, they’ll move up to the next energy level, the next step, but if you give
them the wrong amount they’ll just sit there. The opposite is true as well; electrons can
be in a higher energy state, up on a higher step, and then give off energy when they jump
down. The amount they give off is exactly the same amount needed to get them
to jump up in the first place. How do they get this energy? Light! If light hitting the atom has just the right amount
of energy, the electron will absorb it and jump up. It can also jump down and emit light at
that energy, too. An electron can also jump two steps, or three, or whatever, but it needs
exactly the right energy to do it. But as I said earlier, energy and wavelength
are the same thing, and that’s equivalent to color. So when an electron jumps up or down,
it absorbs or emits a very specific color of light. Not only that, but the steps are different
for different atoms. To stick with our analogy, it’s like different atoms are different
staircases, with different heights between the steps. So when an electron jumps down
a step in a hydrogen atom, it emits a different energy, a different color of light, than an electron
jumping down in a helium or calcium atom. And this, THIS, is the key to the Universe.
Because different atoms emit different colors of light, if we can measure that light, in
principle we can determine what an object is made of, even if we can’t touch it. Even if
it’s a bazillion light years away! And we can. Can you tell the difference between these
two squares? They’re a very slightly different shade of red. Your eye probably can’t tell
the difference, but a spectrometer can. This is a device that can precisely measure
the wavelength of light, and can for example distinguish light emitted by a hydrogen atom
from light emitted by helium. When you hook one of these spectrometers up to a telescope, you can
figure out what astronomical objects are made of. In the case of thin gas clouds in space, the
atoms are basically floating free, rarely bumping into one another. The atoms emit those
individual colors of light, allowing us to identify them. Unlike dense stars, the color of the thinner gas
depends more on what’s in it than its temperature. And this is how we learned what the Universe
is made of. Stars and gas clouds in space are mostly hydrogen, with some helium and
heavier elements thrown in. Jupiter has methane in its atmosphere, Venus carbon dioxide. Everything
in the Universe has its own mix of ingredients, like cakes at a bakery. With spectroscopy,
we can taste them. But wait! There’s more. You’re probably familiar with the Doppler
effect; the change in pitch when, say, a motorcycle goes by. In sound, the wavelength defines
the pitch; higher tones (“eeeee”) have shorter wavelengths, and lower tones (“eeeee”)
longer wavelengths. When the motorcycle is headed toward you, the sound waves get compressed,
causing the pitch to rise. After it passes you, the pitch drops because the wavelengths
get stretched out. The same thing happens with light. If an object
is headed toward you, the wavelength of light from the source gets compressed, shorter.
We say the light is blue-shifted. If it heads away, the wavelength gets longer, and it’s
red-shifted. Apply that to a spectrum, and by measuring that shift we can tell if an
object is moving toward or away from us. Here’s a teaser: This becomes super important
later, when we talk about galaxies. Spoiler alert: The Universe is expanding, and it's
this redshift that allowed us to figure that out. And that’s still not the end of it. With
other spectroscopic techniques we can determine if an object is spinning and how fast, whether
it has a magnetic field and how strong it is, and even how massive and dense an object
is. A vast amount of the fundamental properties of astronomical objects can be found just
by dissecting their light into individual colors. Almost everything we know about the Universe
comes from the light objects in it give off. Pictures of astronomical objects show us their
structure, their beauty, and hint at their history. But with spectra, we can examine
their blueprints. Today you learned that light is a form of
energy. Its wavelength tells us its energy and color. Spectroscopy allows us to analyze
those colors and determine an object’s temperature, density, spin, motion, and chemical composition. Crash Course is produced in association with
PBS Digital Studios. Head over to their channel for even more awesome videos. This episode
was written by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant
is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, the script supervisor and
editor is Nicole Sweeney, the sound designer was Michael Aranda, and the graphics team
is Thought Café.
Wait (possibly stupid question inbound), if we can tell what something is composed of by measuring its energy (wavelength) then surely by having the wavelength red/blue shifted, the reading of what the object is composed of gets changed?
So if something looks like helium but it's moving either towards us or away from us, then it would look like something else, like hydrogen?