Lighting Up the Universe

Have you ever wondered what the world looks like to a bee buzzing around your flowers, or why the sky is blue?

Have you ever wondered how astronomers can take two pictures of the same nebula and end up with vastly different images, or why those colorful nebulae look like gray smudges in a telescope?

Have you ever wondered how light from a star can tell us what it’s made of—and what makes up the air of exoplanets?

If you’ve ever wondered (or are now!) about any of these questions, read on in our series: Lighting up the Universe.

What, exactly, is “light”?

Light isn’t just the colors of the rainbow or the stuff plants use to make food. It’s so much more than that—light is the smallest amount of energy that can be transported. It’s transported by an elementary particle called a photon—a particle that has no size, no weight and travels at the speed limit of the universe (that’s why it’s called the speed of light). 

Light is created when an electron in an atom moves from a high energy, excited state to a lower energy state. The excess energy from this transition is emitted as a photon. This is where it starts to get messy—because a photon is a particle that has a tendency to travel like a wave. Light is actually both a particle and a wave, a phenomenon called “wave-particle duality”.

Because light travels as a wave, it has a wavelength—the distance between the peaks, or crests, of a wave. You can actually see the difference in wavelengths of light without any help at all—the color of light depends on its wavelength! These wavelengths can be smaller than an atom to larger than the entire Earth—but we only see a limited portion of them! The subset that humans see is called visible light—and it’s no accident that we see this subset. Visible light just so happens to be in the wavelengths that transmit through water easily—and water is where we think the first eyes evolved here on Earth. Beyond visible light is the rest of the electromagnetic spectrum, far more than what our eyes can see, but still types of light! Using all of these types of light, we’re able to unravel the secrets of the universe around us.

Now that we have a base to work from, it’s time to start shining some light on our universe!

Why is the sky blue? 

I remember asking my dad this question when I was a kid—and I’m sure I wasn’t the only one! It’s a simple question that seems like it should have a similarly simple solution…

The answer to why the sky is blue has three parts, which all work together to make the blue sky that we see. 

It starts with the Sun. Our Sun sends out many different colors of light that we see mixed together as white light. Each of those colors has a different wavelength. Blue light has a short wavelength, and red has a long one. (For more about wavelengths, check out our previous post in this series.) 

After travelling the 8 light minutes between us and the Sun, the light hits our atmosphere—and scatters from its direct path. How much the light scatters depends on its wavelength—shorter wavelengths (blue) scatter more and longer wavelengths (red) scatter less. 

You can see this effect at home—just shine a white light (I recommend a flashlight) through a clear bowl of water that has some milk or soap added to it. Looking at the light beam from the side, it should appear bluer. The light that makes it out the other side should appear redder—try looking at it on a nearby white wall or piece of paper.

Light does need something to scatter off of—in your experiment, it was the milk or soap. In the Earth’s atmosphere, light scatters off of nitrogen and oxygen molecules.

There’s one last ingredient in making the sky blue—our eyes! Our eyes have color receptors that take in primarily green, red and blue light. This means that even though violet light is scattered more strongly than blue light, our eyes pick up on the blue much more than the violet.

Why is the sky blue? Sunlight scatters off molecules in our atmosphere, with different wavelengths scattering differently, and our eyes are better at picking up blue light than other colors.

Nebulae

We’ve all seen the beautiful, color filled photographs of nebulae from telescopes like the Hubble Space Telescope, like the one above. They capture the imagination and inspire us to look up at the night sky, hoping to see one for ourselves.

Yet, when you see a nebula through a telescope for the first time, it can be a bit disappointing if you were executing to see a miniature Hubble photo through the lens! Through a backyard telescope, nebulae most often appear to be gray smudges—like a bit of dust got stuck on the telescope. So, why do we see nebulae like this?

Much of it has to do with how our eyes work. There are two structures in your eye that help you see, called rods and cones. The cones in your eye are able to discern color and work best in bright light. The rods detect black/white and work best in dimmer light. 

The light that reaches our eyes from a nebula is very faint, so the cones in our eyes do not detect it—but the rods do. Since the rods in your eye cannot detect color, most nebulae look as if they’re in grayscale. The same can’t be said for cameras and telescopes, which we’ll talk about next time!

Don’t let a grayscale nebula deter you though—being able to see a stellar nursery or the remains of a giant star through a telescope is magical, when you know what to expect!

There are only two different nebulae in these photos, although there are eight photos! Why do they look so different?

These images were taken in different sections of the electromagnetic spectrum (see above!). Humans can only see a small part of this spectrum, but our telescopes can see a lot more. Telescopes are also able to gather more light than a human eye can, which gives them much more detailed images. The images here were taken by many different telescopes, using two kinds of representative color—some of the photos are colored by what the nebula is made of, some are colored by the type of light used. Let’s dive in!

The first set of images show the Pillars of Creation in the Eagle nebula. Image 1 is the iconic image of the Pillars of Creation as seen by the Hubble Space Telescope in visible light; specifically, 3 wavelengths. These wavelengths are emitted from oxygen, hydrogen and silicon molecules that make up the clouds of dust in the nebula. Image 2 is also the Pillars of Creation—this time in infrared light! Infrared light is essentially heat, so the cool clouds of gas are less noticeable and the hot stars shine through.

Images 3 through 8 all show the Crab nebula. Image 3 is a visible light image with representative color; blue are regions with neutral oxygen, green is sulphur, and red is doubly ionized oxygen. Image 8 is another visible light image, but without the representative color of the first. Image 4 is infrared, image 5 is radio, image 6 is X-ray, and image 7 is ultraviolet.

By capturing each of these types of light, we can learn all sorts of different things about nebulae—and by using representative colors we have a great way to visualize data from our telescopes. Those colors do lead to a pretty big question though…how do scientists know what anything is made of when it’s light years away?

Cosmic Composition

How do we know what stars and nebulae are made of? It’s impossible to sample every star, planet and gas cloud—instead astronomers use spectroscopy. Looking at the specific colors of light that we see (or don’t see!) emanating from or through an object tells us what that object is made of, because each element has its own special pattern.

These patterns are determined by what’s happening inside the atom. Electrons in an atom aren’t stuck on just one level—they can become excited by absorbing specific amounts of energy and jump up one or more levels. That energy often comes in the form of a photon. Each wavelength of light has a specific energy associated with it. When that energy matches the energy needed to excite an electron, the light is absorbed, and the electron jumps up a level. Since that specific wavelength is absorbed, we don’t see it when the light reaches us. This creates what is called an absorption spectrum.

Electrons don’t stay excited forever. Eventually, they drop back down to their initial states. As they drop, the electrons lose the same amount of energy that it took to get them excited. That energy leaves the atom in the form of—you guessed it—light! This creates the opposite of an absorption spectrum, called an emission spectrum.

When looking at objects in space, astronomers can split the light into a spectrum to see the specific wavelengths it’s made of. Unfortunately, there are few things in the universe made of just one element—so astronomers need to find just the right combination of elements to match the lines in the spectrum. Once that’s complete, we know what a far away star or nebula is made of!