The electromagnetic spectrum sorts out light according to its wavelength. It places the light that we can detect with our eyes—what we call visible light—right in the middle of the spectrum, with red on the right and blue on the left. Looking at the whole electromagnetic spectrum, it’s easy to see why astronomers have wanted telescopes that can study the universe at all wavelengths and all energies.
The electromagnetic spectrum surprisingly only represents a tiny fraction of electromagnetic light. If we move further to the right, to longer wavelengths, we quickly reach something called infrared light, which is past the limits of our kind of vision, but which other animals—like mosquitoes and goldfish—can utilize.
Cool stars, young planets, and even little dust grains drifting through space all emit infrared light. Pushing out to even longer wavelengths, we find ourselves in the microwave and then into the radio regime. These are the very long wavelengths detected by radio telescopes and can be used to study everything from the moments after the Big Bang to supermassive black holes.
But what happens on the other end of the spectrum, as we move to shorter wavelengths?
Wavelength and Energy of Light
To understand what we see and study here we need to remember the relationship between the wavelength and the energy of light.
When we look at the cosmic microwave background: the energy of a photon, E, is equal to h, Planck’s constant, times c, the speed of light, over lambda, the photon’s wavelength. H and c are both constants, and so the equation gives us a clear relationship between the variables of energy and wavelength.
If the wavelength of light gets shorter and the value of lambda goes down, the energy, E, has to increase to keep the equation balanced. This means that longer wavelength infrared and radio light have very low energy, and that shorter wavelength light has much higher energy.
Ultraviolet light is emitted by stars; we know our sun emits plenty of it! Very hot and massive stars actually emit most of their light in the ultraviolet range, making this the perfect wavelength for studying them. Lots of ultraviolet light is also emitted by atoms whose electrons have been excited to higher energy levels.
When those atoms calm down and drop back to lower energies, they release that energy as a photon, and watching atoms release ultraviolet light can tell us a great deal about the composition of distant galaxies and interstellar space.
This article comes directly from content in the video series Great Heroes and Discoveries of Astronomy. Watch it now, on Wondrium.
At even higher energies and shorter wavelengths we find x-rays. X-rays come from things like collisions between fast-moving particles, which we see around black holes or during the supernova deaths of enormous stars, or from super-heated interstellar gas that we can see when galaxies collide.
At the very highest energies and shortest wavelengths we even run into what we call gamma rays, which can be produced during the very strange supernova deaths of some massive stars.
Looking at the whole electromagnetic spectrum, it’s easy to see why we’ve wanted telescopes that can study the universe at all wavelengths and all energies. There are incredible mirrored telescopes which are used to study the light that we can see, and with a few tweaks those same telescopes can be used to study infrared light.
The unique physics of radio astronomy allows us to design antennae and interferometers that peer into this realm of the spectrum. But what about the shorter wavelengths of UV, X-rays, and gamma rays? What do we need to study these?
In our attempt to study UV, X-rays, and gamma rays , we run into two problems. The first is our own atmosphere! One can plot out how much light makes it from space through our atmosphere and all the way to the ground as we go from very long wavelengths to very short wavelengths.
The plot reveals that the light that we can see with our eyes makes it all the way to the ground without a problem, making this a great place to build our classic mirrored telescopes. It’s also pretty easy to study radio wavelengths from the ground. Infrared can get a bit tougher, but can still be managed. The problem begins with the ultraviolet light.
Bouncing Off the Ozone
Anyone who’s ever gotten a bad sunburn might think that plenty of ultraviolet light makes it through the atmosphere and down to our beaches and backyards. And yet, the truth is that most ultraviolet light can’t penetrate our atmosphere. Instead, it bounces off the oxygen molecules that make up our ozone layer.
Similarly, x-rays and gamma rays get scattered by our upper atmosphere, making it very hard to detect any high-energy light from space while staying on the surface of the Earth.
The second problem concerns the physics of ultraviolet and x-ray light. X-ray light may bounce off of the upper layers of Earth’s atmosphere, but we also know that it’s capable of penetrating our very skin and taking pictures of our bones!
Collecting and Studying Light
Consequently, reflecting x-ray light with a shiny mirror, like one does for optical astronomy, just won’t work. Thus, collecting and studying light at these shorter wavelengths means getting creative with how we design and build telescopes, and how we store the light that they collect.
In conclusion, no doubt, studying the sky in the x-ray and UV can illuminate entirely new areas of physics and answer important questions about how the cosmos works. The catch comes when we consider how to detect these types of light, one that can only be resolved by ultraviolet astronomy.
Common Questions about the Electromagnetic Spectrum
Ultraviolet light is emitted by stars; we know our sun emits plenty of it! Very hot and massive stars actually emit most of their light in the ultraviolet range, making this the perfect wavelength for studying them.
Most ultraviolet light can’t penetrate our atmosphere. Instead, it bounces off the oxygen molecules that make up our ozone layer.
X-rays and gamma rays get scattered by our upper atmosphere, making it very hard to detect any high-energy light from space while staying on the surface of the Earth.