- The electromagnetic spectrum includes a range of all types of light, not just what we can see. This range – going from radio waves to gamma rays – is mostly invisible to our eyes.
- Our eyes see just visible light, which includes colors from red to violet. Different colors represent different wavelengths!
- Astronomers use the entire spectrum of radiation from stars and other objects to study outer space. For example, radio waves help map galaxies, while infrared can see through dust clouds and identify cool stars.
The electromagnetic spectrum
When you think of light, you probably think of what your eyes can see. However, the light our human eyes can detect is only a sliver of the total amount of light that’s out there. So, the electromagnetic spectrum is the term scientists use to describe the entire range of light that exists. From radio waves to gamma rays, most of the light in the universe is, in fact, invisible to us.
Light is a wave of alternating electric and magnetic fields. The propagation of light isn’t much different than waves crossing an ocean. Like any other wave, light has a few fundamental properties that describe it. For example, one is its frequency, measured in hertz (Hz), which counts the number of waves that pass by a point in one second. Another closely related property is its wavelength: the distance from the peak of one wave to the peak of the next. In fact, these two attributes are inversely related. The larger the frequency, the smaller the wavelength, and vice versa.
Our eyes see visible light
The electromagnetic waves your eyes detect – visible light – oscillate between 400 and 790 terahertz (THz). To put it another way, that’s several hundred trillion times a second. As an illustration, the wavelengths are roughly the size of a large virus: 390 – 750 nanometers (1 nanometer = 1 billionth of a meter; a meter is about 39 inches long). Our brain interprets the various wavelengths of light as different colors. For example, red has the longest wavelength, and violet the shortest. When we pass sunlight through a prism, we see that it’s actually composed of many wavelengths of light. The prism creates a rainbow by redirecting each wavelength out at a slightly different angle.
But light doesn’t stop at red or violet. Indeed, just like there are sounds we can’t hear, there is an enormous range of light that our eyes can’t detect. In general, the longer wavelengths come from the coolest and darkest regions of space. Meanwhile, the shorter wavelengths measure extremely energetic phenomena.
The coolest part of the electromagnetic spectrum
Astronomers use the entire electromagnetic spectrum to observe a variety of things. Radio waves and microwaves are the longest wavelengths and lowest energies of light. With this in mind, they are used to peer inside dense interstellar clouds and track the motion of cold, dark gas. Radio telescopes have been used to map the structure of our galaxy. Additionally, microwave telescopes are sensitive to the remnant glow of the Big Bang.
Infrared telescopes excel at finding cool, dim stars, slicing through interstellar dust bands. Plus, they even measure the temperatures of planets in other solar systems. The wavelengths of infrared light are long enough to navigate through clouds that would otherwise block our view. By using large infrared telescopes, astronomers peer through the dust lanes of our galaxy into the Milky Way’s core.
Most stars emit visible light
The majority of stars emit most of their electromagnetic energy as visible light, the tiny portion of the spectrum to which our eyes are sensitive. And, because wavelength correlates with energy, the color of a star tells us how hot it is: red stars are coolest, blue are hottest. On the other hand, the coldest of stars emit hardly any visible light at all; they can only be seen with infrared telescopes.
The more energetic ultraviolet light
At wavelengths shorter than violet, we find the ultraviolet, or UV, light. You may be familiar with UV from its ability to give you a sunburn. Astronomers use it to hunt out the most energetic of stars and identify regions of star birth. When viewing distant galaxies with UV telescopes, most of the stars and gas disappear, and all the stellar nurseries pop into view.
Highest energy light: X-ray and Gamma Ray
Then, beyond UV come the highest energies in the electromagnetic spectrum: X-rays and gamma rays. Our atmosphere blocks this light, so astronomers must rely on telescopes in space to see the X-ray and gamma ray universe. X-rays come from exotic neutron stars, or from the vortex of superheated material spiraling around a black hole. As well as, from diffuse clouds of gas in galactic clusters that are heated to many millions of degrees.
Meanwhile, gamma rays – the shortest wavelength of light and deadly to humans – unveil violent events. These include supernova explosions, cosmic radioactive decay and even the destruction of antimatter. Gamma ray bursts are among the most energetic singular events in the universe. Or they are a brief flickering of gamma ray light from distant galaxies when a star explodes and creates a black hole.
See the difference for yourself
As visible light fades to near-infrared, more stars are revealed, plus twin jets blasting out from a young star at the top of this dusty pillar. Different wavelengths of light show more of what is happening in space. Credit: NASA, ESA, STScI. pic.twitter.com/ROKk7hooOI
— Hubble Space Telescope (@HubbleTelescope) August 11, 2023
Hubble and #NASAWebb work together to give complementary views of the star-forming region NGC 346. Fading from visible light to near-infrared and mid-infrared, each image highlights different features: pic.twitter.com/oZIdafgbBX
— Space Telescope Science Institute (@SpaceTelescope) October 10, 2023
Which came first for this Penguin and its Egg? @NASAHubble took the visible-light view on the left in 2013.
On the right side is Webb’s near-infrared image. With its sensitive vision, Webb peers through dust, intensifies bright objects, and highlights unseen stars and galaxies. pic.twitter.com/IYyBoTWm8V
— NASA Webb Telescope (@NASAWebb) July 12, 2024
There is more than meets the eye on the Sun.
In visible light (left), which the human eye can see, the Sun’s surface has dark splotches known as sunspots.
Ultraviolet light (right) is invisible to the human eye but shows sunspots surrounded by bright regions named faculae. pic.twitter.com/DbLMh0s13P
— NASA Sun & Space (@NASASun) October 3, 2022
Bottom line: The electromagnetic spectrum describes all the wavelengths of light, both seen and unseen.