When stargazers go outside at night to look at the sky, they see the light from distant stars, planets, and galaxies. Light is crucial to astronomical discovery. Whether it's from stars or other bright objects, light is something astronomers use all the time. Human eyes "see" (technically, they "detect") visible light. That's one part of a larger spectrum of light called the electromagnetic spectrum (or EMS), and the extended spectrum is what astronomers use to explore the cosmos.
The Electromagnetic Spectrum
The EMS comprises the full range of wavelengths and frequencies of light that exist: radio waves, microwave, infrared, visual (optical), ultraviolet, x-rays, and gamma rays. The part humans see is a very tiny sliver of the wide spectrum of light that is given off (radiated and reflected) by objects in space and on our planet. For example, the light from the Moon is actually light from the Sun that's reflected off it. Human bodies also emit (radiate) infrared (sometimes referred to as heat radiation). If people could see in the infrared, things would look very different. Other wavelengths and frequencies, such as x-rays, are also emitted and reflected. X-rays can pass through objects to illuminate bones. Ultraviolet light, which is also invisible to humans, is quite energetic and is responsible for sunburned skin.
The Properties of Light
Astronomers measure many properties of light, such as luminosity (brightness), intensity, its frequency or wavelength, and polarization. Each wavelength and frequency of light lets astronomers study objects in the universe in different ways. The speed of light (which is 299,729,458 meters a second) is also an important tool in determining distance. For example, the Sun and Jupiter (and many other objects in the universe) are natural emitters of radio frequencies. Radio astronomers look at those emissions and learn about the objects' temperatures, velocities, pressures, and magnetic fields. One field of radio astronomy is focused on searching out life on other worlds by finding any signals they may send. That is called the search for extraterrestrial intelligence (SETI).
What Light Properties Tell Astronomers
Astronomy researchers are often interested in the luminosity of an object, which is the measure of how much energy it puts out in the form of electromagnetic radiation. That tells them something about activity in and around the object.
In addition, light can be "scattered" off an object's surface. The scattered light has properties that tell planetary scientists what materials make up that surface. For example, they might see the scattered light that reveals the presence of minerals in the rocks of the Martian surface, in the crust of an asteroid, or on Earth.
Infrared light is given off by warm objects such things as protostars (stars about to be born), planets, moons, and brown dwarf objects. When astronomers aim an infrared detector at a cloud of gas and dust, for example, the infrared light from the protostellar objects inside the cloud can pass through the gas and dust. That gives astronomers a look inside the stellar nursery. Infrared astronomy discovers young stars and seeks out worlds not be visible in optical wavelengths, including asteroids in our own solar system. It even gives them a peek at places like the center of our galaxy, hidden behind a thick cloud of gas and dust.
Beyond the Optical
Optical (visible) light is how humans see the universe; we see stars, planets, comets, nebulae, and galaxies, but only in that narrow range of wavelengths that our eyes can detect. It's the light we evolved to "see" with our eyes.
Interestingly, some creatures on Earth can also see into the infrared and ultraviolet, and others can sense (but not see) magnetic fields and sounds that we cannot directly sense. We are all familiar with dogs who can hear sounds that humans can't hear.
Ultraviolet light is given off by energetic processes and objects in the universe. An object has to be a certain temperature to emit this form of light. Temperature is related to high-energy events, and so we look for x-ray emissions from such objects and events as newly forming stars, which are quite energetic. Their ultraviolet light can tear apart molecules of gas (in a process called photodissociation), which is why we often see newborn stars "eating away" at their birth clouds.
X-rays are emitted by even MORE energetic processes and objects, such as jets of superheated material streaming away from black holes. Supernova explosions also give off x-rays. Our Sun emits tremendous streams of x-rays whenever it belches up a solar flare.
Gamma-rays are given off by the most energetic objects and events in the universe. Quasars and hypernova explosions are two good examples of gamma-ray emitters, along with the famous "gamma-ray bursts".
Detecting Various Forms of Light
Astronomers have different types of detectors to study each of these forms of light. The best ones are in orbit around our planet, away from the atmosphere (which affects light as it passes through). There are some very good optical and infrared observatories on Earth (called ground-based observatories), and they are located at very high altitude to avoid most of the atmospheric effects. The detectors "see" the light coming in. The light might be sent to a spectrograph, which is a very sensitive instrument that breaks the incoming light into its component wavelengths. It produces "spectra", graphs that astronomers use to understand the chemical properties of the object. For example, a spectrum of the Sun shows black lines in various places; those lines indicate the chemical elements that exist in the Sun.
Light is used not just in astronomy but in a wide range of sciences, including the medical profession, for discovery and diagnosis, chemistry, geology, physics, and engineering. It's really one of the most important tools tha scientists have in their arsenal of ways they study the cosmos.