Light and Astronomy

How Astronomy Uses Light

What do familiar objects look like in different wavelengths of light? Here's Jupiter in three wavelengths of light, taken at three different times. Left is visible light, center is radio frequency, and right is infrared. Carolyn Collins Petersen/NASA/Chandra/Spitzer

Light in the Universe

Light is the energy that helps us detect the universe. It's something we deal with every day. Our eyes "see" (technically, they detect) visible light. What we see with our eyes is only one part of a larger spectrum of light called the electromagnetic spectrum (or EMS). The properties of light are its luminosity (brightness), intensity, its frequency or wavelength, and polarization.

Its speed in a vacuum is the same everywhere in the universe: 299,792,458 meters a second (186,292 miles per second). If it travels through a relatively dense cloud of gas and dust, its speed diminishes, and its direction of travel can be influenced by strong gravitational fields. 

Astronomers 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. 

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 we 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, we see the Moon because light from the Sun is reflected off it. Human bodies also emit (radiate) infrared (sometimes referred to as heat radiation).

If we could see in the infrared, we would look very different to each other. X-rays can be emitted and reflected, and they can also pass through our bodies to illuminate our bones. Ultraviolet light is responsible for sunburned skin.

Each wavelength and frequency of light has special properties that astronomers can use to study objects in the universe.

In addition to reflected light off the Moon, we can study planets in the light they reflect in all wavelengths and frequencies. 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).

In addition, light can be "scattered" off a surface. The scattered light has properties that tell planetary scientists what materials make up that surface. For example, they might see scattered light that reveals the presence of minerals in the rocks of the Martian surface, in the crust of an asteroid, or on Earth. 

Let's look at a few other examples of what astronomers "see" when they look at the universe in wavelengths and frequencies of light across the spectrum.

Infrared light is given off by warm objects such things as protostars (stars about to be born), planets, moons, and brown dwarf objects. If you 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 us a peek at places like the center of our galaxy, hidden behind a thick cloud of gas and dust. 

Optical (visible) light is how WE 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". 

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 what are called "spectra", which are 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.