Spectroscopy: Reading the Rainbow

What Is a Spectrum?

A spectrum is a rainbow! This rainbow is created when a beam of white light is broken into its component colors, such as with a prism. The colors formed are ordered according to their wavelength. When scientists look at this rainbow, they examine how intense the light is in each color. Is blue brighter than yellow, or is this specific red brighter than this other red?

When material interacts with light, properties of that material are stamped on the light. This stamp is like a specific fingerprint for each element and molecule. By examining the intensity of light in each color, scientist can work backward to infer the properties of the material that touched the light along the way.
 

What Is Spectroscopy?

Spectroscopy is the study of the spectra produced when material interacts with or emits light. It is the key to revealing details that cannot be uncovered through a picture. A spectrograph — sometimes called a spectroscope or spectrometer — breaks the light from a single material into its component colors the way a prism splits white light into a rainbow. It records this spectrum, which allows scientists to analyze the light and discover properties of the material interacting with it. Spectroscopy is as crucial as imaging to understanding the universe.

Hubble and Spectroscopy

Hubble is famous for the images captured by its cameras, but it often also relies on its spectrographs. Spectrographs collect data that tell scientists how much light comes out at each wavelength. These data reveal important details about the makeup of atmospheres on exoplanets, the compositions of stars and nebulas, the motion of galaxies and more.

Ultraviolet spectroscopy is one of Hubble’s most unique contributions to the astronomical community, and this capability will not be replaced or superseded by any mission in the near future. Ultraviolet spectroscopy tells us certain things about the universe, while visible and infrared spectroscopy tells us others. By combining Hubble’s ultraviolet spectroscopy with the infrared spectroscopic capabilities of the James Webb Space Telescope, the two telescopes will achieve scientific results together that neither could achieve alone.
 

How Do You Read a Spectrum?

Light carries information about the material with which it interacts. Different materials interact differently with light, and we can use light to understand what something is made of. All matter is made of atoms. Electrons go around the nucleus of an atom at different allowed energies, like rungs on a ladder. Light with the exact energy needed to go between rungs can be absorbed, but not others. Electrons fall down to lower rungs, emitting light at the specific energy of the difference between the rungs. This allows different atoms to emit different colors of light. Sodium’s spectrum does not look like nitrogen’s spectrum — nor like the spectrum of any other element.

All elements absorb and emit specific wavelengths of light that correspond to those energy levels. An absorption spectrum is a spectrum of light transmitted through a substance, showing dark lines or bands where light has been absorbed by atoms, causing a dip in the spectrum. An emission spectrum is made by electrons falling down the energy ladder. It’s what you get when you look at hot gas, which is heated by something out of the line of sight. This heating moves the electrons up the ladder, then when they fall down the ladder some of the light they emit comes to you. This results in bright, colored spikes due to atoms releasing light at those wavelengths.

How Does a Spectrograph Work?

A spectrograph passes light coming into the telescope through a tiny hole or slit in a metal plate to isolate light from a single area or object. This light is bounced off a special grating, which splits the light into its different wavelengths (just like a prism makes rainbows). The split light lands on a detector, which records the spectrum that is formed.

What Has Hubble Found with Spectroscopy?

Hubble’s spectrographs reveals important details of many aspects of our universe. Below are examples of the many spectroscopic findings from Hubble.

• Recording a black hole’s signature, uncovering gas swirling at hundreds of miles per second around a black hole at the center of another galaxy
• Providing first direct detection of the atmosphere of a planet orbiting a star outside our solar system
• Detecting an organic molecule in the atmosphere of a planet orbiting another star
• Finding what were thought to be randomly distributed, nearby primordial clouds of hydrogen may actually be associated with galaxies or clusters of galaxies
• Showing that massive clouds of ionized gas are raining down from our galaxy’s halo and intergalactic space, and will continue to provide fuel for the Milky Way to keep forming stars
• Fingerprinting the distant universe using the light from a quasar, allowing astronomers to probe the raw materials from which galaxies form and determine how this gas was assembled into the complex structures of the present-day universe

Last Updated: May 30, 2019