Spectroscopy's Origins

When we turn on a light in a dark room, it illuminates our surroundings, allowing us to better understand our environment. Astronomers use light in a similar way, but light from astronomical objects not only illuminates the cosmos, it also helps us better understand the very nature of the objects we are looking at.

Spectroscopy, the study of light and its component parts, had its beginnings in the 17th century when Isaac Newton passed a beam of sunlight shining through a pinhole in a covered window through a prism. The prism separated the sunbeam into a continuous series of colors that Newton called a “spectrum.”

Rainbows are the best-known type of spectrum. They form when sunlight bends (refracts) and then bounces (reflects) inside a raindrop. Our Sun emits white light which holds all colors of the rainbow in roughly equal amounts. When sunlight encounters a raindrop, the raindrop acts like the prism in Newton’s experiment. It separates sunlight into its different wavelengths, producing the familiar arc of color we call a rainbow.

But spectroscopy produces more than a simple rainbow arc of color. It can reveal the molecules around stars or in the atmospheres of distant planets. It can tell us their temperatures, densities, and in the case of gas, their pressures. Spectroscopy can also reveal the distance to an object along with the direction and speed it is traveling.

An image of the Southern Crab Nebula with a white superimposed arrow pointing to it labeled "Observed Target Object." Two lines extend down and out, leading to a horizontal line representing its spectrum. Below it, four repeated images of the Southern Crab Nebula are in green, yellow, orange, and red, from left to right. They are, respectively, labeled Oxygen, Hydrogen, Nitrogen, and Sulfur. Below that, white text reads "Detected wavelengths of light correspond to different elements in the Southern Crab Nebula."

Hubble detects the light signatures that correspond to different elements in the Southern Crab Nebula.

NASA, ESA, and J. DePasquale (STScI)

The Electromagnetic Spectrum” shows the shortest, most energetic wavelengths at the left and the longest, least energetic to the right, illustrated as a long, partially colored strip that ends in arrows. From left to right, the types of light are paired with their measurements: gamma ray (0.000001 to 0.01 nanometers), X-ray (0.01 to 10 nanometers), ultraviolet (10 to 380 nanometers), visible light (380 to 780 nanometers), infrared (780 nanometers to 0.3 millimeters), microwave (0.3 millimeters to 10 centimeters), radio (10 centimeters to 10 meters). On the graphic, visible light is represented by a rainbow of colors: violet, indigo, blue, green, yellow, orange, and red. The background is largely purple.

Gamma rays, X-rays, ultraviolet light, visible light (the visible rainbow), infrared light, microwaves, and radio waves are all forms of light (also called electromagnetic radiation). Together, they make up the electromagnetic spectrum. Each band of light has a different range of wavelengths: Gamma rays are the shortest and radio waves are the longest.

NASA, ESA, Leah Hustak (STScI)

Hubble Spectroscopy

All normal matter in the universe either emits, absorbs, reflects, or transmits light. That light, called electromagnetic radiation, includes the rainbow of visible light our eyes can see, the ultraviolet radiation that streams from the Sun, the infrared or heat radiation we feel when we open a hot oven’s door, X-rays, radio waves, and all of the wavelengths in between or beyond.

Because the range of wavelengths available for study is so broad and each one has different requirements to capture, no one telescope can study them all. That means telescopes specialize in a wavelength range, like the James Webb Space Telescop e that studies infrared light, or series of ranges like Hubble.

Hubble’s design and instruments allow it to capture ultraviolet, visible, and a small part of the infrared spectrum called near-infrared. As light from a distant cosmic object enters Hubble, its mirrors reflect and focus that light onto Hubble’s instruments. Those instruments include dedicated spectrographs that separate light into its different wavelengths.

When light from a cosmic object enters one of Hubble’s spectrographs, it passes through a tiny slit in a metal plate that isolates it from the light of other objects. The light then bounces off a special metal grating, called a diffraction grating, that splits the light into its component wavelengths, focusing them onto a detector that records them as a spectrum.

Hubble’s current instruments include two main spectrographs — the Space Telescope Imaging Spectrograph (STIS) that captures wavelengths from ultraviolet through visible and into near-infrared, making it an “all purpose” instrument, and the Cosmic Origins Spectrograph (COS) which measures faint ultraviolet light from faraway sources like remote galaxies— and three cameras — Advanced Camera for Surveys (ACS), Wide Field Camera 3 (WFC3), and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Each of these cameras also have spectrographic capability that allows astronomers to capture an image along with detailed data about the object’s light.

A graphic with a black background and white text across the top that reads "How a Spectroscope Works." A Hubble image of a star cluster is seen near the top, white a white circle labeled as a Target Object. Yellow arrows and a beam point down the image, with just the beam continuing downward through a gray bar. That beam hits a diagonal gray line representing the grating. A rainbow beam of light then extends from the grating, aimed diagonally upward to the right and into a detector, which is also seen in rainbow hues.

When light from a celestial object enters the telescope, it passes through a tiny slit in a metal plate to isolate it from other light. The light then bounces off a special grating – a metal surface of tiny patterns - that splits the light into its constituent wavelengths. The split wavelengths are focused to hit a detector, which records them as a spectrum, and in this example, includes black absorption lines that are indicative of what elements are present.

NASA Goddard/Shireen Dooling

Types of Spectra

Astronomers study three basic types of spectra: continuous, absorption, and emission.

A continuous spectrum.

NASA/JPL-Caltech

A continuous spectrum holds all wavelengths of light across a certain wavelength range. As its name implies, it is uninterrupted with no gaps or intensely bright bands. The hot, compact point of light emitted by stars is nearly a continuous spectrum.

Visible light (ROYGBIV) spectra. An Emission spectrum for hydrogen that has bright lines at about 410 nm, 435 nm, 485 nm, and 655 nm.

An absorption spectrum for hydrogen.

NASA/JPL-Caltech

We see an absorption spectrum when light from an object like a star passes through another medium, like an interstellar cloud or nebula. As the light interacts the nebula, the nebula may absorb certain wavelengths of light while others pass through. The absorbed wavelengths depend on the elements and compounds that make up the nebula. An absorption spectrum holds dark lines or breaks that correspond to the absorbed wavelengths of light.

Visible light (ROYGBIV) spectra. An Absorpotion spectrum for hydrogen and shows dark lines at the same locations.

An emission spectrum for hydrogen.

NASA/JPL-Caltech

An emission spectrum is the result of excited atoms and molecules that emit light. As starlight passes through a nebula, it can heat up the cloud, exciting gas molecules and elements causing them to glow. The resulting emission spectrum holds a series of bright colored lines that correspond to wavelengths of light emitted by the excited atoms and molecules.

Astronomers look at spectra in the form of a graph that generally holds wavelengths along the x-axis and light intensity (luminous flux) along the y-axis. A spectrum's curve, called a continuum, represents the continuous emission of electromagnetic radiation at all wavelengths. A bright line (emission) or dark line (absorption) in the spectra correlates to peaks and valleys on the curve. They reveal the wavelengths where specific atoms emit or absorb light.

Spectra allow astronomers to easily see the intensity of light at precise wavelengths; providing them with important details about the object’s chemical makeup, temperature, density, motion, or physical environment. They offer a detailed way to compare the spectra of any given object over time, allowing astronomers to track the evolution of an object and its environment. Such detail also allows astronomers to compare objects so they can better determine differences in environment, age, motion, and physical characteristics, making each spectrum a fingerprint of the object it represents.

This illustration of a spectrum shows the intensity of light being absorbed. Each peak in the graph represents the darkest part of a dark line in the absorption spectrum.

NASA GSFC/Paul Morris

Hubble observations of Eta Carinae. Left is a colorful image of the double-lobed nebula with a line indicating where the spectrum was taken. Right is the spectrum extending out from Eta Carinae.

Hubble’s Space Telescope Imaging Spectrograph (STIS) captured a spectrum (right) of material ejected by a pair of massive stars called Eta Carinae, while the Wide Field and Planetary Camera 2 took an image of the billowing clouds of gas enveloping the stellar pair (left). The spectrum reveals that one of the lobes contains the elements helium (He), argon (Ar), iron (Fe), and nickel (Ni).

NASA, ESA and the Hubble SM4 ERO Team

Information Hubble Captures with Spectroscopy

Composition
A spectrum's peaks and valleys reveal the elements contained in the objects Hubble observes. Each element has a characteristic signature. Here a spectrum taken by Hubble's Cosmic Origins Spectrograph (COS) of a supernova remnant, the explosive death of a star, reveals sharp peaks that indicate the elements oxygen and carbon.

Hubble was the first telescope to detect and analyze an atmosphere around a planet outside of our solar system. Today, Hubble's wealth of spectroscopic data has helped astronomers better understand the various classes of exoplanets and their environments.
Temperature
Atoms have different energy levels that electrons can occupy. When an atom heats up it absorbs energy and an electron jumps to a higher energy level, becoming "excited". When that electron returns to its normal energy state, it releases a particle of light called a photon. Astronomers use this basic property of matter to determine an object's temperature.

Hubble's Space Telescope Imaging Spectrograph (STIS) long-slit spectrograph viewed the entire ring system of supernova 1987a, dissecting its light and producing a detailed image of the ring in each of its component colors. Each color represents light emitted by specific elements in the ring's gasses, including oxygen (single green ring), nitrogen and hydrogen (triple orange rings), and sulfur (double red rings). Each ring represents an emission line of interest in the element's spectrum.

The ratio of the ring's brightness in different colors, emanating from the same elements, gives a measure of the concentration of the gasses. The light from different elements also identifies gasses at different temperatures.
Density
Pressure and density are directly related. As pressure increases, so does density. Here we see the absorption spectra of three different star types. If you compare the spectrum of an extremely dense white dwarf star to a that of the Sun or a puffy blue giant star, you’ll notice that the spectral lines in the white dwarf are much wider and fuzzier than those in the spectra of the blue giant or Sun.

White dwarfs have roughly the mass of the Sun compressed to the volume of Earth. At the atomic level, pressure in the white dwarf is so high that it disrupts the atoms' energy levels, causing its spectral lines to spread out, allowing astronomers to determine its density.
Motion
As an object moves toward an observer, its light waves "bunch up" in the direction of its motion. The observer sees each wave at an increased frequency, which makes them appear bluer. When an object travels away from an observer, its light waves arrive "spread out." To the observer, the light waves appear to have a longer frequency, making the object look redder. This change in frequency related to motion is called the Doppler Effect.

The unique capabilities of Hubble's Space Telescope Imaging Spectrograph (STIS) allowed astronomers to discover a black hole in the center of galaxy Messier 84. In a single exposure, STIS was able to capture the motions of gas whirling around and into the black hole. The left, blue point in the zig-zag indicates gas moving toward us while the right, red point indicates gas moving away from us.
Distance
This image illustrates how the dark lines of an absorption spectrum are shifted toward the right or red end of the spectrum. This shift is the result of the expanding universe. As light travels from great distances to Hubble's mirrors, it is stretched to longer and longer red wavelengths, or cosmologically redshifted, as the universe expands. Astronomers can look for known features in an object's spectrum, like these absorption lines, to see if they are shifted from their normal position on the spectrum. The difference between their normal position and their new position is called their cosmological redshift.

Hubble's Discoveries

Hubble's high resolution and instrumentation allow it to capture the data necessary to make its unprecedented discoveries. Although spectroscopy had its beginnings in the 1600s with Isaac Newton, it didn't become a part of the astronomer's toolkit until the early 1800s. It wasn't until the late 1960s that astronomers began taking spectroscopic observations from space with the launch of NASA's Orbiting Astronomical Observatory (OAO) program in 1966. When Hubble launched in 1990, it built upon the work of those earlier missions.

Before space-based spectroscopy, ground-based telescopes consistently returned blurred spectra distorted by our atmospheric turbulence. With its orbit above Earth’s atmosphere, Hubble’s instruments not only capture spectacular images, but also spectra with crisp peaks and valleys that have forever changed our understanding of the universe.