The Importance of Spectroscopy in Astronomy

Studying electromagnetic radiation as a function of the wavelength or frequency of the radiation and its interaction with matter is called spectroscopy. Initially, the study originated between the wavelength dependence of the absorption by a gas-phase matter of visible light distributed by a prism.

Spectroscopy also refers to the splitting light technique, wherein light is split into its constituent wavelengths like a prism splits light into a rainbow of colors. However, a spectrum is generally more than a simple ‘rainbow.’ The electron’s energy levels in atoms and molecules are quantized, and electromagnetic radiation’s absorption and emission only occur at specific wavelengths. Consequently, spectra are not smooth but marked by absorption or emission ‘lines.

Radiative energy can be in the form of matter waves and acoustic waves. However, recently, gravitational waves have been associated with a spectral signature in the context of the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Traditional spectroscopy is done using a prism and photographic plates. Still, modern spectroscopy uses diffusion gratings to disperse the light, then projected onto CCDs (Charge Coupled Devices) like those used in digital cameras.

Spectroscopy, primarily in the study of the electromagnetic spectrum, is a central exploratory tool in the fields of physics, chemistry, and astronomy because it allows the investigation and observation of composition, physical structure, and electronic structure at the atomic, molecular, and macro scale, and over astronomical distances.

Astronomical Spectroscopy

Astronomical Spectroscopy

Spectroscopy is also essential in astronomy in measuring the spectrum of electromagnetic radiation, including visible radio and light, which radiates from stars and other celestial objects in the galaxy.

With a stellar spectrum, several properties of stars, such as their chemical composition, temperature, density, mass, distance, luminosity, and relative motion, can be revealed using Doppler shift measurements. Physical properties of other celestial bodies such as active galactic nuclei, nebulae, galaxies, and planets can also be studied.


The spectra of galaxies look like stellar spectra, as they consist of the combined light of billions of stars. Fritz Zwicky’s doppler shift studies of galaxy clusters in 1937 found that the galaxies in a cluster have much faster movement than seemed possible from the cluster’s mass inferred from the visible light.

Zwicky hypothesized that there must be a non-luminous matter in the galaxy clusters, known as dark matter. With this discovery, astronomers have determined dark matter comprises a large portion of galaxies (and most of the universe). 

However, in 2003, four galaxies (NGC 821, NGC 3379, NGC 4494, and NGC 4697) were discovered without a dark matter that could influence the motion of the stars within them. However, the reason behind the lack of dark matter is unknown.

Strong radio sources were found in the 1950s, and they were associated with very dim and very red objects. One of the object’s first spectrum was taken, and there were absorption lines at wavelengths where none were expected. It was found out that it was a normal galactic, highly redshifted spectrum.

In 1964, the spectrum was named by Hong-Yee Chiu as quasi-stellar radio sources or quasars. Today, quasars are thought to be galaxies with extreme energy output powered by supermassive black holes formed in the universe in the early years.

The properties of a galaxy and its age can also be determined when you study and analyze the stars found within it. NGC 4550, a barred lenticular galaxy located in Virgo’s constellation, has a significant portion of its stars rotating in the opposite direction as the other portion. 

That is why the galaxy is thought to be a merger of two smaller galaxies rotating in each other’s opposite directions. The star’s brightness can also help determine its distance to a galaxy, which may be a more accurate method than parallax or standard candles.

Interstellar Medium

The interstellar medium is the matter that fills the space between galaxies’ star systems. Helium, hydrogen, and minor amounts of other ionized elements like oxygen make up 99 percent of this stuff. The remaining 1% comprises dust particles, which are primarily silicates, graphite, and ices. The term “nebula” refers to dust and gas clouds.

Absorption, emission, and reflection nebulae are the three primary forms of nebulae. Absorption (or dark) nebulae are made up of so much dust and gas that they block out the starlight behind them, making photometry tricky. As their name implies, reflection nebulae reflect the light of surrounding stars.

Their spectra are similar to those of the stars in their immediate vicinity, except the light is bluer. Shorter wavelengths scatter more efficiently than longer wavelengths. Depending on their chemical makeup, emission nebulae release light at specific wavelengths.

Interstellar Medium

Universal Motion

Galaxies are created by the gravitational attraction of stars and interstellar gas, and galaxy clusters are formed by the gravitational attraction of groupings of galaxies. Almost all galaxies are shifting away from ours due to the universe’s expansion, apart from stars in the Milky Way and galaxies in the Local Group.

Asteroids, Planets, and Comets

Asteroids, planets, and comets all reflect their parent stars’ light while also emitting their own. Most of the emission from colder objects, such as solar-system asteroids and planets, occurs at infrared wavelengths that we can’t see but can be studied using spectrometers.

Further absorption and emission at specific wavelengths in the gas occur for objects enveloped by gas, like planets and comets with atmospheres, imprinting the gas spectra on the spectrum of the solid object. The spectrum is primarily or entirely attributable to the atmosphere alone for worlds with complete cloud cover or thick atmospheres (like Venus, the gas giants, and Saturn’s satellite Titan).


Spectra and What They Can Tell Us

A spectrum shows the intensity of light emitted over a range of energies. If you have seen a rainbow, then you have seen a spectrum. When sunlight is sent through raindrops, it spreads out to display its different colors.

Spectroscopy is an essential technique in helping scientists understand celestial objects such as neutron stars, black holes, or active galaxies. It can help determine how things produce light, the movement speed, and the elements that compose it. Spectra can come from any energy of light-from low-energy radio waves to very-high-energy gamma rays.

Each spectrum is a threshold of a wide variety of information. For instance, there are many different mechanisms by which an object can produce light, and each mechanism has a characteristic spectrum. If you find this post interesting, we recommend you also read our post about celestial bodies that are visible with high-power binoculars.