Guide to the Electromagnetic Spectrum in Astronomy

Editors: Sabina Sagynbayeva, Jason Hinkle, Ryan Golant

(NOTE: This is an updated and expanded version of an older Astrobites guide to the electromagnetic spectrum – the older guide (edited by Tanmoy Laskar) can be found here)

Astronomy is arguably one of the oldest observational sciences, with astronomical records found in many ancient societies, including Ancient Greece, Egypt, Babylon, and China. What linked all these early astronomers was the use of their unaided eyes to study the heavens. Since the invention of the first optical telescope in the early 1600s, observational astronomy has come a long way. By the early 20th century, large optical telescopes existed on the ground and soon thereafter the first telescopes for detecting radio waves were built in the 1930s. By the 1970s, rocket-borne ultraviolet, X-ray, and gamma-ray detectors allowed us to observe the highest-energy phenomena in the universe. Finally, in the 1980s, detector technology improved in the infrared, meaning that astronomers could now view light from essentially any portion of the electromagnetic spectrum – the wide array of waves that propagate as electromagnetic radiation.

Technology often drives astronomy forward – each time a new window in the electromagnetic spectrum is opened, new scientific discoveries are made. But, while modern telescopes are invariably bigger and better than their predecessors, the basic designs amongst telescopes tuned to see similar wavelengths haven’t changed much. In this guide, we examine each band of the electromagnetic spectrum – from low-energy radio waves up to γ-rays – and address the following key questions:

  • What astrophysical systems emit this radiation?
  • What physical processes does this radiation trace?
  • How is this radiation observed, and what are some real-world facilities dedicated to probing this wavelength range?

Figure 1. The electromagnetic spectrum, from low energy light on the left to high energy light on the right. At the top of the figure, Y and N label wavelengths of light that can and cannot pass through the Earth’s atmosphere, respectively. At the bottom, a temperature bar indicates the typical temperature of an object emitting most strongly at this wavelength. Image credit: wikipedia.org.

Radio

Wavelength: Longer than 1 mm
Frequency: Lower than 300 GHz

Radio waves are the lowest-energy radiation in the universe. Radio light is commonly produced by phenomena such as synchrotron radiation – due to the gyration of charged particles around magnetic field lines – and free-free radiation – due to the deceleration of charged particles in an electric field. Very often, radio waves in astrophysical scenarios trace magnetic fields and regions where particles are accelerated.

Common astrophysical sources of radio waves include the powerful jets produced by active galactic nuclei (AGN) and gamma-ray bursts (GRBs). Additionally, some transient events like supernovae and tidal disruption events (TDEs) emit radio waves. At lower luminosities (i.e., lower intrinsic brightness), radio waves are also commonly seen originating from H II regions, where ionized hot gas surrounds young, hot OB stars.

Fortunately for Earth-based astronomers, most radio waves can easily penetrate through the Earth’s atmosphere, even through clouds.

Radio telescopes operate in two main ways: some facilities, such as the Green Bank Telescope (GBT) and the Five-hundred-meter Aperture Spherical Telescope (FAST), use a single radio dish, while others, including the Very Large Array (VLA), the Square Kilometer Array (SKA), and the Low-Frequency Array (LoFAR), use many radio dishes, combining the signals using interferometry; interferometry effectively turns an array of telescopes into one big telescope with superior resolution.

Further reading on radio astronomy: NRAO, JPL, Wikipedia

Figure 2. Radio image of the central jet in the galaxy M87 taken at a wavelength of 18 cm, or a frequency of 1.6 GHz. The main image was taken with the Very Large Array, while the inset is a higher-resolution image taken with the Very Long Baseline Array and the now-retired HALCA spacecraft. Image credit: wikimedia.org.

Microwave/Sub-millimeter

Wavelength: 300 microns (μm) to 1 mm
Frequency:  1 THz to 300 GHz

The microwave and sub-millimeter (sub-mm) bands occupy the wavelength range between radio and far-infrared light. Processes that emit radio light can also produce emission at microwave/sub-mm wavelengths. Additionally, thermal emission from cold material can produce light in this range.

Perhaps the most well-known example of microwave radiation in the universe is the cosmic microwave background (or CMB), the earliest light we can observe, produced when electrons and free nuclei first combined to form neutral atoms. From our perspective, the CMB has a remarkably consistent temperature across the sky – about 2.725 K, with small fluctuations on the order of 10-5 to 10-4 Kelvin.

Sub-mm emission can come from higher-energy phenomena, such as relativistic jets (fast streams of ionized matter ejected by a compact object, like a black hole or a neutron star). However, these wavelengths of light can also come from very cold dust and gas in star-forming galaxies, particularly those at high redshift (i.e., very distant galaxies).

Some well-known microwave experiments include the Nobel-prize-winning Cosmic Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and Planck. Examples of sub-mm facilities include the Submillimeter Array (SMA) and the Atacama Large Millimeter/submillimeter Array (ALMA).

Further reading on microwave/sub-mm astronomy: ALMA, ESA, Wikipedia

Figure 3. Planck image of the cosmic microwave background. The fluctuations in temperature (illustrated by changes in color) are miniscule, roughly 1 part in 100,000. Image credit: apod.nasa.gov.

Infrared

Far-Infrared

Wavelength: 15 microns (μm) to 300 microns
Frequency:  20 THz to 1 THz

Far-infrared (FIR) emission in the universe comes predominantly from thermal blackbody emission. Wien’s law – which relates the temperature of an object to the wavelength at which the object gives off most of its light – tells us that far-infrared light comes from cool dust or gas. Even for the shortest wavelengths (highest energies) of FIR emission, the typical temperatures are roughly 200 K (-70 ℃ or -100 ℉). Star-forming galaxies and young stellar objects (i.e., protostars and pre-main-sequence stars) are some of the strongest sources of far-infrared emission in the universe. 

Some examples of far-infrared missions are the Infrared Astronomical Satellite (IRAS), the Infrared Space Observatory (ISO), and the Herschel satellite.

Mid-Infrared

Wavelength: 2.5 microns (μm) to 15 microns
Frequency:  120 THz to 20 THz

As the name implies, mid-infrared (MIR) light has a shorter wavelength than far-infrared light, but a longer wavelength than near-infrared light. MIR radiation largely traces cosmic dust, such as the dust surrounding young stars, the dust in protoplanetary disks, and zodiacal dust. The mid-infrared also traces the predominant emission of cool Solar System objects, such as planets, comets, and asteroids.

While MIR light can be seen from the ground (e.g., by the NASA Infrared Telescope Facility (IRTF) and the United Kingdom InfraRed Telescope (UKIRT)), it is difficult to detect due to strong thermal background radiation from the Earth itself. Several space-based observatories have covered the mid-IR, including the Wide-field Infrared Survey Explorer (WISE) and Spitzer. JWST’s MIRI has both a camera and a spectrograph that see mid-infrared light.

Near-Infrared

Wavelength: 0.8 microns (μm) to 2.5 microns
Frequency:  380 THz to 120 THz

Near-infrared (NIR) light is emitted by a wide range of sources, predominantly as blackbody radiation. The emission of cool stars (like M dwarfs) peaks in the NIR; because low-mass stars are the most common stars in the universe (see the stellar initial mass function), many galaxies have their strongest emission in the near-infrared as well.

Near-infrared light can be seen from the ground, in between strong bands of water vapor absorption. Examples of ground-based NIR telescopes include the 2MASS survey, the Infrared Telescope Facility (IRTF), the United Kingdom Infrared Telescope (UKIRT), and the Visible and Infrared Survey Telescope for Astronomy (VISTA). Near-infrared astronomy is also commonly done from space – in particular, the recently-launched JWST will revolutionize near-infrared astronomy with its NIRCam and NIRSpec instruments.

Further reading on infrared astronomy: ESA, JWST, SOFIA, Wikipedia

Figure 4. JWST image of the Southern Ring nebula in the near-infrared (left panel) and the mid-infrared (right panel). Image credit: nasa.gov.

Optical

Wavelength: 350 nm to 800 nm
Frequency:  860 THz to 380 THz

Optical (or visible) light is the radiation that is visible to human eyes. Optical light is commonly produced from blackbody processes, but can also arise from non-thermal sources. Thermal optical emission is often seen from stars and the galaxies that house stars. Ionized gasses can also produce optical emission, but often in the form of discrete spectral lines rather than a continuum of light. More extreme examples of optical emission are the blue continuum and broad emission lines seen in active galactic nuclei.

As our eyes can attest, optical light can be seen from the ground. Several large ground-based telescopes observe primarily in the optical, including the twin W.M. Keck telescopes, the four Very Large Telescopes, and the Southern African Large Telescope (SALT). Examples of optical telescopes in space include the Hubble Space Telescope, Gaia, Kepler, and the Transiting Exoplanet Survey Satellite (TESS).

In recent years, a series of optical telescopes scanning the sky at very short cadences have been built to discover new transient events. Examples of these projects include the All-Sky Automated Survey for Supernovae (ASAS-SN), the Asteroid Terrestrial-impact Last Alert System (ATLAS), the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), and the Zwicky Transient Facility (ZTF). In the near future, the Legacy Survey of Space and Time (LSST), conducted by the Vera Rubin Observatory, will allow us to see even fainter optical sources.

Further reading on optical astronomy: ESA, Wikipedia

Figure 5. Hubble space telescope optical image of M104 (The Sombrero Galaxy). The diffuse glow is the combined light of billions of stars residing within the galaxy. The dark dust lanes surrounding the edge of the galaxy block some of the optical light from reaching us. Image credit: nasa.gov.

Ultraviolet

Wavelength: 10 nm to 350 nm
Frequency:  3e16 Hz to 860 THz
Energy: 120 eV to 3.5 eV

The longest ultraviolet (UV) wavelengths are just short enough to be invisible to the naked eye, while the shortest wavelengths are comparable to the sizes of small molecules. UV emission comes from many processes, including blackbody emission from hot sources and powerful non-thermal sources.

Thermal UV emission commonly comes from hot O stars and B stars on the main sequence, as well as from white dwarfs, the tiny cores left over by dying low-mass stars. Non-thermal UV emission can be seen, for example, in the continuum emission of AGN. Because of its short wavelengths, UV emission is easily blocked (or extinguished) by dust along our line of sight, obscuring many UV sources from view.

Except for the very longest wavelengths, UV radiation cannot be observed from the ground. Space telescopes that observe in the UV include AstroSat, the Galaxy Evolution Explorer (GALEX), the Hubble Space Telescope, and the Neil Gehrels Swift Observatory.

Further reading on ultraviolet astronomy: NASA, Wikipedia

Figure 6. GALEX image of M101 (The Pinwheel Galaxy). The UV light traces newly-formed massive stars, which are primarily born in the spiral arms. Little UV light can be seen in regions of the galaxy that lack massive stars. Image credit: galex.caltech.edu.

X-ray

Wavelength: 10 pm to 10 nm
Frequency:  3e19 Hz to 3e16 Hz
Energy: 120 keV to 0.12 keV

X-ray emission can be produced by thermal emission from very hot sources like neutron stars and by free-free emission from the hot gas in galaxy clusters. X-rays often arise from accretion – or the accumulation of matter – onto compact objects like the black holes found in either X-ray binaries or AGN.

Since their short wavelengths are blocked out by the Earth’s atmosphere, X-rays must be observed from space. Historical examples of X-ray missions include the Uhuru, Einstein, and ROSAT telescopes. More recent telescopes include Chandra, XMM-Newton, NuSTAR, and eROSITA.

Further reading on X-ray astronomy: Chandra, NASA, Wikipedia

Figure 7. The Chandra X-ray deep field, imaged between 0.5 and 8 keV. The colors indicate different energy ranges: red for 0.5-2 keV, green for 2-4 keV, and blue for 4-8 keV. Almost all of the sources in this image are AGN, actively-accreting supermassive black holes. Image credit: chandra.harvard.edu.

Gamma-ray

Wavelength: Shorter than 10 pm
Frequency: Higher than 3e19 Hz
Energy: Greater than 120 keV

Gamma-ray (γ-ray) photons have wavelengths comparable to or smaller than the size of an individual atom. This means that many of the processes that produce gamma-rays are associated with nuclear physics, such as gamma decay. The pair-annihilation of high-energy electrons and positrons can also produce gamma-rays. Additionally, some gamma-rays are the result of the acceleration/energization of lower-energy photons by phenomena such as shock waves and inverse-Compton scattering.

Gamma-rays can be seen from certain classes of AGN with relativistic jets, as well as from compact object binaries, like the aptly-named gamma-ray binaries. Another abundant source of gamma-rays are gamma-ray bursts, which are among the most luminous explosions in the universe.

Gamma-rays must be observed from space. Examples of gamma-ray telescopes include the Compton Gamma-ray Observatory, the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), and Fermi.

Further reading on gamma-ray astronomy: NASA, Wikipedia

Figure 8. Image of the gamma-ray sky as measured by the Fermi telescope. Image credit: svs.gsfc.nasa.gov.

Additional resources

This guide is not intended to be a catch-all resource for the electromagnetic spectrum. Many other excellent resources exist on this topic, including the following links:

Featured image credit: The Foundation of Astronomical Studies and Exploration

About Astrobites

This post was written collectively by multiple members of the Astrobites team. Meet the authors of Astrobites.

3 Comments

  1. very clear and good article easy to understand. Thank you

    Reply
    • Interesting. Will recommend to others

      Reply
  2. Great spread of information, relative graphics and linked sources. This is the next unit for my High School Astronomy course, will share with my students for sure!

    Reply

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