How does astronomy use the electromagnetic spectrum? - PART I

How does astronomy use the electromagnetic spectrum? - PART I

Parts of the electromagnetic spectrum invisible to human eyes reveal a vast amount of information about the universe, but it took a long time for astronomers to learn how to view it.
For thousands of years, humans were looking up at the star-studded night sky using just their eyes sensitive to the optical wavelength of the electromagnetic spectrum. The first telescopes, invented in the early 17th century, enhanced the ability of human eyes by magnifying distant objects.
But as physicists started discovering in the 19th century that there are other, invisible, types of light in the natural world around us, astronomers realized that there must be such light emanating also from the universe.
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Today, astronomers know that the majority of radiation, or light, present in the universe is invisible to human eyes. By looking at the universe in all possible wavelengths, scientists are piecing together a complex picture of the unfathomably vast cosmic environment that we are a part of. It took, however, decades, for instruments to be developed that could detect this invisible radiation from celestial sources.
Here we explain what different parts of the electromagnetic spectrum teach us about the universe.

(Image credit: NASA)
WHAT DO RADIO WAVES TEACH US ABOUT THE UNIVERSE?
Radio astronomy studies cosmic radiation with the longest wavelengths (from less than 0.4 inches to several miles, or 1 centimeter to several kilometers) and was the first kind of astronomy developed that relies on wavelengths other than optical light.
The discovery that radio waves from bodies in the universe lash our planet was made completely by accident. In 1933, a young American radio engineer Karl Jansky, an employee of the famous telephone company Bell Laboratories, was tasked to search for sources of unexplained hiss that sometimes interfered with transmissions of radio messages across the Atlantic Ocean. Jansky found that while some of this noise was coming from sources on Earth, such as nearby thunderstorms, there was a type of signal, constantly picked up by his experimental antennas, that appeared to be coming from what we know today is the center of our Milky Way galaxy, the region where the black hole Sagittarius A* resides. Systematic exploration of the radio universe began soon thereafter.
Astronomers have discovered since that radio waves are emitted by spinning electrons and emanate from all sorts of environments that have the ability to make those electrons spin, Affelia Wibisono, an astronomer at the Royal Observatory Greenwich in the U.K., told Space.com.
"Typically, when you detect radio waves, you're looking at electrons moving through a magnetic field," Wibisono said. "But ionized gas can emit radio waves as well."
By tracing the structure of radio wave-emitting clouds, astronomers were able to map out the entire structure of our galaxy, the Milky Way, as well as other nearby galaxies. They could determine areas with high concentrations of hot young stars, but also study objects obscured by dust, such as black holes that hide in galactic centers. Highly magnetized bodies, such as fast-spinning stellar remnants called pulsars are prime targets for radio astronomy as they send out powerful flashes of radio waves as they spin like superfast cosmic lighthouses.

Famous radio telescopes
As radio waves are the type of electromagnetic radiation with the longest wavelengths, radio telescopes have to be rather large. Vast arrays of radio-antennas, such as the Karl G. Jansky Very Large Array in New Mexico that consists of 28 dishes each 82-foot-wide (25 meters), are the technological standard today. By combining multiple antennas, astronomers create telescopes that have immense apertures that equal the distance between the array's most distant parts, thus enabling the scientist to detect the faintest signals with the best possible resolution.
The Square Kilometer Array (SKA), currently constructed across two locations in Australia and South Africa, will be the world's largest radio telescope by a significant margin once it comes online around 2028. With its thousands of dishes and dipole antennas spanning thousands of square miles of remote land, SKA will survey large areas of the sky at once and detect the faintest signals coming from the farthest reaches of the universe.
The Event Horizon Telescope, famous for taking photographs of black holes, is also a radio telescope, or rather a worldwide network of radio telescopes with an aperture equalling the size of our planet.
Unlike some other types of wavelengths, radio waves mostly penetrate Earth's atmosphere with ease, allowing astronomers to base their equipment on the planet's surface.
However, due to the ubiquity of radio communication technologies in the modern world, radio telescopes are at risk of getting confused by human-made signals. SKA, for example, will therefore be surrounded by a radio-quiet zone where no cell phones and no radio equipment will be allowed.
Constantly searching for better ways to study the universe, astronomers are now seriously considering building a radio telescope on the far side of the moon. Removed from Earth-based sources of human-made radio noise, as well as from Earth's ionosphere (the upper part of the atmosphere which contains ionized gas that absorbs and distorts some cosmic radio signals), such an observatory would provide scientists with the deepest and most undisturbed views into the earliest epoch of the universe.
WHAT DO MICROWAVES TEACH US ABOUT THE UNIVERSE?
The next electromagnetic spectrum band after radio waves are microwaves. As microwaves cover wavelengths between 3.3 feet and 0.04 inches (1 meter and 1 millimeter), the first discoveries of cosmic microwaves were actually made by radio telescopes.
Microwaves have a special, although rather limited place in astronomy. According to the European Space Agency (ESA), the whole sky glows uniformly in microwaves(opens in new tab) with what has been identified as the cosmic microwave background.
This uniformness is unseen in other wavelengths, which reveal the sky in dots and regions of varying brightness. In fact, cosmic microwave radiation is so odd that the researchers who first discovered it in the 1960s (completely by accident during experiments with echo balloons) originally thought it was produced by a telescope defect.
Subsequent research, however, confirmed that the microwave hum was coming from space and that it was nothing less than the residue of radiation released by the Big Bang, the enormous explosion which created the universe some 13.8 billion years ago.
This radiation was originally released in the form of highly energetic, short-wavelength X-rays, but since it took so long to reach us, the so-called redshift effect caused by the expansion of the universe has stretched this wavelength all the way into microwaves.
Microwaves reveal the universe as it looked in its earliest stages. The most sensitive surveys were able to go as far as distinguishing the denser regions of gas and dust that subsequently produced the first galaxies.
Famous microwave telescopes
Microwaves get mostly absorbed by Earth's atmosphere, which means they are best studied by space-based telescopes.
In 1989, after the initial crude Earth-based detections of cosmic microwaves, NASA sent the first dedicated microwave-observing satellite — the Cosmic Background Explorer (COBE) — into space. COBE measured differences in the temperature of the microwave background in various regions. COBE's successor, the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2003, further improved the level of detail of this cosmic microwave map. These observations helped to determine the universe's age with greater precision, according to the European Space Agency (ESA)(opens in new tab), and define the amounts of different types of matter that the universe contained in its earliest years.
ESA's Planck mission, launched in 2009, then completed the task of creating the most accurate map of the cosmic microwave background, which, ESA said, is to some extent "definitive," as some of the measurements cannot be further improved.
WHAT DO SUBMILLIMETER WAVES TEACH US ABOUT THE UNIVERSE?
The submillimeter wavelength sits between the millimeter and infrared ranges. As the name suggests, submillimeter waves have lengths shorter than 1 mm, or 0.04 inches, and up to a few hundred micrometers. Observations in this range partially overlap with the longest wavelengths of the infrared spectrum.
The use of submillimeter wavelengths in astronomy is relatively recent, according to Astronomy Cast(opens in new tab). Detectors used to detect submillimeter radiation are quite similar to those used in radio astronomy, but thousands of times smaller. Technology therefore had to progress enough to make these detectors possible.
Speaking to the Astronomy Cast, an astronomy podcast, American astronomer Pamela Gay said that the use of submillimeter waves in astronomy is limited to certain types of objects and phenomena.
Submillimeter waves penetrate through clouds of molecular gas and dust into star-forming regions, which are obscured from the view of of optical telescopes.
In submillimeter waves, astronomers can observe universe's "natural lasers," regions where highly charged electrons emit laser light as they discharge some of their energy, said Gay. These natural lasers, sometimes called masers, are usually observed in a special type of pulsating variable stars called the Mira stars.
Submillimeter waves are also good at pointing astronomers to some interesting types of organic molecules and do a good job analyzing cold objects such as comets in the solar system, said Gay.
Famous submilimeter telescopes
Because submillimeter waves get absorbed by water in Earth's atmosphere, observatories that study sources of submillimeter radiation in the universe need to be built in high and dry places to prevent water vapor from obscuring their views. In essence, you will find submillimeter telescopes in the same places on Earth where you find the best optical telescopes.
The Atacama Large Millimeter/submillimeter Array (ALMA), operated by the European Southern Observatory is located on the Chajnantor plateau in northern Chile at an altitude of 16,400 feet (5,000 m).
The Submillimeter Array on Hawai'i's Maunakea, which is operated by the Harvard Smithsonian Center for Astrophysics, sits somewhat lower, at 13,450 feet (4,100 m) above sea level.
WHAT DOES INFRARED LIGHT TEACH US ABOUT THE UNIVERSE?
Unlike submillimeter waves, infrared light spans a vast range of the electromagnetic spectrum from 0.04 inches (just below 1 millimeter) on the side bordering with microwaves to 0.75 micrometers on the side bordering with the visible light.
The NASA-led James Webb Space Telescope (JWST), launched on Christmas Day, 2021, thrust infrared astronomy into the spotlight with its ability to see the farthest reaches of the universe.
Infrared light, which is essentially heat, was the first non-visible wavelength discovered, completely by accident, by British astronomer William Herschel in 1800 during his experiments with the visible light spectrum. It took, however, a long time for infrared detectors to become sensitive enough to provide the breathtaking views of the cosmos that JWST is now known for.
The first crude observations of celestial objects in the infrared spectrum focused on the moon and the sun. Astronomers in the second half of the 19th century were able to measure the temperature of the sun's atmosphere as well as the various temperature zones on the moon's surface. By the turn of the century, technology progressed to the level that it was possible to detect heat from the solar system's giant planets Jupiter and Saturn, according to A brief history of infrared astronomy(opens in new tab).
Infrared astronomy, however, didn't fully take off until the second half of the 20th century when more sophisticated detectors were developed, allowing astronomers to analyze heat sources across the Milky Way.
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As the JWST has plentifully demonstrated since the release of its first images in July, 2022, infrared light is good at many things.
Thanks to its ability to penetrate through dust and gas, infrared light reveals what's going on inside of thick dust and gas clouds where stars form. Stars emerging in the middle of these clouds are not yet hot enough to emit visible light, but are warm enough to be detected by infrared sensors.
With such advanced technology as the JWST, astronomers can observe matter that is only several degrees warmer than absolute zero, the temperature of minus 459.67 degrees Fahrenheit (minus 273.15 degrees Fahrenheit), where the motion of atoms stops.
When viewing the Milky Way in infrared light, a hidden galaxy of failed stars, called brown dwarfs, emerges. Brown dwarfs are bodies that are too big to be called planets but are not quite massive enough to ignite nuclear fusion in their cores. Bodies in the farther reaches of the solar system that receive too little solar illumination also spring into view. Even the interstellar medium, the cool gas and dust dispersed between stars and galaxies, can be mapped in the infrared spectrum.
Webb was built with the aim to detect the first light that lit up the universe a few hundred million years after the Big Bang. Although this light had been emitted in the optical wavelength range, the accelerating expansion of the universe had stretched this light into the infrared range thanks to the effect known as redshift. Optical telescopes, even if they were as sensitive as Webb, could therefore no longer see this light.
But the James Webb Space Telescope sees only a small fraction of the infrared spectrum, the so-called mid and near-infrared light, which spans wavelengths from 28.5 micrometers to 0.6 micrometers where the visible spectrum begins.
NASA's recently retired flying telescope SOFIA was a specialist in the longer wavelength type of infrared light, the so-called far infrared, which reaches all the way to 612 micrometers and is best for observing the cool interstellar medium.
Famous infrared telescopes
Both, the James Webb Space Telescope and SOFIA, the current and recently retired (respectively) infrared astronomy flagships, had their predecessor.
NASA's Spitzer Space Telescope surveyed the universe in the mid-infrared and parts of the far infrared spectrum from 2003 to 2020. ESA's Herschel spacecraft complemented this work in the far-infrared spectrum between 2009 and 2013.
An earlier airborne telescope, the Kuiper Airborne Observatory, studied the infrared sky from 1974 to 1995.
WHAT DOES OPTICAL LIGHT TEACH US ABOUT THE UNIVERSE?
Optical astronomy has made enormous leaps since those first early 17th-century telescopes. Enhancing the natural abilities of the human eye beyond imagination, 21st-century optical telescopes are still the backbone of astronomy research.
From giant telescopes occupying remote mountain tops and highland plateaus to orbiting super-eyes such as the iconic Hubble Space Telescope, optical observatories reveal the universe with an ever-increasing level of detail. Some, on the other hand, focus on scanning vast swaths of the sky at once to spot unexpected phenomena, such as supernova explosions of dying stars or approaching asteroids.
Optical telescopes show the universe as it would appear to human eyes. Colors in optical images correspond to the colors human eyes would see. Images from other types of telescopes, such as those imaging the universe in infrared and ultraviolet light, have to be processed by astronomers on the ground, with colors artificially assigned to different wavelengths.
To be visible in the optical wavelengths, objects need to either emit their own visible light or be illuminated by other objects. Planets, moons and asteroids in our solar system are only visible to optical telescopes (and to human eyes) because of the vicinity of our sun.
Optical light can't pass through obstacles, such as thick clouds of dust, which hide some of the most interesting areas of the universe (such as centers of galaxies where supermassive black holes devour huge amounts of material or star-forming nebulas).
Optical light is also somewhat affected by Earth's atmosphere, even though not as much as the infrared and submillimeter wavelengths. While infrared and submillimeter radiation gets mostly absorbed, optical rays get a little dispersed by the molecules in the atmosphere, which means that observed objects don't appear as sharp as they would if the atmosphere wasn't present. This atmospheric blurring limits the accuracy of observations that Earth-based optical telescopes can achieve, even though modern adaptive optics systems installed on the world's best telescopes can to a certain extent make up for this shortcoming.
Aside from complex, costly machines in space and on remote mountain tops, optical astronomy is the most accessible method of observing the sky for amateur skywatchers. Decent backyard telescopes can be purchased for a few hundred dollars and Space.com provides plenty of guides on how to pick the best one for you.
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Famous optical telescopes
The Hubble Space Telescope is the undisputed king of optical astronomy and the source of many images that have gained iconic status. The telescope, launched in 1990, is still going strong and still may have a decade or so of life and fabulous astronomy ahead of it.
The Very Large Telescope (VLT) operated by the European Southern Observatory (ESO) in Chile is one of the most advanced Earth-based optical telescopes. VLT consists of four main telescopes, each with a 27-foot-wide (8.2 meter) mirror, and four 5.9-foot-wide (1.8 m) auxiliary telescopes. The four main telescopes can each detect light that is four billion times fainter than what human eyes can see. The telescopes can also work together as a so-called interferometer(opens in new tab), which increases the resolution to a level that would be achievable with a single telescope with a 426-foot-wide (130 m) mirror.
ESO is currently building the next-generation Extremely Large Telescope (ELT), also in Chile. With a single 130-foot-wide (39.3 m) mirror(opens in new tab), ELT will be the world's largest optical telescope. Once completed, the observatory will be able to gather 100 million times more light than the human eye and provide images 16 times sharper than the Hubble Space Telescope, according to ESO.
The twin Keck Telescopes on the Hawaiian island of Maunakea are fitted with 32.8-foot-wide (10 m wide) mirrors that forced the technical teams that designed and built them in the late 1980s to develop some ingenious technical solutions. Since it wasn't possible at that time to accurately operate a single solid mirror of such a size, engineers made the Keck mirrors from 36 hexagonal segments that work together as a unit with the help of an active optics system. This segmented mirror design is quite similar to the one used for the 21-foot-wide (6.5 m) mirror of the James Webb Space Telescope.
The Large Binocular Telescope in Arizona features the world's largest non-segmented mirror, measuring 28 feet (8.4 m) in diameter.
The Gran Telescopio Canarias on the Spanish island of La Palma off the coast of western Africa, is the world's largest single-aperture optical telescope, featuring a 10.4 m wide mirror.













































