Radio signals keep coming from deep space. Here's what they really are.

When the New York Times first wrote about "mysterious radio waves" from the cosmos in 1933, they made sure to note one fundamental caveat: "No Evidence of Interstellar Signaling."

Indeed, the radio waves weren't signals from aliens. After all, it's never aliens.

Yet today, some 90 years later, you've undoubtedly seen headlines promoting more "strange radio signals coming from deep space" or "radio signals detected from another solar system." They're among the most clicked on internet space stories. In reality, our planet is constantly bombarded with radio waves, which are a type of energy or light naturally produced all over the cosmos (like visible light or X-rays). Radio waves are extremely valuable to detect using giant dished-shaped antennas because they reveal fascinating, extremely far-off events that we can't otherwise see, like stars exploding or black holes munching on cosmic dust. They are normal. And they aren't attempts at intergalactic communication.

"Since the dawn of radio astronomy, this is what astronomers do. They tell people, 'It's not aliens,'" Yvette Cendes, an astronomer and postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, told Mashable.

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These detected radio waves often come from almost incomprehensibly distant galaxies, many light-years away. To us, they're like whispers drifting through the wind. That's why astronomers must use behemoth antennas to find them.

"Since the dawn of radio astronomy, this is what astronomers do. They tell people, 'It's not aliens.'"

"If you took your cell phone and put it on the moon, it would be one of the brightest radio sources in the sky," Cendes explained. "These are very faint signals. The amount of energy collected in radio astronomy's history is less than the energy needed to melt a snowflake."

Even so, they reveal momentous events in our galaxy, and beyond.

Radio telescopes from the Very Large Array (VLA) in New Mexico. Credit: Jeff Hellerman, NRAO / AUI / NSF

What radio waves tell us

It might seem like the great, vast, black ether of space is awfully quiet. Viewed through the lens of visible light (with much shorter wavelengths than radio waves), that's true. But seen through radio waves, it's a wild universe.

"The sky looks serene and calm, but if you look in the radio bands, extremely energetic phenomena are taking place in the universe," Poonam Chandra, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Virginia, told Mashable. Chandra researches the explosion of massive stars in the cosmos.

But radio astronomers don't listen to radio waves, a misperception shown in the still-exceptional sci-fi film Contact. Astronomers like Cendes, who uses radio signals to research black holes that tear objects apart, rely on radio dishes like those shown above to collect these signals as they pass by, ultimately allowing them to view far-off objects or events. There are many different lengths (or "wavelengths") of radio waves — and different events in space emit different radio waves. Essentially, astronomers can "tune in" to different channels that broadcast different cosmic "shows." (To allow for more detailed views of cosmic happenings, often many large radio telescopes are pointed at the same object to essentially create a giant telescope, like at the Very Large Array in New Mexico.)

As this graphic shows on the left, radio waves travel in the longest waves compared to other types of radiation (like visible light and beyond). Credit: NASA

"The sky looks serene and calm, but if you look in the radio bands, extremely energetic phenomena are taking place in the universe."

When we look up at the sky at night normally, we see the tranquil cosmos teeming with twinkling stars and perhaps a nearby galaxy like Andromeda. But through radio telescopes, as shown in the first image below, we spy a view known as the "radio sky." All the bright spots in the image below aren't stars: they're galaxies and quasars (extremely bright objects at the center of galaxies). And that's not all: You can also see clouds of gas from exploded stars.

The sky as seen through a radio telescope. The land and telescopes below are from visible light photography, but the sky above is radio wave detections composed of bright events in galaxies and the aftermath of exploded stars (the puffy cloud-like features). Credit: NRAO / AUI / NSF

Gas emanating from around the black hole at the heart of the galaxy Hercules A as seen through a radio telescope. Credit: B. Saxton / W. Cotton / R. Perley (NRAO / AUI / NSF)

Here are some of the common objects or cosmic events that produce radio waves:

1. Active galactic nuclei, or AGN: Found at the center of galaxies, these are supermassive black holes emitting intense energy into the cosmos. Black holes, which are places where matter has been crunched down into an intensely compact area and have a gravity so intense light can't escape, can release extremely luminous jets of heat and gas (which are manufactured by the material rotating rapidly around a black hole). Not all galaxies contain such vivid objects, but because these black holes are so bright when detected with radio telescopes, astronomers often see them, Cendes explained.

2. Gigantic space explosions: Giant stars frequently explode in space. Huge stars, much larger than the sun, eventually run out of fuel, collapse, and explode in events called supernovae and gamma-ray bursts. The blasts release bounties of energy. Some of this energy travels through space in gamma-rays, which are much more energetic than radio waves, but pass by us quickly, in a matter of minutes or so. But radio waves, which are emitted from the mass blown off by the star, allow astronomers to watch explosions for years. The radio waves give astronomers bounties of information about how a star exploded, and even how it evolved, Chandra explained.

3. Star formation: Astronomers can tune into radio waves to understand how stars form in other galaxies (stars emit certain radio waves during their formation). In 2022, astronomers even detected radio waves from the most distant galaxy yet, born early in the universe. The detection allowed scientists to look back a whopping 8.8 billion years in time and better grasp what this galaxy was broadly like.

4. Fast radio bursts (FRBs): Fast radio bursts are precisely that: pulses of radio waves that last milliseconds. They're often labeled as "mysterious," in part because astronomers are still researching where they come from and how they're made (FRBs happen in the blink of an eye, so they're inherently hard to study.) So far, the evidence suggests these quick radio signals come from "magnetars," which are a type of neutron star (the collapsed core of a star).

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Indeed, objects all over the cosmos are releasing radio waves into space. "There is a lot of radio out there," Cendes emphasized. And unlike stars and most objects in our solar system (like planets and asteroids), radio waves can be viewed during both day and night, and aren't obscured by pesky weather.

Radio waves often aren't too mysterious. Conversely, they're revealing; they reveal what we can't otherwise see, deep in the evolving universe. These radio signals help us understand how other galaxies and stars came to be, shining a light on our own cosmic story, out here on a spiral arm of the Milky Way galaxy.

"They're not aliens, but they're equally exciting," Chandra said.

This story was originally published in February 2023 and has been updated.

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