How dark is dark?
When you go outside on a clear, moonless night, you can see not only lots of stars but also the black space between them—literally space, in this case. Given that you can see stars and that the sky is black, you might think the sky is transparent. But it’s not—at least, not really.
Various molecules, atoms and particulates float around in the air, and these reflect light. During the day, sunlight entering the atmosphere is scattered by molecules such as nitrogen and oxygen at more or less random angles, like pinballs caroming off a bumper, so anywhere you look in the sky, you’ll see sunlight directed toward you. Complicated physics makes these molecules scatter more blue light than red, giving the sky its azure hue.
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During the day, the sky is so bright from this light that no stars (well, except the sun) can be seen. They’re too faint. At night, after the sun sets and its scattered light diminishes, the sky gets darker. But does it get completely dark—that is, with absolutely no light coming from between the stars?
Nope. Even at night, there’s some light in the sky. There’s the moon, of course, but also, depending on where you live, illumination can come from city lights, street lamps, car headlights and other artificial sources that create what we call light pollution. The farther removed you are from those, the darker the night sky will be.
But even then there’s a limit because the sky itself glows! During the day oxygen and nitrogen molecules in the upper atmosphere get energized by ultraviolet light from the sun, and they release that energy as a soft nocturnal glow. Sometimes atoms of these elements collide with each other and combine into molecules, again emitting a faint light. Both processes contribute to what’s known as airglow, which, for example, shows up in photographs from orbit as arcs of color above the horizon. Long photographic exposures taken from the ground can reveal it as well.
Astronomers are always trying to see faint objects in the sky, from small objects in the solar system to galaxies so remote that pondering their distance makes your brain hurt. That’s why observatories tend to be far from civilization, where the sky is darkest. That does make it somewhat inconvenient to build and operate them, but it pays off in science.
Of course, astronomers are scientists, so they like to quantify their observations with actual measurements they can make. When they look at sky brightness—what they call the sky background—they use a unit of brightness per area of the sky: how much light is spread out over a given parcel of celestial real estate. In technical terms, it’s measured in magnitudes per square arcsecond, which is usually written as mag/arcsec2. (Magnitude is a weird unit that astronomers prefer for brightness—the bigger the number, the fainter the object. An arcsecond is a very small angle on the sky; the full moon is more than two million square arcseconds in area.)
A decently dark site in a rural area has a sky background of about 21 mag/arcsec2. In the Rocky Mountains in Colorado I’ve been to sites a bit darker than this, and it’s truly spectacular. Thousands of stars are visible, and the Milky Way is like a river of light across the sky. At the very darkest sites on Earth, far removed from humanity’s wasted sky-directed light, that background can be as dim as 22 mag/arcsec2. I’d love to have a moonless night with my binoculars and telescope at such a place! It must feel like being in space.
That brings up a fascinating, often overlooked point: even in space, the sky isn’t totally dark. It’s much darker above all that airglow, of course, allowing much fainter objects to be seen—which is a big reason we put telescopes in orbit around Earth or the sun. The Hubble Space Telescope is nowhere near the largest observatory ever built, but the dark background it enjoys bestows the ability to see incredibly faint objects.
Because Hubble orbits Earth, it’s still deep inside our solar system, though. And despite the name, the space around the sun isn’t really empty: there’s a lot of junk out there, mostly in the form of dust released by comets fragmenting as they approach our star. The sunlight scattering off this interplanetary dust is called zodiacal light and is roughly 24.5 mag/arcsec2. That’s really faint, but in certain circumstances the zodiacal light can still be visible to the unaided eye from very dark sites. (One of my bucket list items is to see it myself someday, in fact.)
This means that if you want to truly know how bright the sky is, you need to get out of the inner solar system. And it’s best to get as far out as you possibly can.
As it turns out, astronomers have done that. The New Horizons spacecraft flew past Pluto in 2015, returning stunning images and other data about the Kuiper Belt Object Formerly Known as a Planet. New Horizons traveled so far out, in fact, that it left the background glow of the sun, Earth and cometary dust all behind, allowing us to see truly dark, nearly interstellar skies for the first time. Using its Long-Range Reconnaissance Imager, or LORRI, the probe deeply imaged about a dozen different patches of the sky, each located up and away from the Milky Way’s dust-strewn disk to minimize any unwanted galactic glow.
Once the images were sent to Earth, astronomers subtracted all the lingering vestiges of light emitted from stars and reflected from dust in and around the Milky Way and published their results in the Astrophysical Journal. They found that the extremely diffuse background glow was phenomenally faint: lead author Marc Postman of the Space Telescope Science Institute told me it was just 27.42 mag/arcsec2. That’s about 1 percent as bright as the glow seen from the darkest site on Earth!
Still, it’s enough to measure, and that means it has a real, physical source. Eliminating all other possibilities, the astronomers concluded that the light is coming from extremely faint background galaxies scattered throughout the cosmos. Like the billions of too-faint-to-see stars collectively melding to create the Milky Way’s luminous glow in our sky, these distant galaxies combine their ultradim light to make what astronomers call the cosmic optical background.
Such dimly luminous backgrounds have been seen in other wavelengths, too, albeit often from distinctly different objects or processes. The background in high-energy gamma rays is likely caused by massive stars exploding and sending out cosmic rays that interact with the gas inside galaxies, making it glow. The cosmic x-ray background probably arises from actively feeding supermassive black holes at cosmological distances that voraciously gobble down matter and emit energy. The infrared background is caused by distant galaxies that are shrouded in dust, which absorbs light from stars and reradiates it in infrared wavelengths. The cosmic microwave background is the leftover glow from the big bang itself and was crucial to the discovery that the universe had a definitive starting point.
The cosmos is filled with galaxies that glow across the electromagnetic spectrum, creating the ultimate dark-sky limit. The next time you’re outside, gazing up into the darkness, take a moment to appreciate the idea that “dark” is relative. What you see as black sky looks very different to an astronomer, and they can use that fact to grasp the components of the universe.