What is a nebula? Come for the science, stay for the beauty.

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Nebulae are all but impossible to spot in the night sky, but you’d never know it. Through a telescope, they make the most jaw-dropping beauty shots. But what is a nebula in the first place? Have you ever wondered where nebulae come from, or what they’re made of? If so, you’re in good company, and we’ve got a gorgeous gallery of nebulae from the world’s great observatories to prove it.

First, the basics: A nebula is a giant cosmic cloud of gas and dust. The word ‘nebula’ comes from Latin, meaning “little mist.” Before telescopes existed, astronomers considered all such faint deep-sky objects to be nebulae. With the advantage of modern telescopes, though, we can see that some of those colorful points of light are “true” nebular clouds, some are star clusters, and some are entire galaxies.

Astronomers have known about nebulae for at least a thousand years. The first attested mention of a nebula comes from the Persian astronomer Abd al-Rahman al-Sufi, who wrote in 964 about a “little cloud” we know today as the Andromeda Galaxy. Less than a hundred years later, Arabic and Chinese astronomers marveled at a supernova that created the Crab Nebula. Since then, thousands more have been identified and described in surveys like the Abell Catalog of Planetary Nebulae and the Messier catalogue.

Nebulae are mostly found within galaxies—specifically, in interstellar space. There’s often at least one star at the core of a nebula, whether new or recently deceased, but sometimes a giant molecular cloud turns up starless but sporting its own subtle glow. Astronomers estimate that there are about twenty thousand nebulae in the Milky Way alone, but only about 3,500 have been confirmed because of the obscuring cloud of gas and dust hovering around the center of the galaxy. The closest nebula to Earth is the Helix Nebula, which is about 700 light years away from Earth.

Space telescopes are invaluable for studying nebulae, which are often faint and dim. Earth’s atmosphere is like an Achilles heel for telescopes. Light pollution and moonlight are enough to drown out a nebula altogether; worse, some frequencies of light can only be observed from space. Nevertheless, astronomers can and do perform in-depth studies in the visible spectrum, using terrestrial telescopes like the 90-cm NSF telescope on Kitt Peak or the European Southern Observatory’s Very Large Telescope (VLT) in the Chilean Andes. Compared or composited, when taken together they can tell scientists much more than they reveal on their own.

Two different views of the Horsehead Nebula. On the right is a view of the nebula in visible light, taken using the European Southern Observatory’s Very Large Telescope in Chile (eso0202a). The new image on the left shows the nebula in the infrared, using observations from Hubble’s high-resolution Wide Field Camera 3.
Credit: NASA, ESA, and the Hubble Heritage Team (AURA/STScI); ESO

Astronomers name nebulae for what they look like through a telescope (examples include nebulae named for dumbbells, horseheads, tarantulas and crabs), or where they appear in the sky (for example, the Orion Nebula). With the aid of high-powered telescopes, it’s now known that many different types of nebula exist, but they tend to fall into just a few main classes.

Supernova Remnants

Many nebulae are formed by a supernova: the violent explosion of a dying star. When stars explode, they cast off their outermost mantles of gas and plasma, creating clouds, rings, and plumes. New stars rise like a phoenix from the ashes of the star that came before.

The Crab Nebula, a supernova remnant nebula. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)

One of the most striking examples of a supernova remnant is the Crab Nebula, found in the constellation of Taurus. Sometime between April and June of 1054, a so-called “guest star” flared into being, visible to the naked eye where no star had been discernible before. Two years later, it had faded from its peak brightness, leaving behind it a smudge of diffuse color. Today, it’s comparable in brightness to Titan, one of the moons of Saturn. (Neither Titan nor the Crab Nebula are visible to the unaided eye, but with a dark sky and cooperative weather, you can find them with binoculars.) The explosion stripped off the star’s outer layers, and the stellar core—now a neutron star—lies in the center of the nebula.

In the 1920s, astronomers like Edwin Hubble noticed that the Crab Nebula was getting bigger in photos of the night sky. Hubble did the math and realized that the universe was expanding at a meaningful fraction of the speed of light.

Planetary Nebulae

An explosive fate awaits stars of less than about eight solar masses. Planetary nebulae are a type of supernova remnant formed when Sun-like stars first swell up to enormous sizes and then explode. When it happens to the Sun, its mantle will be flung outward to form a planetary nebula, while its core will linger on as a white dwarf.

Named because their color and size made them resemble planets through early telescopes, planetary nebulae also tend to be round or oval in shape. However, because these stars go through a phase of repeated explosions at the end of their lives, they often have a subtle structure of nesting layers. Often, these fine features are only visible through larger telescopes.

The Cat’s Eye Nebula, a planetary nebula formally known as NGC 6543. According to NASA, “Observations suggest the star ejected its mass in a series of pulses at 1,500-year intervals. These convulsions created dust shells, each of which contain as much mass as all of the planets in our solar system combined (still only one percent of the Sun’s mass).”
Credit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA)

Space telescopes like Webb, Hubble, and the Chandra X-ray Observatory are valuable tools in the investigation of planetary nebulae. Their abilities go far beyond the simple observation of stars and nebulae themselves. For example, Chandra can pick up X-rays emitted by the shock wave after a supernova.

Stellar Nurseries

When stars are born, they collapse from massive clouds of dust and gas. Sometimes, the very same clouds that form a supernova-remnant nebula serve as ready material for the formation of new stars. (That’s how our own Solar System formed: out of the ashes of a predecessor star, our presolar nebula was shocked into collapsing by another neighboring star that exploded in a supernova.)

Star clusters enveloped in a shroud of cold dust and gas can light it from within, like car headlight beams visible through a cloud of fog. This type of nebula is called a reflection nebula, because its internal light is scattered or reflected.

Credit: Davide De Martin/ESA/ESO/NASA. Image Processing: Photoshop FITS Liberator

Sharp-eyed stargazers may be able to make out the Pleiades, a brilliant cluster of young blue-white stars in the constellation of Taurus. Harder to discern without a telescope is the reflection nebula surrounding the cluster.

Emission Nebulae

Many nebulae have high-energy stars inside, or a massive explosion in their past. However, not all nebulae have such a violent origin story. The soft light from a nebula may also come from the ambient radiation released by the nebular cloud itself. This type of cosmic cloud is known as an emission nebula. One of the most famous is the Eagle Nebula, which contains the Pillars of Creation. (More on the Eagle Nebula below.)

Where reflection nebulae are cooler clouds illuminated by stars, emission nebulae are their own light source: radiation from the nebular cloud’s ionized gas.

Dark Nebulae

On the other end of the spectrum, there are nebulae that barely shine at all. Dark nebulae are cool, dense clouds of matter that partially obscure brighter areas behind them. Often it’s atomic helium, molecular hydrogen, and grains of dust and ice. Many such clouds are at just a few degrees Kelvin shy of absolute zero. Even through the JWST, whose active cooling lets it see things below 40 Kelvin, dark nebulae are difficult to find. In the visible spectrum, they mostly just look like silhouettes. Paradoxically, some dark nebulae like the Coalsack Nebula are visible to the unaided eye, because they’re just huge clouds of cold dust blotting out regions of the Milky Way.

Most nebulae are found within galaxies, including dark nebulae, but there are exceptions. Some molecular clouds float at the outer edge of a galaxy’s gravitational influence. One type, the integrated flux nebula, is lit only by the distant light of stars in a galaxy such as the Milky Way. Faint and cool, they may take hours of exposure time to get a good image.

Hybrid Theory

Often, on closer inspection, nebulae turn out to be much more than meets the eye. These smudges of color become troves of scientific data. Take for example the Orion Nebula. Forming the middle “star” in Orion’s sword, the Orion Nebula is the only nebula visible to the naked eye. By the time of Galileo, astronomers had made sketches indicating a cloud illuminated from within by a cluster of stars. Spectroscopic observations from the 1920s showed that it had features of an emission nebula, giving off its own glow separate from that of the stars in the cluster. Space telescopes completed the hat trick, uncovering evidence of dark nebula features.

The Orion Nebula.
Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

In one of the most detailed astronomical images ever produced, NASA/ESA’s Hubble Space Telescope captured the above image of the Orion Nebula. According to the Hubble telescope team, “This extensive study took 105 Hubble orbits to complete. All imaging instruments aboard the telescope were used simultaneously to study Orion. The Advanced Camera mosaic covers approximately the apparent angular size of the full moon.”

One enduring scientific puzzle: the nebula’s color palette, a variety of colors from deep red to green and blue-violet. The red hue is evidence of ionized hydrogen, which gives off visible light at a wavelength of 656.3 nm. Likewise, the green is from a rare “forbidden” behavior of electrons in doubly ionized oxygen. Shades of blue and purple come from the massive O-class stars embedded in the cloud.

True Colors

Finding a nebula in the sky is relatively easy, but sometimes what’s visible through binoculars or an optical telescope can be underwhelming. So why are all those press-release images so pretty? It may come as a surprise to hear that many of the classic telescope images of nebulae are colorized, but there’s much more going on than simple artistic license.

According to NASA, scientists will choose to colorize a telescope image in a couple of ways. One workhorse transform commonly used with beauty shots from the Webb telescope is a simple frequency shift. The JWST makes its most important observations in the infrared, which human eyes can’t see. Researchers take wavelengths of light that are invisible to our eyes and transpose them onto the visible spectrum (wavelengths between 400-700 nm), with longer wavelengths represented as redder colors and shorter wavelengths toward the blue end of the visible spectrum.

Different wavelengths correspond to different chemical elements in this deconstructed composite image, taken from observations made with the Hubble Telescope.
Credit: Dean Salman & the ESA/ESO/NASA Photoshop FITS Liberator

In addition to colorizing images by wavelength, some telescope images are color-coded by chemical composition. A wash of fuzzy pastel clouds can become a detailed portrait of a nebula’s chemistry, using spectrographic analysis. Image processors assign each spectral band a color, usually red, green, or blue, depending on the closest color to the band. With computer processing, scientists can integrate and compare multiple telescope images of the same target, from the infrared band to ultraviolet light and X-rays. The resulting composite image is both beautiful and useful.

We Promised: The Eagle Nebula

What better way to end than with some examples of said beauty and utility? After all that reading, we’ll leave you with a few glamour shots of our favorite nebula: Messier 16, better known as the Eagle Nebula. For example, this gorgeous 2014 Hubble composite of the Pillars of Creation, a star-forming region within the Eagle Nebula.

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Compare that to a 2022 survey of the same area, captured with the JWST’s infrared instruments:

Credit: SCIENCE: NASA, ESA, CSA, STScI. IMAGE PROCESSING: Joseph DePasquale (STScI), Anton M. Koekemoer (STScI), Alyssa Pagan (STScI)

Where the Hubble image is in visible light, showing more of the gas and dust, the JWST’s near infrared camera brings out the innumerable stars. More than eight thousand are inside the cluster alone, mostly concentrated in a lumen inside the nebula’s molecular cloud.

Found in the constellation Serpens, the Eagle Nebula is about 6,500 light years away. In its wider dimension, the nebula is about seventy light years from edge to edge. Astronomers currently estimate that the nebula is less than two million years old.

If the word “nebula” still has any meaning to you, you’re doing better than us.
Credit: NASA, ESA, CSA, and STScI

This image of the “Cosmic Cliffs,” a different region in the Eagle Nebula, was captured by the JWST’s NIRCam. For a sense of scale, the orange “peaks” are about seven light years tall.

So, what’s your favorite nebula? Did we miss a really good one? We’d hate to sleep on these cosmic jewels, so let us know in the comments.