What are all the different types of nebula in astronomy?

When most people think about astronomy, they think about the common, light-emitting objects familiar in our night sky: stars, planets, and perhaps even galaxies. Once in a while, here on Earth, we’ll encounter a few other objects that — like planets — reflect the Sun’s light to appear visible either to our eyes or our telescopes: objects like satellites, asteroids, or comets. However, if we look out into the Universe with our telescopes, both within our own Milky Way as well as beyond, into the grand abyss of deep space, we’re likely to encounter a new class of object: objects that aren’t compact and solid, but rather are faint, extended, and difficult to resolve. Astronomers call these objects nebulae.

For a long time, we had only conjecture to tell us what these objects might be. A few of them, as telescopes became more advanced, turned out to be nothing more than clusters of stars. Others turned out to be galaxies far beyond the extent of the Milky Way: “island Universes” full of anywhere from thousands to trillions of stars. These objects, colloquially sometimes called “faint fuzzies” after their appearance through a telescope, not only were fixed and unmoving, but came about through a vast array of astrophysical phenomena. Over time, we began classifying them, recognizing and understanding their properties, and placing them into their proper context within the great cosmic story. Here are what the different types of nebulae known to astronomers actually are.

Behind the dome of a series of European Southern Observatory telescopes, the Milky Way towers in the southern skies, flanked by the Large and Small Magellanic Clouds, at right. Although there are several thousand stars and the plane of the Milky Way all visible to human eyes, there are only four galaxies beyond our own that the typical unaided human eye can detect. We did not know they were located outside of the Milky Way until the 1920s: after Einstein’s general relativity had already superseded Newtonian gravity.

Long ago, astronomers had only their naked eyes to use as tools for exploring and cataloguing the Universe. Originally, there were only four types of astronomical object known:

• the Sun,
• the Moon,
• the stars,
• and the planets,

with a notable feature of planets being that they — unlike stars — both failed to twinkle the way stars do, and also that instead of remaining fixed relative to the other objects in the sky, they appeared to wander, or change position, from night-to-night. However, long before the invention of the telescope, extended objects could be identified in the dark night sky. Andromeda, now known to be the nearest large galaxy to our own, was arguably the first, and was soon joined by others: the Large and Small Magellanic Clouds, as well as the Triangulum nebula. All four of those nebulae turned out to be galaxies.

With the advent of the telescope, many more nebulae appeared. Some took on spiral or elliptical shapes: the most common shape for galaxies found throughout the Universe. Galaxies aren’t “nebulous,” however, because they’re made of gas. Instead, they’re composed of enormous numbers of stars themselves, much like the star clusters found within our own Milky Way. Although many galaxies have nebulous regions within them — gas, dust, and even ionized material — most galaxies, overall, only appear to be nebula-like in nature. In reality, they’re self-luminous objects, powered overwhelmingly by stars.

However, many objects with nebular appearances truly are composed of diffuse, extended gases. In reality, a nebula is intrinsically diffuse, not just apparently diffuse. While some galaxies really do have extended emissions, rendering them truly “nebulous” in nature, they’re only a small subset of the full suite of galaxies present across the Universe.

Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. They also glow red, covering the whole galaxy, thanks to hydrogen emissions. This particular galaxy, M82, the Cigar Galaxy, is gravitationally interacting with its neighbor, M81, causing this burst of activity. With copious winds and ejecta, this galaxy has a large amount of nebulosity, and is classified as a starburst galaxy: where practically the entire galaxy is undergoing a star-formation episode.

The one thing all nebulae have in common is this: they’re made of non-stellar material, or more than merely stars alone, and they either absorb, reflect, or emit light. These, in fact, correspond to the three main classes of nebula that were first known to astronomers: absorption nebulae, reflection nebulae, and emission nebulae.

If your gaseous material is dense, cold, and in front of a background source of light, it’s likely going to create an absorption nebula, more commonly known as a dark nebula. These appear all across the Universe, and many famous examples abound. If you look at the plane of the Milky Way on a dark, moonless night, you’re likely to see not just a large streak of brilliant stars illuminating your view, but what appear to be dark, opaque clouds of material that block the starlight from reaching your eyes. These are indeed dark, absorption nebulae: dense molecular clouds of gas that fail to allow visible light to penetrate through them.

The famed pillars of creation in the Eagle Nebula are dark nebulae. The various Bok globules found in star-forming regions as well as in the halo of the Milky Way are examples of dark nebulae. And the Horsehead Nebula, a dark cloud of opaque material silhouetted in front of a bright background of illuminated gas, is a dark nebula as well. In general, if you’re a cold, dense cloud of material located in front of an otherwise light-emitting source, that’s how you make a dark nebula.

Dark, dusty molecular clouds, like Barnard 59, part of the Pipe Nebula, appear prominent as they block out the light from background objects: stars, heated gas, and light-reflecting material. In the young Universe, prior to an age of ~550 million years, a large fraction of atoms were not ionized, and so should be very efficient at blocking the light even from hot, newly-formed stars. Dark nebulae must be located in front of (i.e., in the foreground of) luminous objects in order to silhouette them, and appear as dark.

Some clouds of gaseous matter found in space, however, don’t absorb light very well, but rather do an excellent job of reflecting it. In many cases, the main difference between a dark nebula and a reflection nebula is simply orientation and location: instead of being located well in the foreground of a luminous source of matter, which is the typical configuration for a dark nebula, a reflection nebula is a cloud of gas that’s located very nearby to an otherwise intrinsically bright, luminous light source, like a hot, young, blue star. If that gas cloud is located just a short distance behind the light source, it typically makes a reflection nebula; if it’s located well in front of the light source, it typically makes a dark (or absorption) nebula.

Although any bright source of light can light up a cloud of gas and turn it into a reflection nebula, the brightest, hottest, most luminous stars tend to be the short-lived, high-mass O and B class stars in the Universe: the ones that must have been created only recently, from star-forming episodes that occurred only within the past few million years. (Although red giant stars can also create reflection nebulae.) The Flame Nebula is a famous reflection nebula, the blue portion of the Trifid Nebula is a reflection nebula, and the dusty nebulosity found within the Pleiades — itself a famed, relatively young star cluster within the Milky Way — reflects the light from the stars in its vicinity. Anytime a neutral cloud of matter reflects light from the stars in its vicinity, a reflection nebula can result.

This portion of the Cosmic Reef composition highlights the blue reflection nebula created by winds blown off of a hot, massive, giant blue star that are then illuminated in reflected light by the original star that created it. The Wolf-Rayet star that powers it may be destined, in short order, for a stellar cataclysm such as a core-collapse supernova, but we can only see the presence of the cold, expelled gas from its outer layers as they reflect the starlight from the luminous, blue star powering it.

And lastly, there are what are known as emission nebulae. Instead of a cool or even a cold cloud of neutral matter, like reflection or dark nebulae, emission nebulae are very hot: so hot that the electrons within the atoms and molecules found inside are kicked up into an excited state or even ionized, enabling the production of emission lines — largely but not exclusively from common elements like hydrogen — as electrons recombine with atomic nuclei and/or cascade down the various energy levels.

Unlike reflection nebulae, emission nebulae are typically red or pink in color, as hydrogen — the most common element in the Universe by far, making up ~92% of all atoms by number — emits a characteristic “red line” as it de-excites from the n=3 to the n=2 energy level. This typically indicates very hot temperatures: temperature of ~10,000 K or more. In some rare but exceptional cases, emission nebulae can take on a green color: corresponding to electrons recombining with doubly-ionized oxygen atoms and de-exciting through the various energy levels, a phenomenon requiring temperatures of ~50,000 K or more.

Emission nebulae are found ubiquitously in active star-forming regions, with that red color from hydrogen atoms representing the overwhelming majority of emission nebulae. Sometimes, in fact, neutral, cool gas and hot, ionized gas are found together, creating instances where emission, reflection, and even absorption (i.e., dark) nebulae can all be found within the same region of space.

Near Orion’s Belt, the reflection nebula known as the Flame Nebula (left) as well as the star-forming emission nebula known as IC 434 (in red) are joined by a series of dark molecular clouds in the foreground that create spectacular silhouettes known as dark nebulae. The Horsehead Nebula (at center) is arguably the most famous dark nebula of them all.

However, there are other types of nebulae as well: nebulae that don’t arise from loose, diffuse, extended clouds of gas common to star-forming (or potentially future star-forming) regions. Instead, the humble star can give rise to nebulae in an enormous variety of ways.

One example of how a star can generate extended emission comes from a class of astronomical objects known as Herbig-Haro objects. These young, massive stars often produce stellar outflows: collimated gaseous ejecta that flow out of the young star (or star system) itself. Originally confined to just a few known objects, there have recently been dozens of them identified in the Orion Nebula alone thanks to the incredible power of the James Webb Space Telescope (JWST).

The young stars that create these Herbig-Haro objects aren’t always found alone, as isolated stars or sets of stars together, but are often found as bright young stars within a complex of extended, diffuse gas. For reasons still unknown, the jets and outflows coming from these Herbig-Haro objects often line up, even across different star systems found within the same gas complex. Herbig-Haro objects, as they expel this material back into the interstellar medium, are an important source of dust production in the Universe, where that ejected material can later become a dark, reflection, or emission nebula down the road.

Ultra-hot, young stars can sometimes form jets, like this Herbig-Haro object in the Orion Nebula, just 1,500 light years away from our position in the galaxy. The radiation and winds from young, massive stars can impart enormous kicks to the surrounding matter, where we find organic molecules as well. These hot regions of space emit much greater amounts of energy than our Sun does, heating up objects in their vicinity to greater temperatures than the Sun can.

Nebulae can also arise — although usually on scales that are much smaller than the scales of typical star-forming regions — from the formation of individual stars, the evolution of single or pairs of stars, and the deaths of stars of all types.

When individual stars first form, they usually do so as the result of a contracting cloud of sufficiently massive gas. Sometimes, the cloud is low-enough in mass that only a single, central star will wind up forming; other times, the cloud is sufficiently massive and extended that it will not merely contract toward its center, but will fragment, producing large numbers of stars, with potentially widely varying masses, all across the molecular cloud that collapsed to form them.

When this contraction occurs, the gas heats up. Even where a sufficient amount of material gathers to lead to the formation of new stars, it’s a rather slow process, often taking tens of millions of years for a protostar to collapse sufficiently to ignite nuclear fusion in its core. During this stage, these protostars form within protostellar nebulae, which are their own class of extended object. As fusion ignites within the cores of these objects and they evolve into full-fledged stars, the disks around those stars evolve as well: going from circumstellar disks (disks around the stars) to protoplanetary disks (disks where planets form within them) to debris disks (disks of dusty material that persist even after planet-formation has ceased). All of these are further examples of nebulae and nebulous regions.

This selection of strongly silhouetted protoplanetary disks from within the Orion Nebula was published in 2000, back when 38 of Orion’s proplyds were then known. At present, some ~150 are now known.

On the other hand, you can also create nebulae and nebulous regions from the evolution and deaths of stars. For “low-mass” stars, or stars that are born with no more than about eight-to-ten solar masses worth of material, stellar evolution is a slow, gradual process that leads to many fascinating nebular stages.

As a star burns through the hydrogen fuel in its core, it eventually begins to run out. When this happens, the core of the star contracts, causing it to heat up and grow. Eventually, the hydrogen fuel in the star’s central core gets completely spent, leaving only a “shell” of hydrogen fusion around the star’s center. As the central part of the star contracts and heats up, the outer layers of the star swell. When the central core heats up sufficiently to initiate helium fusion, the outermost layers of the star begin to get ejected, or gently “blown off” into the interstellar medium surrounding the star. As helium fusion persists, greater amounts of mass get ejected, where it continues to “hang out” in the environment surrounding the star.

Over time, the bright, shining central star begins to illuminate the ejecta, creating an extended object known as a preplanetary nebula. (Some older astronomers still call it a protoplanetary nebula, but that term has fallen out of favor in recent decades, to avoid confusion with the protoplanetary disk phenomenon described earlier.) Then, when the star reaches the true end of its life cycle, the remnant outer layers are expelled completely, and then heat up to become ionized as the central remnant of the core contracts to become a hot white dwarf. A true planetary nebula has then formed: the end state of Sun-like stars, as far as we know.

This compact, symmetric, bipolar nebula with X-shaped spikes is known to have a binary system at its core, and is at the end of its asymptotic giant branch phase of life. It has begun to form a preplanetary nebula, and its unusual shape is caused by a combination of winds, outflows, ejecta, and the central binary at its core. Without heating from the central, dying star, all of these features would not be visible to even the Hubble Space Telescope’s eyes, as the ejected material has yet to become self-luminous through ionization.

Furthermore, there are the nebulae that result from stellar cataclysms, or violent events that trigger a star or stellar remnant’s destruction. When a sufficiently massive star reaches the end of its life cycle, for example, it doesn’t just gently blow off its outer layers while its core contracts down and heats up. Instead, when the star’s core contracts once its helium reserves are exhausted, it continues to fuse still heavier elements: carbon into neon, neon into oxygen, oxygen into silicon, and silicon into iron, for example. When silicon fusion completes, the inner core implodes, leading to a runaway fusion reaction for the outer layers that triggers a supernova event. While the core may leave a neutron star or a black hole, the remainder of the star, usually corresponding to 80-90% of the total initial mass, becomes its own kind of nebula: a supernova remnant.

Other stellar cataclysms can also produce stellar remnants. Kilonovae, for instance, occur when two neutron stars collide. They can not only produce gamma-ray bursts, but can leave extended remnants that are not only rich in heavy elements, but are thought to be the dominant mechanism for producing the heaviest elements of all found in nature. Tidal disruption events result from massive objects, such as stars, being shredded as a result of passing too close to a black hole. These, too, can produce extended, nebulous remnants. And finally, white dwarf collisions — a different mechanism for creating supernovae — can create supernova remnants as well, although in this case, there’s no neutron star, black hole, or any type of collapsed stellar object that remains.

This full-scale view of the Crab Nebula, from upper-right to lower-left, spans about 11-12 light-years in extent at the nebula’s distance of ~6,500 light-years. The outer shells of gas are expanding at around ~1500 km/s, or about 0.5% the speed of light. This is perhaps the best studied supernova remnant of all-time.

Finally, there’s an exciting, new type of nebula that became known to astronomers only over the past two decades: integrated flux nebulae. Whereas most of the nebulae found in the Milky Way are the result of gaseous collection of matter that are illuminated or heated by compact sources of energy, either externally or internally, they instead behave like galactic cirrus clouds, where they’re illuminated by the combined energy (i.e., the integrated flux) of all the stars in the Milky Way, combined. These integrated flux nebulae are incredibly faint; instead of appearing prominently through a telescope, they require (at least) hours of long-exposure observations to be detected and seen at all.

However, these nebulae are very much real, as long exposure campaigns have revealed their composition to be primarily made up of dust particles, hydrogen atoms, and carbon monoxide molecules, along with other compounds. For reasons that are not yet fully understood, these integrated flux nebulae are particularly prominent near the north and south celestial poles, with a gorgeous one found surrounding the North Star: Polaris.

This image shows an example of an integrated flux nebula: diffuse clouds of gas in the galactic halo, sort of like cirrus clouds in Earth’s atmosphere, that reflect the faint, cumulative light from all the stars in the Milky Way galaxy. Note the bright blue star, Polaris, and how little of the nebula is affected by reflecting its blue light. A globular star cluster also appears nearby.

Although nebulae come in an enormous variety of shapes, colors, temperatures, and compositions, as well as coming from a huge variety of sources, they all have something in common: extended, diffuse, gaseous (and/or dusty, and/or ionized) distributions of matter. They are more than just stars and collections of stars; they are much more than mere compact, self-luminous objects. They are clouds that can be hot or cold, neutral or ionized, reflective or absorptive or even radiative.

And what’s remarkable to ponder is that, despite all the different types of nebulae that we know of, this list may not even be exhaustive. Many galaxies — particularly in the early Universe — are forming stars so rapidly that the entire galaxy itself is behaving as an emission nebula: a characteristic normally found in star-forming regions that can apply to the entire galaxy during a starburst event. Active black holes produce gas, jets, and outflows, and might someday be considered to be nebulae of their own. And it’s possible that the halos of gas found in the extreme outskirts of galaxies, the circumgalactic medium, may inevitably be nebulae all unto themselves.

Even though we’ve come extraordinarily far in our understanding of the Universe, there are very likely novel classes of extended, diffuse, nebulous objects just waiting to be discovered.