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What is a Nebula…Besides a Foggy Thing in Space

By Pierre Markuse

What exactly is a nebula, and which different types of nebulae can we see in space?

Nebulae (Nebula is the Latin word for cloud) are one of the most interesting, and most imaged, objects in astrophotography. They come in many different shapes, sizes, and colors. And while many of them may look alike, the way they came to be can be quite different. Let us take a look at some of the common types of nebulae.

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When galaxies were nebulae

Before we got really good at identifying nebulae, other objects were often misidentified as nebulae. Spiral nebulae was a term for objects which we now know are actually spiral galaxies. You may have heard the now outdated name Great Andromeda Nebula for the Andromeda Galaxy (M31). The reasons for these wrong identifications are rooted in the lower resolution of telescopes available at the time of detection, as well as different opinions on the true size of our Milky Way Galaxy and the Universe as a whole.

If you would like to know more about the history of the term, you should read about the Great Debate that took place at the National Academy of Sciences in Washington in 1920. Just a few years later Edwin Hubble found Cepheid variable stars in the Andromeda Galaxy (M31), allowing him to pinpoint the distance and showing that M31 was indeed a galaxy separate from our Milky Way Galaxy.

General structure of a nebula

Generally speaking, nebulae are interstellar clouds of dust and/or ionized gases, mostly hydrogen and helium. Hydrogen and helium are the bulk of the interstellar medium, which, by mass, consists of about 70% hydrogen, 28% helium, and 1.5% heavier elements (all of which are called metals by astronomers and astrophysicists).
While there are true gigantic nebulae, Lyman-alpha blobs can be hundreds of thousands of light-years in diameter, most of the better-known nebulae fall within the range of under a light-year to a few ten thousand light-years, still a pretty impressive variety.
Another distinction, and one especially important for astrophotographers, is the brightness of the nebula, or to be more precise, whether it is emitting or reflecting light at all.

Introducing: dark nebulae

Yes, there are nebulae that are not emitting light. They are called dark nebulae or — more fitting — absorption nebulae. As the latter name implies, they absorb the light of objects behind them, and by doing so reveal themselves to us, as their darkness stands out against the rest of the sky. An example would be the Rho Ophiuchi cloud complex with its dark nebulae, which can be seen in this image.

Dark nebulae in the Rho Ophiuchi cloud complex. Image: Dylan O’Donnell

The dark patches are the dark nebulae. Interstellar dust grains absorb and scatter the light from objects behind them, a process called extinction. Depending on the size and density of the nebulae, they appear more or less dark. While dark nebulae quickly extinct the visible light, they are more transparent to radio waves and infrared light and therefore, using radio and infrared astronomy, we can look behind dark nebulae.

Imaging dark nebulae can yield really impressive images as the one you have just seen, still, most astrophotographers prefer brighter objects. So let us take a look at some nebulae of the shiny kind.

Emission nebulae and reflection nebulae

Although emission nebulae and reflection nebulae are two different kinds of nebulae, we will talk about them together, as they have one thing in common important to astrophotographers, they are — more or less — bright, do not hide in darkness, and sometimes they are conveniently located next to each other.

Reflection nebulae

The mechanism behind reflection nebulae is pretty straight-forward. Light from a star gets reflected by tiny dust particles. The color of a reflection nebula depends on the color of the light the star is emitting but usually ends up to be somewhat bluish, because scattering is more efficient for blue light than for red light.

Take a look at this example of a reflection nebula, IC 2631, illuminated by the star HD 97300.

Reflection Nebula IC 2631 imaged by the MPG/ESO 2.2-metre telescope. Image: ESO CC BY 4.0

And if you look closely at this image, you will not only see the very obvious bluish reflection nebula but also dark nebulae above and below the reflection nebula. This region of space is filled with dust and gas, causing these dark nebulae and also supplying the material for star formation. The illuminating star HD 97300, itself a young star, might at some point be joined by new stars being born from this material.

Protoplanetary nebula IRAS 20068+4051. Image: ESA/Hubble & NASA CC BY 4.0

Protoplanetary (or preplanetary) nebulae, like the one in the image to the left, are another type of reflection nebula. They are a short occurrence in the late evolutionary phase of intermediate-mass stars, during which the star is illuminating the circumstellar envelope it shed during this phase. This reflection nebulae can transition into emission nebulae if the star reaches high enough temperatures, and the ultraviolet radiation it emits is strong enough to ionize the circumstellar envelope, resulting in a planetary nebula.

Emission nebulae

The next type of nebulae would be emission nebulae. Emission nebulae come in different types. What all of them have in common however, is the mechanism that makes them glow. Simply speaking, emission nebulae are clouds of ionized gas, emitting light of different wavelengths.

In most emission nebulae, strong ultraviolet radiation of nearby stars is breaking neutral hydrogen atoms (other gases such as helium and oxygen are also possible, but hydrogen usually makes up the biggest part) into hydrogen nuclei and free electrons (a process called photoionization). Those nuclei and free electrons then recombine at an excited state. When the excited neutral hydrogen atoms return to their lowest energy state, they are emitting photons at wavelengths equivalent to the energy difference. Which in case of hydrogen is at 656.281 nanometers, a wavelength in the red part of the visible spectrum, and the reason for the oftentimes reddish-pinkish appearance of emission nebulae in visual light images. Take a look at this image of the Lagoon Nebula (M8, NGC 6523), an HII region in the constellation of Sagittarius, about 5,000 light-years away from Earth.

Lagoon Nebula (M8), an HII region in the constellation of Sagittarius, showing the telltale reddish-pinkish glow of ionized hydrogen. Image: Dylan O’Donnell

Those HII regions are a common sight in the Universe. You can usually find them in the arms of spiral galaxies as well as in irregular galaxies. Recent star formation generated the stars necessary to ionize the regions and give them their reddish glow. The dense group of stars at the center of the image is the open cluster NGC 6530.

Another form of emission nebulae are planetary nebulae, which I have mentioned a bit earlier when talking about protoplanetary nebulae. Planetary nebulae (PN) are created by red giant stars. Late in their evolution, those stars shed their outer layers and the core of the star, now called a planetary nebula nucleus (PNN), ionizes the shed layers with its immense ultraviolet radiation, creating a glowing nebula. This phase lasts for about 10,000 years, during which the core cools down more and more, ending its life as a white dwarf star, no longer putting out enough ultraviolet radiation to ionize the ejected layers and the planetary nebula vanishes. Depending on the temperatures and gases involved, planetary nebulae can have different colors. They also come in different shapes and sizes, but usually are within a few light-years in diameter and most of them are spherical/elliptical or bipolar in shape.

Take a look at this collage of planetary nebulae.

99 Planetary Nebulae (and a Protoplanetary Disk). Imaged by the Hubble Space Telescope. Image: Hubble data processed by Judy Schmidt CC BY 2.0

Keep in mind, those nebulae are depicted at their apparent size, so it is not a true comparison of their actual size. They are also not always imaged in the visible part of the spectrum, so the actual appearance in visual light could be different. And as you can see, even though most of them adhere to the spherical/elliptical or bipolar shape, they can still look very different from each other.

Let us take a look at a single planetary nebula imaged from Earth by an amateur astronomer, the Helix Nebula (NGC 7293), located in the constellation of Aquarius, about 700 light-years away from Earth. It has a diameter of roughly six light-years.

The Helix Nebula (NGC 7293), a planetary nebula in the constellation of Aquarius. Image: Dylan O’Donnell

The next type of emission nebulae are supernova remnants (SNRs). They consist of ejected material from their stellar progenitors (the stars that went supernova) and the interstellar material the expanding shock wave is sweeping up. Aside from visible light, supernova remnants can be very bright in radio and X-ray emissions. Core-collapse supernovae also lead to the formation of neutron stars or black holes, which can be found in the remnant. Take a look at this image of Cassiopeia A (Cas A), a supernova remnant (SNR) in the constellation Cassiopeia. It is a composite image, made using Chandra X-ray data (blue and green), Hubble near-infrared data (yellow) and Spitzer Space Telescope infrared data (red).

A false color image of Cassiopeia A (Cas A) using observations from both the Hubble and Spitzer telescopes as well as the Chandra X-ray Observatory. Image credit: NASA/JPL-Caltech

Aside from those big, space-based telescopes, supernova remnants can also be a rewarding target for amateur astronomers. Take a look at this image of the supernova remnant Simeis 147, the Spaghetti Nebula, highlighting the reddish emission of the hydrogen, ionized by the shockwave of the supernova.

Supernova remnant Simeis 147, located 3,200 light-years away in the constellation of Auriga. Image: © 2016 Nicolas Kizilian – Used with permission

Wolf-Rayet nebulae are another type of emission nebulae. They surround Wolf-Rayet stars and are driven by the strong stellar winds of the Wolf-Rayet stars interacting with the outer layers of hydrogen, ejected by the stars earlier in their evolution. Take a look at this image of the Wolf–Rayet star WR 31a, located about 30,000 light-years away in the constellation of Carina (The Keel) with a clearly visible, spherical, blue Wolf-Rayet nebula surrounding it.

Wolf-Rayet nebula surrounding WR31a, imaged by the Hubble Space Telescope. ESA/Hubble & NASA Acknowledgement: Judy Schmidt CC BY 4.0

Great, but what does a nebula really look like?

Well, that depends on what you understand as “really look like.

We can image nebulae in the visible part of the spectrum and while that is the part of the electromagnetic spectrum you can see with your eyes, it doesn’t really reflect what you would see with your naked eyes.

One of the best examples of gathering more light to see fainter objects: The Hubble eXtreme Deep Field (XDF), combining over 2,000 images with a cumulative exposure time of 22.5 days. It shows about 5,500 galaxies, the faintest just reaching ten-billionth the brightness of what the human eye can see. Image: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team

Nebulae are usually far away and not as bright as you might expect — more on that soon — so astronomers taking images try to collect more light (more photons, to be exact) by using telescopes with a big aperture, and by taking images with a long exposure time, sometimes even combining multiple of these long exposures. Doing so, a nebula that would look like a somewhat grayish blob to your naked eyes (or a nebula that wouldn’t be visible to you at all), can suddenly shine brightly in all the glory you know from many images.

Now is that what it really looks like?

Let’s jump ahead 500 years into the future. Instead of a car, there is a small spaceship in your garage. Determined to see a nebula up close, you set a course for the Lagoon Nebula and 20 minutes later you reach your destination, not a bad time for a 5,000 light-year-trip. But when you look out of the window all you can see is maybe a faint red glow in the distance, no bright nebula like in the brochure. But what about the images you have seen? Why does it look like that?

The reason for this somewhat disappointing experience of your trip has to do with the density of the nebula. Here on Earth the air you breath contains about 3×10^19 molecules per cubic centimeter (mostly nitrogen and oxygen). In a nebula like the Lagoon Nebula, an HII region, you just have about 100 to 10,000 atoms of hydrogen per cubic centimeter, even less than you would achieve in a typical vacuum here on Earth. So in other words, the nebula is a pretty thin and really spread out object. And unlike on Earth, where you focus the light of the whole nebula with the optics of a telescope onto a detector (a camera) and then collect photons for minutes, hours, or even days, you just have your eyes, can’t change your exposure time, and the nebula is not a neatly focused spot but its light is spread out all around you, covering the whole sky. Depending on the size of the nebula, and how dense it is, you might be able to see a faint glow in the far distance, although it would be entirely possible that you could not see the nebula at all while flying through it with your spaceship.

When taking a look at dark nebulae, we see that the density of dust particles in them is extremely low. Being at the center of a dark nebula you would see a curtain of blackness (or heavily reddened and dimmed stars, depending on the size of the nebula) in the far distance to the edges of the nebula. The light from the stars outside the nebula has been absorbed by light-years-thick layers of thinly spread dust, but you would have no problem seeing your immediate surroundings. The experience would not be unlike fog here on Earth. On a foggy day you can go outside and may have problems seeing cars more than 50 meters away from you, and beyond 100 meters it just looks like a solid white wall, still you can see your hand in front of your eyes just fine.

Why is it called nebulas one time and nebulae the other?

As mentioned before, the word nebula, meaning cloud, is a Latin word. Now when forming the plural of that word, some people prefer doing it the English way, adding an “s” to the word.

Others prefer doing it the Latin way, in which nebula gains an e in the nominative plural form. Both forms are regarded correct, but nebulae seems to be more prevalent.


Recommended further information

Aside from following the links in the article, I recommend the following sources for more information on fluffy nebulae:

About Pierre Markuse

Pierre Markuse
Pierre Markuse

Interested in all things related to science, Pierre Markuse loves to write and does so mostly on Google+, where he is writing featured collections on Astronomy and Astrophysics as well as Space/Space Technology. Always impressed by the public’s interest in science, he knows about questions raised by images, and how to explain them to a curious audience.

Aside from his love for space, Pierre also likes to take a look at Earth and processes image data from Earth observing satellites like Landsat or Sentinel to share on his Space Technology collection and Flickr.

See Pierre’s work on Google+ and on Flickr, and follow him on Twitter.

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