What is a Nebula?

Beautiful colour images of nebulae grace astronomy books and websites and have spread to mainstream culture. They are now familar to the public but what are these gaudy celestial spectacles?

Dark Nebula: The Taurus Molecular Cloud, in the constellation of Taurus (The Bull), lies about 450 light years from Earth. This image is a composite of observations made with submillimetre wavelengths, which reveal the heat glow of the molecular cloud (shown in orange tones) and in visible light to reveal the rich background of stars. (Image credit: ESO/APEX (MPIfR/ESO/OSO)/A. Hacar et al./Digitized Sky Survey 2. Acknowledgment: Davide De Martin)

So what is a nebula?

Nebula is latin for ‘cloud’ so it is the word astronomers use to call huge clouds in space. Note that historically it was used to describe any fuzzy-looking thing in the sky so older books, from say pre-1960, might call a galaxy nebula too but this isn’t done now.

Nebulae come in several types. Giant Molecular Clouds are the most common but least noticed because they don’t look very exciting, being big, dark and cold clouds of dust and gas. An example would be the Taurus Molecular Cloud. However some GMCs can become star-forming regions. These are quite literally stellar nurseries. In these cases the energy from new stars lights up the surrounding gases and they can look spectacular. Just take a look at the Carina Nebula (NGC 3372), if it wasn’t for all the new stars there it would be a murky dark mass, but just add their radiation and you get this amazing expanse of light and colour.

Bright Nebula: This broad panorama of the Carina Nebula, a region of massive star formation in the southern skies, was taken in infrared light using the ESO’s Very Large Telescope. (Image credit: ESO/T. Preibisch)

Can we see any? The Orion nebula is a very famous nebula and we know that it is full of stars. The stars form deep inside the nebula then as they light up radiation and particles flowing out of them blow away the surrounding gas and dust. Looking deep into the Orion Nebula with the Hubble Space Telescope reveals that the material is churning around in wonderful wispy patterns. This process will eventually disperse the nebula but in the course of it the nebula gets sculpted into weird shapes, like fingers, fairytale castles and so on. This is happening a bit in the Orion Nebula but is really noticeable in another famous one, the Eagle Nebula in the summer constellation of Serpens. There are some extraordinary spires of dust and gas there. The Carina Nebula isn’t as well-known but there are some lovely structures there. All of these are transitory features which will one day be gone.

An image of the Orion nebula, a stellar nursery.. (Image credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)

What about other types of nebula? Planetary nebulae are another significant kind. The name is a historical accident as they have nothing to do with planets. These are the gravestone of dead stars. Basically when an average star is ending its life it blasts material from its surface in a series of outbursts (spread over many thousands or even millions of years). Even just the tiny (about the size of Earth) white-hot core of the star is left, surrounded by shells of glowing coloured gas.

Eye of Sauron: ESO's Visible and Infrared Survey Telescope for Astronomy (VISTA) has captured this unusual view of the Helix Nebula (NGC 7293), a planetary nebula located 700 light years away. The telescope's infrared vision reveals strands of cold nebular gas that are mostly obscured in visible images of the Helix. (Image credit: ESO/VISTA/J. Emerson. Acknowledgment: Cambridge Astronomical Survey Unit)

What do planetary nebulae look like? Before I get to that, I’d better say that the material the star ejects doesn’t come out equally in all directions. This means as three-dimensional objects planetary nebulae are commonly shaped like barrels or hourglasses. When we see one from Earth we might see it end-on so it will appear as set of glowing concentric rings, like the Helix and Ring Nebulae, and this is how most people think of planetary nebulae. However seen from other perspective planetary nebulae can look very different, but they always give an impression of stuff exploding from a central source. A really good example is the Butterfly Nebula, which really does look like some stellar cataclysm.

This beautiful structure is the remains of a massive star that ended its life in a supernova explosion some 11 000 years ago. The core of the star collapsed, forming a pulsar, while the outermost layers were ejected into interstellar space, producing the lacy filaments in the image. This supernova remnant is located some 800 light years away, in the southern hemisphere constellation of Vela (The Sails). (Image credit: ESO/J. Pérez)

Any other types of nebulae? Really big stars, ten or more times the size of the Sun, end their lives in one titanic explosion called a supernova. This is them literally blowing apart. Afterwards most of the stuff that made up the star expands into space in an enormous incandescent mass forming a type of nebula referred to as a supernova remnant. One recently formed object of this type is the Crab Nebula which is less than a thousand years old. As time marches on the debris from the exploded star spreads and thins out. An example of these older and more diffuse nebulae would be the Vela Supernova Remnant.

Nebulae come in in many forms but whatever their origin they are surely some of the great splendors of the Universe.