A beginner’s guide to nebulae

Majestic and mysterious, nebulae are some of the most beautiful deep-sky objects to be found in the night’s sky. But what are these space clouds? And how are they created?

The Orion Nebula

What is a nebula?

Nebulae (sing. nebula) are clouds of dust and gas that float in space. They can measure lightyears across in size but are very diffuse – a cloud the size of Earth would still only weigh a few kilograms.

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As they are clouds, blown about by the interstellar winds of our Galaxy as well as their own turbulence, they often have weird and wonderful shapes.

Nebulae can vary hugely depending on how they were created, but many of the clouds are strongly related to the lifecycles of stars. Some are crated when a star dies, others allow them to be born.

What are the different kinds of nebulae?

There are many different kinds of nebula which fall into four main categories:

Within these categories there are emission nebula, which are illuminated by the glowing hot gas they contain, and reflection nebulae, which reflect the light of nearby stars.

Do stars form in nebulae?

Not all nebulae form stars, but all stars (that we know of) started life in clouds of dust known as star-forming regions.

The interstellar medium is the name given to the gas and dust that fills the spaces between stars.

Over hundreds of millions of years this medium begins to clump together to form a cloud, a nebula.

If the gas in the clouds becomes dense enough, it starts to collapse under its own weight, increasing the density even further.

In the resulting turbulence, knots of gas form, their gravity attracting even more gas so they ever larger until they become dense enough to form a star.

The Pillars of Creation
Buried in the Eagle Nebula is a region of intense star formation. Hubble took an image of the region back in 1995, and their column shaped form earned the region the name “The Pillars of Creation”.
NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Do nebulae look different in real life compared to pictures?

The popular idea of what a nebula looks like is often based on the spectacular images taken by the Hubble Space Telescope and other world class observatories, which can lead to disappointment when new astronomers see a faint and colourless nebula through the eyepiece.

Nebulae aren’t very bright so look dim to the human eye, but when astrophotographers take an image, they can leave the camera for several minutes to soak up as much light as possible, and use filters to enhance the colour.

While visually observing nebulae might not be as visually arresting as a picture, nothing’s quite the same as knowing you’re seeing it for yourself

What do a nebula’s colours mean?

While most nebulae’s natural colour is faint at best, they do have some colour based on the gases within them.

When you heat a gas, it often emits a spectral line – light with a very precise colour that is specific to the kind of gas being heated.

Narrowband filters

There are many elements that glow within a nebulae, and each of these elements might have several spectral lines. However, some of these are very faint, or aren’t at wavelengths visible to the human eye.

While professional telescopes can be configured to pick these up, most amateur astronomers use a handful of filters.

Spectral line

Filter name

Colour of line

Wavelength

Sulphur

SII

Red

672nm

Hydrogen alpha

Ha

Red

656nm

Oxygen

OIII

Green

501nm

Hydrogen beta

Hb

Blue

486nm

 

Hydrogen beta isn’t used as much as the other three, however, as it is a weak spectral line.

While it’s still very useful for observing a nebula visually, it’s relative dimness means its rarely used to create images.

Astronomers use a technique called narrowband imaging, where they use filters to pick out only these spectral lines. They can even use this technique to pick out colours that aren’t visible to the human eye.

Professional astronomers use these images to trace where certain types of gas are within a nebula.

Meanwhile, image processors assigning each spectral band a colour – either red, green or blue, depending on which is closest to the spectral line – then blend the frames together to create full colour images.

Narrowband imaging the swan nebula through SII, Ha, OIII and He filters.
An image of the Swan nebula taken by the Hubble Space Telescope. The top four images are taken through specific filters, which are then combined to create the bottom photographs. Using different filters creates a very different view of the nebula.
Dean Salman & the ESA/ESO/NASA Photoshop FITS Liberator

How to nebulae get their names?

Like many astronomical objects, most nebulae have multiple names. There are several catalogues that list nebulae along with other deep-sky objects.

The most commonly used are the Messier catalogue, the NGC (New General Catalogue of Nebulae and Clusters of Stars) and the IC (Index Catalogue). Most nebulae appear in at least one of these, if not all three.

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Many nebulae also have a popular name, which usually refers to where they are found (The Orion Nebula) or what they look like (The Witch Head Nebula).

The Witch's Head Nebula
The clouds of the Witch’s Head Nebula have an uncanny resemblance to a woman’s cackling face.
NASA/JPL-Caltech