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I’m sure we’ve all seen the breath-taking imagery from the Hubble Space Telescope of
beautiful and multicoloured nebulae.
It almost seems as if a contemporary artist flicked different colours onto a canvas.
But this isn’t just a pretty picture.
What you are seeing here are actual structures, tens to thousands of light years across.
But how are these images so colourful?
Does it really appear like that in real life?
To answer this question, let’s give you some context.
Nebulae may appear solid, or maybe even like clouds, but that is far from reality.
Nebulae are extremely diffuse gas clouds; on average you may get about 10-100 particles
per cubic centimetre.
This is far less than any artificial vacuum produced on Earth.
To give you a sense of how little that really is, a comparable cubic centimetre at sea level
on Earth contains 10 quintillion particles.
That’s 10 to the power of 19 particles.
On the other hand, if you were to have a nebula the size of Earth, it may only have a total
mass of a few kgs.
Orion, perhaps the brightest nebula in our sky, is just about bright enough to be seen
with the naked eye on a clear night.
However, nebula can be a lot brighter too.
If the Tarantula Nebula, found in the Large Magellanic Cloud, was as close to us as the
Orion Nebula is, it would even be visible during the day.
But if nebulae are so sparse, how can we see them at all?
It’s important to remember the scales that are at play here.
While nebulae are sparse, they are also massive.
HII regions, which are a specific type of nebula, are known as stellar nurseries.
These massive clouds can collapse and coalesce, and when they do, they form stars.
A single nebula can contain enough mass to create hundreds to thousands of stars, and
it is actually these newly formed stars that light up the gas in a nebula.
Solar radiation shoots away from stars from within the nebula, ionising the nebula’s
particles, which releases this energy as a specific light wavelength.
This means HII regions are known as something called emission nebulae, because ionised atoms
within are emitting their own light.
The process happening in an emission nebula is comparable to what happens in a neon light,
where electricity ionises neon within the bulb, causing it to light up.
However, in a nebula, it’s not neon that lights up.
In fact, in the Hubble images, each colour shows a different ionised atom.
Unfortunately though, I can’t tell you a blanket rule for colours in Hubble images.
So, while in this image of the Carina Nebula reds correspond to sulphur atoms, greens to
hydrogen, and blues to oxygen, in this image of the Swan Nebula, blues correspond to visible
light blues, greens to oxygen, and reds to infrared and hydrogen.
That’s because of the way Hubble takes its photos.
Hubble is a primarily a visible light telescope, but its optical range does also extend into
the ultraviolet and infrared.
When scientists look at a nebula, they often want to see what it is made of, so will photograph
the object using specific filters.
So, say they want to see hydrogen in a nebula, they will image the nebula using a filter
that lets light being emitted along 658nm wavelengths through, or in other words, the
emission band of hydrogen.
After that, they are left with a black and white image.
Then, they may decide to image the nebula again, this time looking for sulphur along
672nm wavelengths, and then again looking for oxygen along 501nm wavelengths.
Scientists then assign colours to each of these different photos, and combine them together,
to produce the colours you see here.
As you browse Hubble images on their website, you can see for yourself what the colours
have been assigned to by looking to see what wavelengths of light were used.
For Hubble images of nebulae, they are pretty much always a false colour.
However, Hubble does also take natural colour images too.
The Sombrero Galaxy, for instance, had three images taken in blue, green and red, or RGB.
So, the colours you see here are how you would also see this galaxy, were it bright enough
for you to see it.
So, if Hubble’s images of nebulae are false colour, what would they really look like to
us?
Generally speaking, a lot more red.
The European Southern Observatory also looks at a lot of the same objects Hubble does,
although often using different colour filters.
Going back to the Carina Nebula, on ESO’s website we see that the colour bands are RGB,
or natural light, so we known that what we are seeing here is the Carina Nebula as we
would see it.
Here’s ESO’s version of Orion, except this time with ultraviolet and hydrogen overlaid
on top of natural colours.
So far, we’ve only discussed the most famous nebula type, HII regions, or emission nebula.
However, it’s worth mentioning that there are a few other types of nebula too.
You also have reflection nebula, where the energy from stars isn’t enough to ionise
the nebula, instead it reflects or scatters the starlight, meaning these nebulae tend
to have similar frequency spectrums to nearby stars, often appearing slightly bluish.
You also have dark nebulae, or nebulae with no stars around them to illuminate them.
These appear almost pitch black.
All in all, nebulae are not as colourful as you may have initially thought, but to me,
that doesn’t make them any less interesting.
Looking through these filters means we can understand a lot more about nebulae than we
otherwise could have, and they do make for beautiful images!
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