Posted by: Kash Farooq | June 18, 2011

Why is the Orion Nebula red?

The Orion Nebula is very photogenic:

Orion Nebula

Orion Nebula (Messier object 42 - M42) is a star forming region about 1300 light years from Earth. The Hubble Space Telescope used all imaging instruments onboard simultaneously to get the data to create this image.

And you don’t have to use Hubble to take great images of the Orion Nebula. This image was taken with a small 80 mm backyard telescope:

Orion nebula (M42)

'The Sword of Orion'. Image by John Love, used with permission. Click image for more information and a larger version.

Very pretty, isn’t it?

Inspired by something James O’Malley asked on Twitter, I want to answer the question: why is it red?

The Orion Nebula (also known as M42 – Messier 42) is an object that you may be able to see just below Orion’s Belt. You can easily see it with binoculars. Through binoculars it appears as a fuzzy/foggy patch, as if your binoculars aren’t properly focused.

M42 is basically a large cloud of gas with stars actively forming in it. It is known as an H II region.

The “H” in H II region stands for hydrogen, and the II means that the hydrogen is ionized – i.e. it has lost its electron (H I stands for atomic hydrogen – i.e. a hydrogen atom with electron present).  As the stars form they start emitting enough energy to strip electrons from the hydrogen atoms in the surrounding cloud. A hydrogen atom without its electron is essentially a proton – so the cloud becomes rich with protons and electrons.

So, we have a cloud of gas with electrons and protons zooming around it.

Now that I have explained what the cloud consists of, I need to backtrack a bit and introduce emission and absorption spectra. We can create a spectrum by passing light through a prism or diffraction grating.

When light passes through a thin gas, light at specific wavelengths is missing. The missing lines are known as absorption lines and the whole thing is referred to as an absorption spectrum.

Hydrogen absorption spectrum

Hydrogen absorption spectrum with missing light at specific wavelengths, and hence at specific energies.

So, something is removing the light at specific points. But what?

Imagine a hydrogen atom with an electron. The electron exists at its lowest quantum mechanical state of n=1 (known as the ground state). To help you visualise this, you can imagine the electron is whizzing around at an n=1 distance from the nucleus of the atom. This isn’t exactly how the electron behaves, but this explanation will do for now. If the hydrogen atom receives a very precise amount of energy (i.e. light at a specific wavelength), the electron will jump to the next state (n=2) and that precise amount of energy will be absorbed. This gives us an absorption line in the continuous spectrum we see above.

If the electron returns from n= 2 back to n = 1, it emits that same precise amount of energy as a photon of light. This time we see an emission spectrum:

Hydrogen emission spectrum

Hydrogen emission spectrum - the atom has emitted the same energy (and hence the same colour) it originally absorbed.

The same thing happens for, say, n=2 to n=3, or even for n=1 to n=3. However, for these transitions a different amount of energy will be absorbed (and emitted when the electrons return to a lower state).

Whether we can actually see an absorption or an emission spectrum depends on the angle we are viewing the light source from:

When do see an emission spectrum vs. an absorption spectrum

We see an absorption spectrum if we are viewing the source directly through the gas cloud, and an emission spectrum if we are viewing the gas cloud from an angle. We see a continuous spectrum if there is no intermediate material. Image credit: Freedman and Kaufmann, Universe.

Now back to our cloud of protons and electrons.

If the electrons and protons meet, they may recombine – the proton captures the electron to form a hydrogen atom again and the electron cascades down to the ground state. About 50% of the time the electron is captured into quantum state n=3 and quickly falls down to state n=2.

Electron moving from quantum mechanical state n=3 to n=2

Electron moving from quantum mechanical state n=3 to n=2, releasing a photon of light.

As the electron falls from n=3 to n=2 the atom emits the exact amount of energy that a hydrogen atom always emits when its electron makes this transition; a photon of light with a wavelength of 656.3 nm.

And light with a wavelength of 656.3 nm is red light. The right hand line you can see in the emission spectrum above is created by an electron transitioning from n=3 to n=2. This line is known as the Hα line (i.e. the hydrogen-alpha line). The whole series of lines is known as the Balmer Series.

And this is why the Orion Nebula is mainly red. There are lots of protons and electrons recombining, with many electrons transitioning from n=3 to n=2, and this transition emits a photon of red light.

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Space Scenery: Orion




  1. […] of Orion Nebula, since I found weird to have a blue and a red picture. And I found out that the real color is Red due to Halpha […]


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