Starship Asterisk*

The 3/14/05 APOD and a multitude of other images have made me wonder: What makes so many nebulae red?

Does anyone know?

You can break down nebulae into two groups: emission and reflection. The Trifid Nebula (M20) contains examples of both.




The reddish-pink color signifies an emission nebula. The cloud is largely made of H2. When ultraviolet light from nearby hot O stars strikes the H2 molecules, the electrons in these molecules are forced to higher energy levels. When the electrons return to their original levels, energy must be released. Every atom that releases photons this way does so in a specific wavelength; H2 happens to emit photons in the wavelength we see as red.

Emission nebulae work in much the same way as neon lights or flourecent bulbs...you could say, the biggest flourecent bulbs in the universe... 8)

The blue color is caused by dust clouds that scatter the blue part of star light in the same way that our atmosphere scatters the blue part of sunlight. The lower frequencies of light keep on going, while the light with the frequency that coresponds to what we see as blue goes in all directions (and looks beautiful in photographs). The key to scattering is that the particles in the cloud are the same size as the wavelength of light.

Scattering works sort of like this: the photons "bump" the dust particles and "shake them up" going off in random directions...however the dust does not actually absorb and reemit the photons.

Hope that helps.

My favorite example of the scattering of blue light (and one of my favorite astronomical images of all time) is the Pleiades Cluster (M45):

Note there is also a third group, absorption.

In the image posted above:

The dark areas are masses of dust absorbing light.

True indeed! And it's those dark, hot, dense regions of nebulae where protostars are formed.

I thought I'd add that several years ago the red emulsion layer in some color films was able to capture light over longer periods of time relative to the other two emulsion layers resulting in inaccurate coloring of nebulae. This was common in many older astrophotographs including the nebula in Orion which were often overly pink whereas more recent and color-accurate images depict some of the other hues present in this nebula. This characteristic of film is called reciprocity and is always taken into consideration (quantitatively) when new films for astrophotography are evaluated by labs. I still remember when I first saw this nebula through my new 10" reflector and it was greenish!

What makes a green color? Why do some of these particels absorb light and some reflect light?

Acutally, the red is not from H2 molecules, but rather HII (pronounced as "H two," thus the common confusion), or ionized ATOMIC hydrogen gas which is recombining.

As for the green emission nebulae, the green light comes from ionized oxygen. Same process, different wavelength.