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):
You can break down nebulae into two groups: [i]emission[/i] and [i]reflection[/i]. The Trifid Nebula (M20) contains examples of both.
[img]http://antwrp.gsfc.nasa.gov/apod/image/0204/trifid_aao.jpg[/img]
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):
[img]http://antwrp.gsfc.nasa.gov/apod/image/0312/m45kite_gendler_c1.jpg[/img]