Emission Nebula

Emission nebulae do just what the name suggests: they emit radiation. We can see these by the spectrum that we observe as we look at these clouds.

Emission nebulae are clouds of gas and dust that surround or are in front of a hot star. The radiation from the star heats up the surrounding cloud, exciting the electrons in the atoms of the gas and dust. As the electrons lose energy, they emit radiation.

These nebula are typically red because of the strong hydrogen-alpha (Hα) lines which occur when an electron falls from second excited state (n=3) to first excited state (n=2).

Emission nebula always found near stars because they must emit radiation to glow.

Rosette Nebula, an emission nebula in Monoceros (Rhinoceros)

Image Credit:

"NGC 2244 Rosette Nebula" by Andreas Fink

Spectroscopy

How can we tell one star from another? We use what is called the spectrum of the star. Recall from before that we discussed the spectral type of a star (here and here) and how that tells us what the temperature of the star is. Well, what exactly is a spectrum?


In a nutshell, the spectrum of a star or any light source is just all the wavelengths of electromagnetic radiation spread over all frequencies. When the light source has a spectrum without breaks in it, we call that a continuous spectrum. In reality, continuous spectra do not exist in nature. Some spectra are composed of discrete lines and they are called emission spectra. And finally, there are spectra that have gaps in the continuous spectra which are called absorption spectra.

 

So what exactly creates an emission spectrum or an absorption spectrum? The electrons in atoms have discrete energies that they can be at, i.e. that can't just orbit around the nucleus of the atom at any distance. We call these discrete energies quanta and these electrons at these quanta can either absorb a photon (if the photon has the right energy) or emit a photon if the electron is in the "excited" state.

Energy States for a Generic Atom

Absorption of A Photon

Emission of A Photon

 

 

What is unique about spectra, is that all element have a unique absorption or emission spectrum. By looking at where certain lines are, you can determine what elements are in the star. The strength of the gaps or the lines will depend on the concentration of the material in the star or cloud. The measurement of the spectral lines for a star help us determine the spectral type as well as how the star is moving (using the Doppler Effect).