P Cygni Profiles

P Cygni profiles are great diagnostic tools for anyone studying anything related to star formation. But, like with many astronomical tools and concepts, they aren't the easiest thing to research on the web with a simple Google search.  The knowledge I have now, the knowledge I'm about to share with you, was gathered over a couple of weeks from internet searches, textbooks, and conversations with professional astronomers.

What are P Cygni profiles?
P Cygni profiles are a spectral pattern named for P Cygni, a bright variable star in the constellation Cygnus. It's one of the most luminous stars in our galaxy (\(L = 610000 L_{sun}\)).  That's cool and all, but P Cygni is more than just a really bright star.  It has a massive outflow, which means matter is flowing away from it.  This outflow is the cause of the star's characteristic profile: a blueshifted absorption line and a redshifted emission line (I'll explain what those terms mean in the next section).

 

Example of a P Cygni profile from wikipedia's P Cygni page

 

Why do they exist?
The P Cygni profile is the result of the Doppler Effect.

Have you ever stood by as an ambulance passed you with its siren on? If you have, hopefully you'll remember what I'm about to describe. If you haven't, hopefully you have a good imagination. As the ambulance approaches you, the sound the siren makes gets higher-pitched. As it moves away from you, the sound gets lower. Why? Picture sound waves coming off of the siren and moving toward you through the air. When the truck is approaching you, the sound waves are getting pushed together, and because shorter wavelengths correspond to higher-pitched noises, the siren sounds higher. The opposite is true for the waves coming off of the back of the truck as it moves away from you.

That's the effect with sound, but it's more or less the same with light waves.

All of the information we get in astronomy comes from photons. In this case, photons are coming from the outflow and hitting our telescope. The gas along our line-of-sight (directly between us and the star), shown with the blue arrow, is being bunched together much like the sound waves in front of the ambulance, so the wavelengths of those photons become shorter. Any photons coming from gas that's moving away from us (shown with the red arrows) have slightly longer wavelengths than they would usually have.

Unlike sound, this doesn't result in a higher or lower pitch, or even in a lighter or darker color. This results in blue- and redshifted data, respectively. That just means that the features shift left (blueshifted) or right (redshifted) along the wavelength axis.

Now you might be thinking, "Okay, that explains why there are two bumps, but why does one of them go below the line and the other goes above it?"

Great question!  The one that goes below the line is called an absorption line.  This means that the photons we're seeing were absorbed by electrons on their way to us. In astronomy, we see absorption lines when there's cold gas between us and a hot source, or in this case, gas between us and a star.

The one that goes above the line is an emission line. This means that the photons were emitted by an electron.

How can we use them? 
So now you know what P Cygni profiles are and why they happen. That's only half the battle.  Now we have to understand what makes them so useful.

Imagine that, instead of one star, there were, let's say, billions of them. Hell, let's say we have a galaxy, or two. Let's go one step farther and say we have two galaxies and they're colliding. (We're done saying things now.)

When we observe these merging galaxies, we see a P Cygni profile, and that lets us know that there are outflows.  But outflows (or streams of matter flying away from an object) can be caused by more than just one phenomenon.  Star formation and Active Galactic Nuclei (black holes) both cause massive outflows. How do we figure out what's causing the outflows we see in our merging galaxies?

P Cygni profiles!  It turns out that AGN-driven outflows and star formation-driven outflows move at really different speeds.  We can find those speeds by measuring the intensities of the absorption and emission lines.

This is just one example of how P Cygni profiles can be used (it just happens to be the way that I'm using them in my research).  But no matter how you use them, P Cygni profiles have this tendency to provide both a question and its answer, and I think that's pretty damn beautiful.