I often refer to what we call the interstellar medium as the galaxy’s “backstage,” and I do that for a reason: for the most part, we can’t see it.
The backstage of any theater isn’t part of the show. You, as part of the audience, never see it. But you see evidence of it, when new props appear as the play progresses through scene after scene and the actors interact with their backstage.
The same thing happens with the interstellar medium. It’s not the hidden area behind the stars of the galaxy. (Ha, get it? Stars?) In fact, more often than not it’s actually the one hiding stars from view. But we can’t see it…unless we study how stars interact with it.
One way to do that is to look at reflection nebulae—evidence of the light from bright young stars reflecting off the dust of the nebula. That qualifies as interaction.
And in the case of emission nebulae, hot O-type stars ionize the hydrogen gas of the nebula. I’d say that’s interaction, too.
Even dark nebulae can technically be seen, since we see them as shadowy clouds silhouetted against background nebulae or stars.
But sometimes, it’s not that simple. Sometimes, we have to rely on the galaxy’s props to guess at what must be stored backstage. And that means studying stellar spectra.
Here, it looks like the chromosphere is the visible surface of the sun, with the photosphere just below. Really, we never see the chromosphere. If you ever look through a solar telescope at the sun, the photosphere is the surface that you see.
The sun is structured a lot like the Earth, just in that it has a core, a dense region between the core and the surface, a “surface” layer, and a few atmospheric layers. The chromosphere is part of that solar atmosphere. And you never see it.
Meet the sun: a G2 class star towards the middle of its lifespan.
Wait a second…G2? What does that even mean?
It’s all part of a way astronomers break down the billions of stars in the sky and organize them by temperature. They can use a star’s spectrum to figure out what it’s made of, and that helps them figure out how hot it is.
But really…being able to read stellar spectra (plural for spectrum) is only so helpful. There are billions. It helps to have an organizational system.
That way, if an astronomer sees a stellar spectrum that looks a certain way, they can know immediately that it’s a certain class of star.
Astronomers know that if white light passes through a prism and is bent, it’s separated out into its component colors—the colors of the rainbow.
Astronomers also know that when light interacts with atoms, the building blocks of the universe, the atoms absorb photons of light and reemit them—but in a different direction.
Put these two bits of knowledge together, and astronomers now have everything they need to understand spectra (the plural for spectrum).
A spectrum is something I’ve covered in previous posts. In astronomy, it means the wavelengths of electromagnetic radiation spread out so we can analyze them individually. And it’s an astronomer’s most valuable tool.
So, what exactly is a spectrum, and how can we use it to analyze radiation from space and learn more about the universe?