Presentation: interstellar medium

In previous presentations we've discussed the sizes of stars, and their source of energy, but consider the space between stars--it is not empty, but contains clouds of hydrogen gas and dust... (Video link: "Stars and Nebulae.")

...which may sound rather untidy, but this is a good thing, as the hydrogen gas in these clouds may eventually form new stars, and the dust may become planets around these stars. (Video link: "Zooming into NGC 3603.")

In this presentation we will discuss the different types of clouds in the interstellar medium, and how stars form these clouds.

First, distinguishing between three different types of clouds, or nebulae.

An emission nebula is a cloud of hydrogen atoms, illuminated by a nearby star. We have an observer in space, with a spacesuit on (because that was important to some students). Photons with sufficient energy from the star will be absorbed by the hydrogen atom's electron, freeing it. As the electron falls back down to lower orbitals, it may give off a violet photon, or a blue photon, or a red photon (in the visible light range, among a multitude of other possible photons). These violet, blue, and red photons are perceived by the observer's naked eye as a pinkish lavender color.

Conversely, if you see a pink cloud in space, then you know that it is composed of predominately hydrogen.

A reflection nebula is a diffuse cloud of tiny dust particles, similar in size to cigarette smoke particles. Again a nearby star illuminates this cloud, but short-wavelength blue light and long-wavelength red light behaves differently. Red light kind of just "wiggles" through the dust cloud, but blue light gets scattered in all directions. To the observer's naked eye, the cloud will appear blue--much like cigarette smoke does.

Again, conversely, if you see a blue cloud in space, it must be composed of diffuse, tiny dust particles.

A dark nebula is a thick cloud of large, clumpy dust particles, thick enough that light from a nearby star...is completely blocked by the cloud.

So if you see a dark splotch in space, it is composed of thick, clumpy dust particles. (Or would it instead be a "rip" in space? What type of light would you look for to convince you that there are stars being blocked behind it, instead of "nothingness" behind it? Extremely short wavelengths, or long wavelengths?)

Let's start our picto-quiz--what type of nebula is this? What is it made of?

What type of nebula is this? What is it made of?

What types of nebulae is this? What two types of material is it made of? By the way, this is also known as the "Horsehead Nebula"--like a chess piece horse, instead of a My Little Pony^TM horse. Or does it look more like a Dementor from J. K. Rowling's Harry Potter book series?

What types of nebulae is this? What parts of this is composed of what? Do you see "Thor's Helmet?"

What type of nebula is this? What is it made of? When you look at this nebula, what does it look like to you?

Second, star formation in the interstellar medium.

Note only pink, blue, and brown/black colors in this cloud. Any other colors that you see in real and simulated views of outer space are "fake" in that they are either enhanced false colors, or mere artistic impressions. Think about that when you pick up a science magazine, or what a documentary or science-fiction movie on television.

A molecular cloud is cool enough that atoms and molecules are moving slowly such that if they collide, then will bind to each other rather than bounce of each other. If you wait long enough, these molecules will form larger and larger clumps, such that their gravity will get stronger and start pulling in even more material, to build stars (and perhaps planets), given enough time. This process is "self-starting" as given enough time, a cloud will eventually settle down and start building stars, although this will be excruciatingly slow. (Video link: "Star-Forming Region.")

Can't wait that long? As we'll see in a later presentations, some stars will explode as they die. The shock wave from this explosion as it expands out into space will force clouds of gas and dust already present to smash together and "kick-start" the formation of stars (and perhaps planets). (Are these "true" or "false" colors in this computer simulation?) (Video link: "The Veil Supernova Explosion.")