Defining a Molecular Cloud

I'm going to try to answer a few questions [this post is in-progress and will be updated]:

1. Why are molecular clouds called molecular clouds?



2. What distinguishes a molecular cloud from similar objects (e.g. cores, clumps, HII regions)?



3. What do molecular clouds look like?


4. How do astronomers look at molecular clouds?


5. Why are astronomers look at molecular clouds (why are they interesting)?


6. What do molecular clouds have to do with star formation and planet formation?


7. How are molecular clouds involved in galactic evolution?

1. Molecular clouds are regions in space with densities high enough and temperatures low enough that molecules can form. The most common molecule is Hydrogen (H₂), simply because hydrogen is the most common atom. The next most common is Carbon Monoxide, CO, that is ~10⁴ times less common. It is the easiest to observe, though, because H₂ can only emit light and energy in 'forbidden' transitions that don't happen very often.

2. Size, temperature, and composition all distinguish molecular clouds from other nebulae.

An H II region is very hot - in it, hydrogen atoms are ionized by radiation from a nearby O-type star. In regions hot enough to ionize hydrogen, all molecules are destroyed unless they have already collected into larger dust particles.

A core is a much denser bunch of gas than a molecular cloud. Cores are dense enough that their outsides shield their insides from the radiation of the rest of the universe and their insides cool to very low temperatures. At low temperatures, the pressure supporting the cloud against collapse is lower, and stars can form via gravitational collapse.

I don't really know what clumps are... they're supposed to be something intermediate between cores and clouds, but I don't know what their distinguishing features are.

3. It depends on what wavelength you're looking in. In the optical, where our eyes can see, they look dark - they're seen because they absorb light from behind them. In the infrared, the hot ones glow, but the cold ones are still invisible. If you go all the way to the millimeter, all molecular clouds glow, no matter how cold, but in order to see them there has to be a lot of material. I'll add some pictures here later.

4. Lots of ways. Optical telescopes aren't the best choice, though. With near-infrared images, we can detect molecular clouds by counting stars and noticing when there aren't as many in some regions. This technique is called "NICE" and works because there are so many stars in the sky and dust in molecular clouds includes some, but not all, of their light. In the far infrared, we can see hot dust glowing, but this is very difficult because our atmosphere is opaque in the infrared - it's like trying to look through a brick wall. We need telescopes in space to be able to see anything at these wavelengths. In the millimeter, there are some 'holes' in the atmospheric absorption, sort of like color filters, that we can see through to detect the coldest emission.