Cold hydrogen atoms (not molecules) can perform a low-energy state change. If the single electron flips its spin, a very low energy photon is emitted. It has a wavelength of 21 cm, which is about 8 inches. This wavelength is HUGE compared to interstellar dust particles, so the dust does not interfere with the 21 cm energy as it moves through space. This means that we can "see" hydrogen clouds right through dust clouds if we are looking ar 21 cm.
The Milky Way Galaxy is interesting in 21 cm radiation. The plane of the Galaxy obviously contains a lot of neutral hydrogen.
There more complicated molecules out there. Remember that a molecule is two or more atoms of various elements stuck together to make a compound. Things like water, carbon monoxide and ammonia are found in the interstellar clouds. Not much of it, mind you, but it is there in small amounts. Each of these molecules can emit specific radio emission lines that allow identification of the molecules.
Stage 1 of a star's life is a cloud of interstellar medium that decides to contract. Actually, the cloud doesn't "decide" to contract; gravity makes it happen. The cloud may have very low density but it is very large, so it can contain many solar masses of material. If the density reaches a critical value, gravity will take over and the cloud will begin to shrink under its own gravity. See page 300. Note that physical theory indicates that the cloud should fragment as it collapses, making a some number of smaller units that continue to collapse.
So what makes the cloud density reach the critical value? Motion of the cloud could do it if the temperature stays low enough (10 K). At that temperature there is very little thermal pressure to impede density increase. Light pressure from nearby hot stars might compress the cloud. Blast waves from a star exploding nearby might also do it.
Gravitational collapse releases gravitational energy, which is converted to heat as the gas is warmed by compression. If the heat cannot escape, the temperature will increase until thermal gas pressure is sufficient to resist the gravitational force and stop the collapse. The cloud is, however, thin and transparent enough that any photons produced from this energy can escape without problem, thus carrying off the heat and keeping the cloud cool. The collapse will continue. See page 302.
The collapse goes on. After a few tens of thousands of years from the start, the core region finally gets dense enough from compresssion that it becomes opaque, and the heat radiated cannot get out. The central temperature begins to rise. The opaque region will develop a "surface" which we will call the photosphere, like the Sun's.
Study the text's description of Stage 4; it is very good. Note that this forming protostar, which is not yet burning hydrogen, can have a luminosity 1,000 times that of the Sun. Gravity can release a LOT of energy, and gravity is the ONLY energy source the protostar has at this stage.
A collapsing cloud can produce stars of all masses. It appears that the little M dwarfs (cool and small) are most numerous. Among the lot can be some objects of less than about 0.08 (1/12) solar mass. These little things never get hot enough to burn hydrogen; they collapse so slowly that their cores are able to lose enough heat so that the core never gets hot enough. They are called "brown dwarfs." The core becomes so dense that the atoms are packed close enough together that quantum forces stop the collapse before the core gets hot enough to burn hydrogen.
The Galaxy may be infested with brown dwarfs, but we will have a hard time proving it. They are so faint that seeing them is VERY difficult. We resort to detecting them by looking for them orbiting larger real stars by looking for Doppler shifts in the larger star's spectrum. This reveals the presence of the brown dwarf.
It is difficult, but sometimes brown dwarfs can be directly images. This is a photo of a brown dwarf orbiting around the star HD3651. It is an infrared image at 1.65 microns. The dwarf is about 480 AU from the star and has about 50 Jupiter masses.
Star clusters have a few properties that make them VERY useful for studying the formation and evolution of stars.
Here's a story that, in a way, illustrates the problem of studying the processes of stellar formation and evolution. Imagine yourself as the science officer of an interstellar exploration spaceship. Your mission is to explore the galaxy looking for life.
You have arrived at a promising G-class star that has a system of planets. Observations indicate that the outer planets are too cold and unfavorable, while the two innermost planets are too hot. You decide to check out the third planet and find obvious evidence of life. Close inspection reveals cities, roads, and a LOT of artifical radio radiation. This planet is definitely infested with intelligent life. Your job is to learn as much as possible about it.
Your means of data collection is to take pictures of the life forms - LOTS of pictures. You have about one rotation period of planet to do this, but your sensors are capable of doing it. When you are done you have a HUGE database of images of these creatures. As you leave this planet and continue searching, you will study these images and try to figure out as much as possible about these life forms. You'll be surprised at what you can learn.
The first step is to sort the images according to the appearance of the life form shown. Some are long and legless (snakes), some are small and have a lot of legs (insects), some appear to walk on four legs (LOTS), some walk on two legs, and so on. You also look carefully at their overall appearance. Most seem to be consistent - with one notable exception. The two-legged ones vary wildly in external appearance. Aside from the fact (that you notice) that they come in two definite types which have slightly different shapes, their external color and decor varies wildly. You see red, green, black, white, stripes(!), patterns(!), and other amazing designs. This, clearly, is an artificial outer covering, or garment, and you have found the intelligent life form.
Now what? Well, continued examination and classification of the images reveals that these beings come in a variety of sizes. The smallest ones observed are about 18 inches tall (long). None any smaller are seen. They come in a continuous range of height up to a maximum of about six feet. A few taller than that are seen, but not many. You have observed the life cycle of the beings. The smallest ones must be the young ones, particulary as they are almost never seen alone. These beings grow to a mature height of about six feet and then stop growing. You also notice that the two different type have different average heights.
If you continue to study, you might notice that the mature beings don't all look alike. Their skin and hair have different appearances. Some have smooth skin and some have rougher, more wrinkled skin. Some have dark hair of top while others have grey or white hair. Counting reveals that as this change progresses from dark hair to grey hair and skin from smooth to wrinkled, the numbers of beings decreases. This must be the end of the life cycle.
You have observed the beings' entire life cycle, from infant to mature to older and elderly. You can see how it goes. There are, however, two things about this life cycle that you will NOT get from ths anaysis. Note these carefully. You will not find out where the young ones come from (although your experience will other life forms might let you guess) and you will not find out how long the life cycle is in time. A one-day sample is not enough for that.
Astronomers face a similar problem: the formation and evolution of stars takes so long that we have no chance of watching it happen. We have really good data covering less than 200 years, which is an instant in astronomical terms. So how can astronomers figure out what is happening? The answer is: use physics. Even though a star is large, hot and harboring a vast nuclear reaction, it is a LOT less complicated than you are!
Physics can predict what a collapsing cloud will do. After all, it's only a cloud of hydrogen, helium, and small amounts of other stuff in roughly known quantities. Physical theories worked out over time predict the phenomena we observe. On the other hand, just try to use physics to predict whether someone likes to sleep late, prefers Budweiser to Miller, or likes Jay Leno.