A very hot and devastating fire, maybe a volcanic eruption, burns down the forest. Almost everything is killed, even meters into the soil all the roots, tubers and seeds are incinerated, even the bulk of the bacteria are sterilized. The soil is naked and exposed to the weather and rapidly erodes. But soon, fast-growing grasses and other ground cover move in, stabilizing the ground and rapidly replacing the topsoil and the community of microbes that make it a suitable habitat for plant life. Quick-growing trees are next to colonize the burn, trees that are tolerant of sunlight and poor soil and drainage and which grow quickly, establishing themselves. Later, in the shade of this first generation, trees that are more tolerant of shade appear and flourish. They in turn, cast their own shade, making it difficult for the sun-tolerant species to continue to spread. Eventually the forest achieves a relatively stable state, with certain populations of certain species–what foresters call a climax forest. This all happens too slowly to be noticed by the casual observer–we simply don’t live long enough. To the average man, only a snapshot of the process is visible in his lifetime, only by comparing many forests is it possible to reconstruct the history. But foresters know this, because they study many forests and have learned to untangle this evolution.

Something analogous to this happens with stars. The analog to the fire is the cloud. A massive cloud of hydrogen gas floating in the galactic disk is affected by some passing event. It might be the shock wave of a nearby supernova, or perhaps the passing of a gravitational density wave (we call them “spiral arms”), racing around the galactic disk, or even the gas pressure of another galaxy colliding, passing through the disk. The shock of this event compresses the gas, and in places along the cloud gravity takes over, causing the cloud to start collapsing differentialy. Some clumps, depending on the gas density, collapse faster than others, and gravity takes over. A process of hierarchical aggregation begins, big clouds merge and collide, split up and reform, disperse and accumulate. Its all driven by gravity, the cloud is massive, but of very low density, and after the initial disturbance it starts clumping together. In some places, stars form, they condense out of the collapsing cloud like raindrops in a thunderstorm.

What are stars? They are defined as collections of gravitationaly collected gas massive enough to collapse into spheres of hot gas, and eventually plasma. They glow with the gravitational energy released by the infalling gas until densities, temperatures and pressures in their interiors produce thermonuclear fusion in the hydrogen, converting it into helium. The back pressure of this reaction stops the gravitational collapse, and the star reaches equilibrium. The light escaping at its surface forces away the remnants of the infalling gas and what remains is a continuously exploding thermonuclear bomb which will remain stable until it runs out of hydrogen fuel to burn.

If the star wasn’t massive enough to generate the conditions for fusion in its core, and if it fails to receive extra material from the surrounding cloud, it will remain as a ball of hot gas, slowly cooling and radiating its gravitational energy into space. These we now call “brown dwarfs” or “failed stars”. Even smaller (i.e., less massive) failed stars will become objects somewhat similar to our own gas giant planets. Depending on their mass and age, they may be hot and bright (“hot Jupiters”), still unloading the gravitational potential energy of their formation, or like our own Jupiter, be very cold. The dividing line between brown dwarf and the least massive, but still stellar, red dwarf appears to be about a tenth of a solar mass. Below that, you get a failed star, above it, you get a hydrogen-burning self-powered thermonuclear power plant. That is a star.

We know very little about these failed stars because they are so faint and difficult to detect. How many there are, and what proportion of the stellar population they make up, is still a mystery. But red dwarfs are easier to spot, especially nearby, and we have collected enough statistics to know they are the most common stars by far. Most of the Galaxy’s mass is tied up in red dwarfs. In fact, we have learned that the least massive stars form in the highest numbers, and the more massive a star is, the rarer it tends to be. We call this the mass function, a formula that gives the number of stars as a function of their mass at birth. The mass function is a power law, it tells us really massive stars are very rare. Internal instabilities prevent stars much bigger than about 100 solar masses from forming. In fact, although our own sun is nearer the bottom of the mass range, it is much more massive than most of the other stars in the Galaxy.

When all stars are born, they quickly (after a few million years) settle down into a stable state called the Main Sequence. This is named for a line on the Color-Magnitude, or Hertzprung-Russell
diagram that plots stars’ temperature vs their brightness. Stars on the main sequence tend to stay there, relatively stable, until they run out of hydrogen in their cores and then are forced to evolve, that is, they alter their internal arrangements and structure until other products of hydrogen fusion can be fused as an alternative energy source, or until they eventually die. So a main sequence star (like our own Sun) will essentially stay there for most of its lifetime, until it runs out of gas. After that, it evolves off the main sequence and the fun starts. Our own sun has been on the main sequence for about 4.6 billion years, and it probably will remain there for another 5 billion years or so. During that time it will remain relatively stable in its mass, energy output, brightness, color, temperature, size and surface activity. After that, it will change all those characteristics drastically, sometimes catastrophically, as it tries to sort out its internal instabilities. After a star evolves off the main sequence, its days are numbered. And they will come fast and abruptly.

Now all stars eventually start off on the main sequence soon after birth, and eventually they all evolve off of it, but while on the M-S they will remain more or less unchanged. Now, where on the M-S they happen to wind up depends on their mass at birth. That is, the least massive M-S stars are at the cool, faint end, and the most massive at the hot, bright end. And how long they remain on the M-S also depends on their initial mass. The most common red dwarfs, the lightweights, will remain on the M-S for many billions of years, the massive bright blue giants will evolve off the M-S after just a few MILLION years (usually by going supernova!). Middle-of-the-road stars like our sun will stay on the M-S for several billion years, after which they will swell up to become red giants, go through a period of erratic variability, a planetary nebula phase, and eventually become little gravitationaly powered faint red dwarfs and fade into obscurity.

I’ve given you enough key words here that with Wikipedia and Google you can go as deep into this as you like. Most of what I’ve outlined here is the result of 20th century astronomy, and it is truly a triumph of the human intellect. Considering there have never been more than a few thousand astronomers working in the world at any one time, this narrative I’ve described is something the human race can be really proud of. We took a walk in the climax forest and we figured it out. Maybe not totally and completely, but we have a vague idea how it all fits together. I’m really proud of us, as a species, for having been able to do that. And I hope I can communicate some of that pride to you by sharing what I know of it.