WARNING: very long post follows
Originally posted by Mindseye:
That's really the gyst of my question. In the World design sequences, there is a 1 in 1.25 chance of a Red Dwarf star(which will have baked any planets it had) and a 1 in 33 chance of a Main Sequence star. I get the idea that "normal" stars should be uncommon, but on average 1/subsector? What's the point? If someone has a better ratio (die result range) I would love to hear it.
I think you're a little confused here. Red dwarf stars are main sequence stars, just down at the low end mass and luminosity-wise. They are not going to be baking any planets unless said planets are
exceedingly close to them.
Main sequence stars run from very hot and bright (O, B, and many A stars) through sunlike stars (dimmer A class stars, and G stars) and down to dimmer and (relative to the sun anyway) very dim stars (class K and M stars). Main sequence stars are sometimes referred to as dwarfs in contrast with giant and supergiant stars. So you will see sunlike stars sometimes referred to as "yellow dwarfs" and class M stars referred to as "red dwarfs" -- they're relatively dim and red.
Main sequence is the state that stars spend most of their time in. The larger a star is, the hotter it "burns" (actually fusion, of course) and the faster it goes through its supply of hydrogen and other fusable elements. As RainofSteel pointed out, red dwarfs burn coolest and slowest so even though they have less hydrogen by mass, they last a long, long time. OTOH high-mass stars are the brightest and hottest and are short-lived -- the biggest of them may spend less than a million years on the main sequence, while the dimmest stars will be there for many billions of years.
Once a star has burned a significant portion of the hydrogen at its core into helium, there is no longer enough hydrogen to sustain energetic fusion at its core. With less energy being produced, gravity begins to pull the star into its core. As it collapses, the core heats up due to compression until (if the star is big enough) it gets hot enough for helium to begin to fuse into heavier elements (otherwise it continues to shrink and to cool as a "white dwarf" until it someday becomes a "black dwarf" -- a slowly cooling ball of densely compressed gas).
Helium fusion produces much more energy than hydrogen fusion does so once helium ignites at the core of the star it expands to be much biggerthan it was during its main sequence phase, thus becoming a red giant.
That's when it starts baking planets. The star goes through it's supply of helium much faster than it went through its hydrogen so this phase is much shorter than the main sequence phase. If the star is much less than the mass of our sun, once it burns through it's helium (producing oxygen, nitrogen, and carbon) it will slowly dwindle down into a "white dwarf" and on towards a "black dwarf." This is the eventual fate of our sun.
If the star is several times the size of our sun it will produce enough heat as it collapses to fuse carbon, oxygen, nitrogen, etc. into even heavier elements and it will expand again out to super giant size. These super giant stars will eventually reach the point where they are fusing stuff into iron at their cores -- this is fatal. Fusion into iron or heavier elements takes more energy than it produces. The reaction sucks energy out of the core of the star and within literally minutes it begins to collapse upon itself. Once everything comes crashing down into the core you end up blowing the star apart in a supernova and the remnant left behind after all the outer layers are blasted away will become a neutron star, or if the star was
very large, a black hole -- surrounded by a rapidly expanding shell of gas and debris.
Those last few moments are where virtually all the elements heavier than iron are created by fusion and then blasted out away from the star to eventually form worlds and all the other non-star stuff in the universe.
Now, how common are various kinds of stars? A principle you'll see throughout nature is that the bigger something is, the fewer of it there are. Given a square mile of forest, there will be uncountable zillions of microorganisms, million of insects, thousands of field mice, dozens of rabbits, and a couple of deer.
There are more grains of sand than pebbles, more pebbles than boulders, more boulders than hills, more hills than mountain peaks, etc.
In the solar system there are vast numbers of meteoroids, billions of small asteroids, dozens of worlds and moons, four terrestrial planets, two small gas giants, a couple of successively bigger gas giants, and one star.
Stars -- There are vastly more dim stars (red dwarfs) and almost stars (brown dwarfs) than sunlike stars, vastly more sunlike stars than really big stars, and the red giants and super giants are exceedingly rare.
Numbers:
Blue and Red Supergiants (type I & II) -- massive (like, freakin' HUGE) stars at the end of their lives. 0.000025% of stars. About 1 in 4,000,000 stars.
Red Giants (type III) -- Stars nearing the end of their lives, fusing helium, oxygen, nitrogen, carbon, etc. at their cores. Average mass is about 1.2 solar masses (sols). 0.5% of stars or about 1 star in 200.
White Dwarfs -- Burned out stars, average about 1 solar mass. 8.75% or almost 1 in 11 stars.
Black Holes and Neutron Stars -- Cinders. Exceedingly rare. Perhaps .001% and .0001% respectively (I don't have actual numbers here) That would be 1 in 100,000 and 1 in 1,000,000 stars.
The rest are basically main sequence stars. I'll break those down by class:
Spectral Class O Stars (Blue Giants) -- These are so big, even as main sequence, that they're called giants. They lose whole suns worth of mass as solar winds (solar hurricaines?) blowing their outer layers off as they age. Lifespans of only thousands of years before they die spectacularly. They average about 25 solar masses. 0.0000025% or 1 star in 40,000,000.
Spectral Class B Stars -- Average about 5 solar masses. Still very hot and very short lived. Probably not enough time in their lives for planets to form out of the gas and dust orbiting them. 0.075% or 1 star in 1300
Spectral Class A Stars -- Still too hot, big, and short lived for life-bearing planets to have time to form around them -- life might get as far as oceans of yeast and stuff before they die. Average about 1.7 solar masses. 0.75% or 1 star in 130.
Spectral Class F Stars -- The hottest stars likely to harbor life-bearing planets. Average of 1.2 solar masses. 3% or about 1 star in 33.
Spectral Class G Stars -- Sun-like stars - yellow dwarfs. Average about 0.9 solar masses. 6.5% or about 1 star in 15.
Spectral Class K Stars -- Cooler and dimmer than our sun. Smaller ones will probably tide-lock planets in their habitable zones. Average about 0.5 solar masses. 13% or about 1 in 8 stars.
Spectral Class M Stars -- The smallest and coolest stars -- red dwarfs. Habitable planets will be tide-locked except under very unusual circumstances (like Mercury's 3:2 lock with the Sun). BTW recent research suggests that tide-locked planets with a decent atmospher can be very life-friendly -- they may be the most common kind of life-bearing planets in the universe. (But that's for another post.) 67.5% or about 2 out of 3 stars.
Brown dwarfs -- Bigger than jupiter, too small to sustain fusion, these are very hot compared to planets (glowing red) but cool compared to stars. We don't really have a count for them since they're hard to find (so very dim compared to stars) but the proportion of brown dwarfs to stars (of any kind) is probably at least a similar ration to that of red dwarfs to all other stars -- say perhaps 3 brown dwarfs for every star.
