Tuesday, October 15, 2013

Traveller Tuesday: Stellar Classification

One of the many ironies of running a science fiction game while being an artsy-fartsy type is that I lack the proper background in science to give the system a proper "crunch".  This is made worse by the fact that I have some degree of innumeracy -- specifically, my eyes tend to glaze over and my brain makes a droning noise whenever I see lots and lots of numbers together, especially without explanatory text.

Which is a complicated way of telling you that understanding Stellar Classification has boggled me for a while, because Wikipedia is fond of throwing out charts like this:

.. and even though I read the text again and again, I don't get it. I once had someone explain it to me using very simple terms, and I finally got it -- for a moment -- and then because I didn't immediately use it, I lost the knowledge. Which led to an embarrassing incident in my last game session, where I couldn't figure out where on the damn chart the system primary was, so I would be able to tell my players if it was a red dwarf or a red giant.

They probably don't care, but it's important to ME.  Mostly because, as I said before, I don't get the math, but I can do ambiance damn well, so setting details like "a low, angry red sun fills the sky" make the game more fantastic.

All of that is lead-in for a guest article by one of my players, Brook "Tanuki" West, who sent me a lovely article explaining how Stellar Classification works in simple terms, and now I can refer to it in the future.

In other words, Guest Post!

The following is adapted from a couple of posts I made on TravellerRPG.com back in 2005, where I posted as Tanuki.

Warning - long post follows.

There's a lot of misunderstanding about stars and related stellar objects, in part brought about by problems with the star generation parts of the original GDW Book 6: Scouts.

I attempted to clear some of that up in the article that follows, and offer a set of Star Generation Tables that, while still somewhat exagerating the number of unusual stars out there, does give a more realistic spread for gaming.

Unfortunately, some of the errors in Book 6 have propagated into more or less canon atlases of star systems for play. Anyone using these atlases may wish to tinker with the balance of main-sequence stars to giants and other star types.

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.

You will often see starts referred to as G2V or F5III. Stars are classified first by letters in the order (from hottest to coolest) O, B, A, F, G, K, M - which sequence may be remebered with the mnemonic "Oh Be A Fine Girl, Kiss Me." The letters are not in alphabetical order because they were assigned by astronomers who only had spectral information back in the early days of 19th century astronomy. This has been further refined by adding the numbers 0 through 9 to the letters. Thus G type stars (of which our star, Sol, is one) run from G0 through G9 (from hottest G to Coolest G). Sol is generally considered to be a G2. After G0-G9, we get K0-K9, and then M0-M9. M9 stars are the coolest stars actually undergoing fusion.

You will also see roman numerals, thus Sol is a G2V. The roman numeral suffixes indicate the relative size of stars. Type I stars are supergiants; stars that may be as large as our whole solar system. Type II stars are bright giants, type III are giants, type IV are subgiants. All these stars are in the last stages of their lives. Type V stars are main sequence stars, like our sun and this is where stars spend the majority of their lives. Others you might occasionally see are type VI stars (subdwarfs), and type VII (white dwarfs). These latter roman numerals are not used much though.

Subdwarfs (type VI) are rare stars, seen mostly out in the galactic halo, which have essentially no elements other than hydrogen and some helium (this is referred to as "metal poor") They are three to five times dimmer than expected of their spectral class, and will probably have nothing but captured asteroids or planets in their systems. They can be ignored for gaming purposes.

You will occasionally see small letters following the roman numerals. These may fine tune the classification further. Don't worry about them.

Main sequence is the state that stars spend most of their time in. Most main sequence stars are called dwarfs in contrast to all the giants out there. While giants are rare, they are so bright they can be seen for vast distances, so they were over represented by the 19th century astronomers who gave us much of our modern astronomical nomenclature.

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. Red dwarfs burn coolest and slowest so even though they have less hydrogen by mass, they last a long, long time. On the other hand, 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 bigger than 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 around the mass of our sun or smaller, 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 atoms 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.

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.

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:

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.

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

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.

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.

Class G Stars -- Sun-like stars - yellow dwarfs. Average about 0.9 solar masses. 6.5% or about 1 star in 15.

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.

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). By the way, recent research suggests that tide-locked planets with a decent atmosphere 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.

And now, a quick and dirty set of tables using (more or less) the percentages given in the post above:

Roll two six-sided dice (one high and one low) for each applicable table (the usual 6 x 6 Traveller matrix applies here).

Table One: Common stars

11-13 -- White Dwarf (burned out star)
14-53 -- class M (dim red star)
54-62 -- class K (dim orange star)
63-64 -- class G (yellow Sun-like star)
65 -- class F (bright white star)
66 -- class A (very bright white star) or roll on Rare Star Table

Table Two: Rare Stars

11-42 -- class A (very bright white star)
43-44 -- class B (huge blue-white star)
45-65 -- Red Giant (bloated dying star)
66 -- class O (ginormous blue-white star) or roll on Rare Giant Star Table

Table Three: Rare Giant Star Table
(roll one six sided die)

1 -- class O (ginormous blue-white star)
2 -- neutron star (cinder)
3 -- black hole (really crispy cinder)
4-5 -- Red Supergiant (really bloated dying star)
6 -- Blue Supergiant (dying ginormous blue-white star)

Really, all you need is the first table and even that over-represents class A stars. The last couple of tables make the rare stars much more common than they really are but will certainly do for game play -- trying to keep things simple.


  1. Well, I missed it on consecutive read-throughs, so the fault isn't entirely yours. Want me to fix it?

  2. The Evil Dr. Ganymede also has a good post on this subject. Check it out:



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