Starting with Stars

TOC

1. Why Stars?
2. I Give You...the Stars!!!
   • Hertzberg-Russell (H-R) Diagram
   • What H-R Diagrams Mean for Worldbuilding
   • All UR Stars ᴙ Belong 2 Us!
3. What the Numbers Mean
   • Stellar Luminosity
   • Stellar Radius
   • Stellar Mass
   • Planet Distances
   • Absolute & Bolometric Magnitude
     • Apparent Brightness
   • Amount of Life
4. Examples
   • Example #1
   • Example #2
5. Summary

Why Start with Stars?

tl;dr: Because you have to start somewhere, and stars are important for everything.
My tendency for most things is to look for root causes and the most basic fundamentals. To put it in another way, I don't just wonder about things and stuff, I wonder what is behind those things and stuffs, and then go on to wonder what lies behind those.
(Warning: here be speculative digression)

Perhaps that is the reason behind why I've always been attracted to both worldbuilding, science, culture, religion and spirituality—although more for the joy and wonder of the mystery than to find answers and control... see what I'm doing there? I can't help it.

As I have meditated and created worlds, systems, and cultures, that tendency to ask "What is behind that?" has led me to come to a conclusion that the most fundamental part of building worlds starts with a star—a sun. In a nutshell, everything on a world—including cultures, peoples, animals, plants, all life, resources, geography, etc.—depends on that planet. And that planet was created and depends on the star that made it and that it (hopefully) still orbits. Going the other way from top-down, the galaxies and clusters all come down down to stars as their fundamental units. Everything is made of stars or former stars.
So why not focus on atoms? Well, just about everything that is was made in stars. Since the Big Bang, when only hydrogen and maybe some helium were all the normal matter that made the universe, its first population of stars created all other elements in their gravitational forges. But still, why stars? Why not clouds of gas and dust? Why not dark matter and dark energy, which make up most of the universe? Good questions. So we'll relate it back to the making of worlds. We know hardly anything about dark matter and energy, and the matter we do know about comes from and is influenced by stars. The beautiful stellar gas and dust that make nebulae are important, but we do need something a bit more than just matter. So I would like to focus my theory like this:

A star is that fundamental point where matter and energy come together in its most basic form in the universe to do stuff.

In a way, it's kind of like Newton and Einstein. The vast majority of our daily lives does just fine in a Newtonian perceptional reality, from driving automobiles to dropping anvils. If we need to travel at relativistic speeds, we can get out Einstein's fine china, but we need to begin somewhere in our understanding. We can built up to it. It's kind of a practical matter. Once we understand the rules, then it makes it easier to bend them. Break them.
I just want to reiterate: a star is where things do stuff. That means energy. Everything on our planet is not just matter, but energy, and how it flows (or doesn't) makes a world of difference.
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Q: But I'm making a fantasy world. Does this still apply?

tl;dr: Yes, especially if your world has other differences.
Yes. Even if most of your fantasy world takes place underground, you are still on a planet and that planet is most likely going around a star. Remember, everything is connected, even the seemingly significant things your denizens don't see or hear or smell. And even though your planet might be very Earth-like, if it has something like a longer year or if your sun is green or larger in the sky than Earth's, there are consequences and reasons for that. You might want to investigate why that is, what happens if that is the case, and how that impacts other aspects of your world.
In addition, by exploring how all of the parts of your world interact, even the ones that don't seem at first all that relevant, can lead to new ideas.
It might seem like a lot more work, but coherent systems that your readers and/or players engage in are more likely to be easier to accept, play in, and expand. Nothing kills the buzz like when your willing suspension of belief is shattered in a "wait...what???" moment. The work will pay off down the road.

Q: But I'm making a world orbiting a black hole singularity!!! Does this still apply?

tl;dr: Yes. More so!
Yes, because at one time, it was a star. The properties of radiation (if not outright luminosity) and especially gravity still follow many of the same laws of physics. If you want to know how to play with an ultra-massive space object, knowing something about stars is a good way to go!

Q: But I'm making a world set on an asteroid floating in between stars in the cold, cruel absence of all light and heat in the Void. Does this still apply?

tl;dr: Maybe. But it wouldn't hurt! Much...
You got me there. Good luck with that. But knowing something about stars might still be recommended. Interstellar space is devoid of a lot. And...well...that can get a little boring, don't you think? If your asteroid is something like Oumuamua, you'll definitely want to know something about stars!

Q: But my worldbuilding happens all in underground caverns. Who cares what the sun looks like in my world?

tl;dr: You and your critters and characters should.
Whatever star your world orbits is still going to be a factor, even if your characters don't have a clue that there is a ball of nuclear gas igniting a few million miles away. Your planet, moon, comet, or asteroid was made in a star. And whether it orbits very close or very far away is going to likely affect the temperature. A rogue planet without a star in the blackness of the intergalactic void with only its internal radioactive isotope decay (a.k.a. geothermal heat) to keep it warm is going to do battle with the single-digit Kelvin of deep space. Conversely, a blazing supergiant a mere fraction of A.U. away will still wreak gravitational havoc as your little planet tries to keep itself from quaking apart. Remember, just because your world's sun isn't visible doesn't mean it's irrelevant. Again, everything is connected.
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Stars

If you live where I am these days, we call it "that big, warm, bright thing up in the sky". Most of the rest of you just call it "the Sun." But besides heat and light, one of the most important but least recognized things about a sun or star is its mass—how much "stuff" it's made out of. I'll present the things you as a worldbuilder might be most interested in first. And the more exotic your star, the further we can tweak things. So, I'm going to try present this as simply and briefly as possible, so accuracy might suffer a little as a result.
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Hertzberg-Russell (H-R) Diagram

Don't panic at that name, but if you want to know more, remember H-R Diagrams. Basically, these two guys looked at the temperature and brightness (luminosity) of a bunch of stars and put them on a graph. They saw patterns, especially this line of them. Turns out, our Sun is on that line, so they called it the Main Sequence of Stars. What makes that important is that we'll just limit our focus on that line because that's most likely where we will have our world. Another nice thing is that all of the important stuff about Main Sequence stars is nice, related, and (better yet) I put it on a table for you.

Idealized Properties Table of Main Sequence Stars

This is an idealized table of data used for the creation of stars and solar systems.
 Type = Spectral Type (Class O and B are not shown for practicality. UV light would vaporize surface water)
 Habitable Zone Conditions: ♀ = Venus-like, ♁ = hot, normal, and cold Earth-like, ♂ = Martian
 Lum. = Stellar Luminosity (in sols) ; Mass & Radius in sols ; Temperature in degrees Kelvin ;
 Abs.M. = Absolute Magnitude ; Bol. = Bolometric Magnitude ; AU = astronomical unit

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Type	Lum.	Radius	Mass	Temp	♀	hot ♁	♁	cold ♁	♂	Abs.M.	Bol.	Life (B-yrs)
A0	63.0	2.30	2.60	9600	5.74 AU	6.46 AU	7.94 AU	11.46 AU	12.09 AU	0.8	0.3	0.47
A1	53.0	2.18	2.43	9650	5.27 AU	5.92 AU	7.28 AU	10.51 AU	11.09 AU	1.0	0.6	0.60
A2	44.5	2.08	2.27	9700	4.83 AU	5.43 AU	6.67 AU	9.63 AU	10.16 AU	1.2	0.9	0.72
A3	37.0	1.98	2.13	9367	4.40 AU	4.95 AU	6.08 AU	8.78 AU	9.27 AU	1.4	1.1	0.88
A4	30.0	1.88	2.01	9033	3.96 AU	4.46 AU	5.48 AU	7.91 AU	8.35 AU	1.7	1.4	1.04
A5	24.0	1.80	1.90	8700	3.54 AU	3.99 AU	4.90 AU	7.07 AU	7.46 AU	1.9	1.6	1.20
A6	20.2	1.74	1.84	8400	3.25 AU	3.66 AU	4.49 AU	6.49 AU	6.85 AU	2.0	1.8	1.30
A7	16.8	1.68	1.77	8100	2.96 AU	3.34 AU	4.10 AU	5.92 AU	6.25 AU	2.1	1.9	1.40
A8	13.8	1.62	1.71	7800	2.69 AU	3.02 AU	3.71 AU	5.36 AU	5.66 AU	2.3	2.1	1.63
A9	11.2	1.56	1.66	7500	2.42 AU	2.72 AU	3.35 AU	4.83 AU	5.10 AU	2.4	2.3	1.87
F0	9.00	1.50	1.60	7200	2.17 AU	2.44 AU	3.00 AU	4.33 AU	4.57 AU	2.6	2.5	2.10
F1	7.80	1.43	1.55	7050	2.02 AU	2.27 AU	2.79 AU	4.03 AU	4.26 AU	2.8	2.7	2.45
F2	6.71	1.36	1.50	6900	1.87 AU	2.11 AU	2.59 AU	3.74 AU	3.95 AU	2.9	2.9	2.80
F3	5.71	1.30	1.45	6733	1.73 AU	1.94 AU	2.39 AU	3.45 AU	3.64 AU	3.0	3.0	3.10
F4	4.80	1.25	1.40	6567	1.59 AU	1.78 AU	2.19 AU	3.16 AU	3.34 AU	3.2	3.2	3.40
F5	4.00	1.20	1.35	6400	1.45 AU	1.63 AU	2.00 AU	2.89 AU	3.05 AU	3.3	3.3	3.70
F6	3.33	1.17	1.29	6350	1.32 AU	1.48 AU	1.82 AU	2.63 AU	2.78 AU	3.6	3.6	4.45
F7	2.74	1.14	1.23	6300	1.20 AU	1.35 AU	1.65 AU	2.39 AU	2.52 AU	3.8	3.8	5.20
F8	2.23	1.11	1.18	6200	1.08 AU	1.21 AU	1.49 AU	2.15 AU	2.27 AU	4.0	4.0	6.07
F9	1.80	1.08	1.13	6100	0.97 AU	1.09 AU	1.34 AU	1.93 AU	2.04 AU	4.2	4.2	6.93
G0	1.45	1.05	1.08	6000	0.87 AU	0.98 AU	1.20 AU	1.74 AU	1.83 AU	4.4	4.4	7.80
G1	1.20	1.021	1.04	5850	0.79 AU	0.89 AU	1.10 AU	1.58 AU	1.67 AU	4.6	4.6	8.65
G2	1.00	1.000	1.00	5700	0.72 AU	0.814 AU	1.00 AU	1.443 AU	1.52 AU	4.7	4.7	9.50
G3	0.892	0.996	0.99	5633	0.68 AU	0.77 AU	0.94 AU	1.36 AU	1.44 AU	4.8	4.8	10.33
G4	0.792	0.990	0.97	5567	0.64 AU	0.72 AU	0.89 AU	1.28 AU	1.36 AU	5.0	4.9	11.17
G5	0.700	0.980	0.95	5500	0.61 AU	0.68 AU	0.84 AU	1.21 AU	1.27 AU	5.1	5.0	12.00
G6	0.619	0.966	0.93	5467	0.57 AU	0.64 AU	0.79 AU	1.14 AU	1.20 AU	5.2	5.1	12.67
G7	0.545	0.950	0.91	5433	0.53 AU	0.60 AU	0.74 AU	1.07 AU	1.12 AU	5.4	5.2	13.33
G8	0.477	0.932	0.89	5400	0.50 AU	0.56 AU	0.69 AU	1.00 AU	1.05 AU	5.5	5.3	14.00
G9	0.415	0.912	0.86	5275	0.47 AU	0.52 AU	0.64 AU	0.93 AU	0.98 AU	5.7	5.6	16.50
K0	0.360	0.890	0.83	5150	0.434 AU	0.488 AU	0.600 AU	0.866 AU	0.914 AU	5.9	5.8	19.0
K1	0.318	0.860	0.78	4975	0.408 AU	0.459 AU	0.564 AU	0.814 AU	0.859 AU	6.1	6.0	22.0
K2	0.279	0.830	0.74	4800	0.382 AU	0.430 AU	0.528 AU	0.762 AU	0.805 AU	6.3	6.1	25.0
K3	0.243	0.802	0.70	4683	0.357 AU	0.401 AU	0.493 AU	0.712 AU	0.751 AU	6.7	6.3	28.3
K4	0.210	0.776	0.66	4567	0.331 AU	0.373 AU	0.458 AU	0.661 AU	0.698 AU	7.0	6.4	31.7
K5	0.180	0.750	0.62	4450	0.307 AU	0.345 AU	0.424 AU	0.612 AU	0.646 AU	7.4	6.6	35.0
K6	0.156	0.742	0.60	4325	0.286 AU	0.321 AU	0.395 AU	0.570 AU	0.602 AU	7.8	6.9	43.5
K7	0.133	0.726	0.57	4200	0.264 AU	0.297 AU	0.365 AU	0.526 AU	0.556 AU	8.1	7.2	52.0
K8	0.112	0.704	0.54	4083	0.242 AU	0.272 AU	0.335 AU	0.483 AU	0.510 AU	8.3	7.4	60.7
K9	0.093	0.676	0.51	3967	0.220 AU	0.248 AU	0.304 AU	0.439 AU	0.464 AU	8.6	7.6	69.3
M0	0.075	0.640	0.47	3850	0.198 AU	0.223 AU	0.274 AU	0.395 AU	0.417 AU	8.8	7.8	78.
M1	0.0584	0.591	0.43	3675	0.175 AU	0.197 AU	0.242 AU	0.349 AU	0.368 AU	9.5	8.1	94.
M2	0.0439	0.539	0.39	3500	0.152 AU	0.171 AU	0.210 AU	0.302 AU	0.319 AU	10.1	8.3	110.
M3	0.0315	0.483	0.34	3400	0.128 AU	0.144 AU	0.177 AU	0.256 AU	0.270 AU	10.6	8.6	130.
M4	0.0212	0.423	0.30	3300	0.105 AU	0.118 AU	0.146 AU	0.210 AU	0.222 AU	11.1	8.8	150.
M5	0.0130	0.360	0.25	3200	0.0825 AU	0.093 AU	0.114 AU	0.165 AU	0.174 AU	11.6	9.2	200.
M6	0.0076	0.272	0.18	2900	0.063 AU	0.071 AU	0.087 AU	0.123 AU	0.133 AU	12.1	9.5	250.
M7	0.0036	0.202	0.13	2700	0.043 AU	0.049 AU	0.060 AU	0.086 AU	0.091 AU	14.1	10.7	725.
M8	0.0008	0.150	0.10	2500	0.021 AU	0.023 AU	0.028 AU	0.041 AU	0.043 AU	16.0	11.8	1200.
M9	0.0004	0.116	0.08	2200	0.0145 AU	0.016 AU	0.020 AU	0.029 AU	0.0305 AU	17.0	12.4	4600.
M9.5	0.0001	0.100	0.08	1900	0.0072 AU	0.0081 AU	0.010 AU	0.0144 AU	0.0152 AU	18.0	13.0	5600.

Now, don't get overwhelmed. Yes, there's a lot of stuff on there, but it's stuff we'll go back to in order to look it up.
The first thing you might notice is the Stellar Class. Just know it's not alphabetical and goes from hottest to coolest in this order: OBAFGAKM. There are more letters, but these are the one's we'll start off with first. The thing about stars (and lots of other kinds of light) is that blue is actually hottest, with yellow, orange, and red progressively cooler and dimmer. We just think of hot fire being "warm" colors. But anyone who uses a natural gas stove will notice blue flame. Blue means hotter.
On the Main Sequence, the values that matter most line up really nicely. I made an idealized table (above) for the perfect star along that line. Now, the universe being as it is, it doesn't really line up this neat. But barring slight variations, the table will give you believable and workable results. Otherwise, we'll have to go into the not-so-simple stuff where it gets messy right quick. Yeah, I'm a stellarist. I admit.
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What This Means for Worldbuilding

Our Sun is a G2 star. Fantasy worldbuilders, listen up! If you are making a world where your sun is exactly like our real Sun and your planet is just like Earth except with different continents and oceans and maybe dragons, you need not go further. But keep in mind, your sun is a G2 star.
For the rest of you, we need to pay attention to luminosity and mass mostly. Radius isn't as useful. Temperature is just on there because...well...H&R thought it was necessary, but luminosity is going to be our driver when it comes to the comforts of our home planet. The rest of the table will probably be what you might be most interested in, however. This is the distance a planet needs to be from its main star to be like the planets we know. I've included everything from Venus to Mars to give you an idea. The theoretical Habitable Zone, which is the region from a star where water exists in that familiar, life-affirming way we all like so much, lies between what I labelled as "hot" and "cold" Earth.
I've also included Absolute Visual Magnitude (which has to do with how bright a star appears...that's much later) and the star's life. Big hot blue stars tend to burn out very quick, so that darn slow evolution doesn't have much time to really get going and make whales and petunias. Cool red dwarfs, however, can hang around and see a dozen stars like our sun come and go before giving out. That's one reason why there are so many more Class M stars—longevity.
The numbers are given in such a way as to make it really easy and relatable to what most people know. Earthlings, in most cases. Luminosity, radius, and mass are all in relation to our Sun's brightness, radius, and mass. So, an ideal G2 star is 1.0× the Sun's brightness, size, and mass. Well, of course it is. A K2 star is only 27.9% as bright and warm, 89% as big, and 83% as massive. A huge A0 star will fry your eyes and boil them at 63× the brightness and heat, be 2.3× as big, and 2.6× as massive! A purdy blue star might be romantic, but it's mighty hard to snuggle if you're boiling that close to it!
As far as planetary distance, the measure is called an Astronomical Unit, which is a nerd's way of saying Earth's average distance from the Sun. 1.0 means you are roughly the same distance away as the Earth is. Trust me, it's a lot easier this way.
For example, lets say we want to be different and say our star is a G0 star, which is slightly hotter and bigger than our beloved Sun. Well, your planet will have to be 20% further out than our Earth. So what? Well, this will mean a longer year. Longer seasons means a different calendar. A different calendar may mean a different culture. Even different biology. See how everything starts to be more connected? Let's say, "screw that, I want a year to be a year just like Earth". Well, then you'll more likely be a much hotter planet. Hotter planet might mean more deserts, no ice caps or snow for Festivus, which means no hot cocoa, which may collapse any truly descent civilized society. Everything is still connected. Your world gets a bit more complicated. But, guess what? Your world just also became potentially more interesting!
Want a naturally blood-red star in your sky? Well, now we're playing with the spectral type. You shouldn't just say 'hocus pocus' (even if your wizards do) without knowing more about star chemistry and physics. Well, you can, but worlds for our readers are like magic—they ought to follow consistent rules or it can backfire and get more confusing, not less*.
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"Whoo-hoo!!! Now I can rule teh Univerz^™!" :3

Now, hold on there. Here's the biggest caveat of all. This is an idealized table. I know I keep saying that, but what does it mean? It means that while this is helpful in slapping together stars faster for a quick RPG, it not meant to be a hard law and give you "the answer" either. So if you happen to be a Hollywood producer that would rather not bother NASA, the ESA or a real astronomer, and you see my little chart here (hey, I can dream, can't I?), realize that real A0 stars are actually going to be much, much hotter and brighter so you shouldn't just slap those down willy-nilly. This also means that stars are a lot more diverse than what we're giving them credit for here. If you take a good look at an H-R Diagram you see other clusters. There are Big Red Giants like Arcturus and Aldebaran, Supergiants like Betelgeuse and Deneb, and little white dwarfs like in the Hobbit movies a star that orbits Sirius

Wait, there's a star that orbits Sirius, the Dog Star? Yep. It's called Sirius B. Seriously! (Sorry, bad astro-nerd pun. I couldn't help myself.)

.
So, yeah, you can have that giant purple-green star one AU away and not fry your peeps, but why is that? Not "just because that's how I want it and my wizards are so friggin' all-powerful that we can." It might seem exotic, but really those kinds of worlds are really just clichés with a different set of curtains. And not knowing how everything interrelates ends up leading to the same old status quo. It's the connections, not the window dressing, that makes for interesting worlds.
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What the Numbers Mean

Right now, it might seem that this is overwhelming. Or maybe underwhelming, since it just looks like a big, boring table of numbers.
First of all, the way these numbers are presented are in terms we can understand them. They are all in relationship to our Sun. Sometimes, you'll see it as Sols (or solar units, ☉), but that just means "times our own Sun's values". So a F4 star is going to be 4.8 times as bright and hot as our Sun, 1.25 times its size, and 1.4 times its mass. A G5 star is going to be 70% of our Sun's or 0.70 ☉, 98% its size, and a mass = 0.95 ☉ (95% of our Sun's mass). By putting these terms in relation to our Sun, it makes the equations a whole ton easier too. No fussing about with kilograms or meters or Imperial gallons! (Later we'll do this for Earths and stuff too).
So, who cares if our whitish F4 is bigger and hotter and our yellow G5 is smaller and cooler? Well, here are some things to consider:
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Luminosity

Basically, you can think of luminosity as how bright a star is. But luminosity is also pretty closely linked with how much energy it outputs. That mostly affects your planet's temperature as well as how much light it's getting. Plus, let's say you're pretty fond of solar energy. A higher luminosity means more solar power! This becomes even more of an issue when it comes to space probes that rely on solar panels. The luminosity of your star is going to affect your probe panel's efficiency. So sending a solar-powered probe to a world orbiting a M8 star is probably not going to do so well.
But yet another thing to consider is that more luminosity is going to affect other things on your planet, even fantasy-based worlds. Plants are basically the first and best solar-powered lifeforms. Luminosity is going to be a factor there. And it's also going to affect other things on your planet, like weather. Now, much of our Earth's weather is also a factor of the atmosphere, but understanding how a star's luminosity drives the atmosphere not only makes it easier to understand your world, it's also handy in debunking those who deny our own Earth's atmospheric global temperature is warming!
For instance, I calculated that the Habitable Zone (sometimes called the "Goldilocks Zone" for being not too hot nor too cold for our porridge Earth) is somewhere between about 1.51 times as hot as it is now (I didn't say it would be comfortable) to just a bit cooler by half. We use luminosity to find how far away from the star this is. We'll use good old G2 Sun to figure our own habitable zone:
a_hot = (L / 1.51)^0.5 ...or... a_hot = √[(L / 1.51)] for the hot Earth inner limit and
a_cold = (L / 0.48)^0.5 ...or... a_hot = √[(L / 0.48)] for the cold Earth outer limit
...where a is the distance in astronomical units (AU, the average distance between the Earth and Sun),
  L is luminosity in solar units (see, so much easier than candelas per square centimeter! I'm on your side, honest.)
Luminosity will get more interesting in later articles.
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Stellar Radius

Okay. Let's face it. For Main Sequence stars, stellar radius isn't going to play much of role. Not like supergiant stars like Betelgeuse whose diameter is so large, if it was in place of our Sun, it would swallow the Earth. It's radius, center to edge, is 1.2 times the orbit of Earth! It's photosphere, where all the shiny rays come out, is larger than the orbit of Jupiter. To blow your mind further, if you were to somehow shine a really, really, really bright flashlight from one end of Betelgeuse to the opposite end, it would take the light ten freaking minutes just to get there!
But, on the other hand, if we are putting our world or space colony on a planet orbiting very close to a Main Sequence Spectral Class M star, and you'd be curious to know just how big that sucker looms like a drop of blood in our alien sky, we could do some (shutter) math.
We can calculate diameter like this:
D_star = L^0.5 × (T_sun² / t_star²)
...where D_star is the diameter in solar units (Sun sizes),
  L is luminosity in solar units,
  T_sun is the temperature of our Sun (5770 K is good to use)
  t_star is the temperature of our star (which we can look up on the table if we know it)

Or we can do it like this (basically the same thing):
R_star/R_sun = (T_sun/T_star)² × √[(L_star/L_sun)]
...where R_star is the diameter in solar units (Sun sizes), (R_sun = 1.0 solar units)
  L is luminosity in solar units,
  T_sun is the temperature of our Sun (5770 K)
  t_star is the temperature of our star

If you check the table above, you won't get quite the same answers, but close. This is for various reasons, such as the idealized curve and I may have used slightly different stellar temperatures. We really don't need these equations except now you know how stellar size is determined. By measuring a star's temperature and luminosity, astronomers can determine a star's size and, more importantly, use its size to determine its temperature at the photosphere.
But you wanted to know about size. Right! That's trigonometry.

Show spoiler

[COMING SOON]

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Mass

Mass is basically how much "stuff" something has. It's not exactly weight, but you get something's weight if you involve gravity (my arch nemesis). It's a fundamental property of matter, so I don't have an equation to determine it for you. But it is extremely important when it comes to orbits.
This is a big problem for a lot of Hollywood movies. Everyone thinks that once you are in space, there is no gravity. This is so wrong! I can rant about that later, but of all things, mass is most important here. Mass is going to determine just how fast or slow your planet is going to whiz around its sun. And its going to be important if you have more than one sun.
The data we get in the table above is observed data. We have a basic idea of how massive a star and by correlating this to the H-R Diagram, we can come up with a pretty good guess. It's helpful that a lot of stars are binaries, which is to say there are a lot of double stars. Watching how they interact and then using a practical version of good old Newton's Second Law (F = m × a) and correlating that to spectral type gives us our idealized data. Just keep in mind that mass is very important when we get to orbital mechanics (don't panic).
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Planet Distances

Included in our little table are a few handy-dandy little columns that help us understand what the star is doing to a planet. We used an equation above to figure out our Sun's Habitable Zone where water is in a range to support some kind of life. Being the kind person I think I am, I've included those distances in AU for a planet to experience what Venus, Mars and Earth might given different stellar luminosity.
For example, if we want a planet to orbit a K0 star, but feel all the comforts of home, we find K0 on the chart and follow across the row to the column under "Earth" and we see it will be 0.600 AU or 60% the distance our Earth is from our Sun. This will mean a shorter year, and we'll learn how to figure that out when we get to orbital mechanics.
BIG NOTE

We are assuming that there are no additional effects from other objects. In a system with just one star, this is it. In a system with multiple stars, it depends on how close and hot that other star is. If there are lots of other stars nearby (I'm looking at you, Firefly/Serenity 'Verse) those additional stars make the worlds there very, very hot and bright.

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Absolute and Bolometric Magnitude

Here's the little problem with bright and dim stars. They are all at different distances. You could have a really dim star, but if it's in your figurative face, it looks really bright. Likewise, a huge mother-of-a-sun is blindingly brilliant, but lives on the far, far side of a galaxy far, far away. It appears like a little dot, if you can see it at all. How bright a star looks to you in the night sky as you are trying to impress your love interest is called Apparent Magnitude. But what if we want to compare stars on an equal footing? How do astronomers fix this?
Somebody decided, "Hey, let's just figure out what a star would look like at a fixed distance and give it a number." And all the astronomers cried tears of joy and say, "Lo shall it be that unto the Apparent Magnitude we shall give it the small letter 'm' and unto the Absolute Magnitude, the big letter 'M'!" And there was much rejoicing and attempts at trying to impress love interests which often failed, and the physicists shouting "I was using those letters to represent mass you idiots!" were drown out. I think someone had a bottle of wine. Of course they did because little is it known that astronomers always have a bottle of wine hidden somewhere. That's the real reason telescopes were invented, because Galileo was a cheeky slush.
Anyway, the relationship between absolute and apparent magnitude goes a little something like this:
M = m - 5 × (log₁₀ (p - 1), where p is the distance in parsecs (just convert light years by dividing by 3.2617)
Okay, I get it. The logarithm is a bit scary, but that's why calculators were invented. This can lead to negative numbers for really, really bright things. For example, our Sun has an apparent magnitude of −26.74. The star, Vega, is pretty close to 0. Dim stars are around 4 or 5. Just think like golf, smaller numbers is brighter.
But there's more than meets the eye with magnitude. Visual magnitude is a measure of the little bit of electromagnetic radiation we like to call "light" (and others might call "the stuff you see with"). But stars put out more than visual light. The further you get into the Class F and Class A stars, the more of that radiation is going to come from the ultraviolet light—the part that causes either tan lines or skin cancer...or both. In the Class K and Class M direction, more of that radiation is infrared—basically turning these stars more and more into heat lamps than reading lights.
So this is another aspect of your world dependent on stars. That pretty orange or blue star might also influence evolution on your planet. Plants might develop to take more advantage of that UV, and animals might have to develop techniques to combat its harmful, carbon-pummeling effects. Plants under Class M stars might be more black than green. And what about eyes? Maybe animals develop sensory organs that either take advantage of these different kinds of radiation or ways to blind predators.

Why should I care about Magnitude?

Here is a big mistake I see too often. Let's say your nerd astronomer guy discovers a brand new Class A star just a measly 40 light years from Earth (as a gaming company did for a certain popular Sci-Fi television/movie show). Nobody knew it was there, right? Well... sorry, but unless it was hiding behind a giant space whale this whole time, you either didn't just discover it or you didn't think about it.
Let's look at our Class A-list Hollywood Star. We'll use the real star Rigel as reference, only instead of being 860 light years away, lets bring it to our local 40 l.yr. First, the easy part: to convert light years into parsecs just divide 40.0 by 3.2617 to get about 12.264. Subtract 1 to get 11.264. Next, we'll use the visual magnitude of our real A0 star, Rigel, of about m_v = 0.12.
Get out your scientific calculator and find the LOG function (short for log₁₀). The log₁₀(11.264) is about 1.052 if you round it a bit. Now multiply by 5 to get 5.26. Okay, so absolute magnitude is M = m_v - 5.26 or M = 0.12 - 5.26. The absolute magnitude of our Hollywood Star is -5.14. In contrast, our Sun is a +4.83. At 10 parsecs (32.6 light years) away, our Hollywood Star would look brighter than Venus at its brightest as a Morning Star. In contrast, you wouldn't even be able to see our Sun at that distance if you looked from a typical suburban back yard. At just over 11 (40 light years) away, the Hollywood Star would still be visible to our naked eye during daylight. It would stick out. Unless...you know...giant space whale.
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Apparent Brightness

This is a more relatable equation related to magnitude, particularly when it comes to Class M stars. But it'll also give you a better idea just how quickly you can go blind around a Class A star too, with or without giant space whales.
I = L/a²
...where a is how far your planet is in AU (astronomical units or Earth distances)
  L is luminosity in solar units,
  I is "intensity" or how bright the star actually looks at that distance

So if you are at a comfy 0.114 AU from your M5 star putting out 0.013 times Sun's luminosity, it'll be about as bright. But at, say Venus' orbital distance (0.72 AU), you'd better get out your night goggles because it will only be 2.5% as bright as a sunny Earth day. That is far less than a full moon as seen on Earth.

More Than Meets the Eye

If you dig a little deeper—and in science, it is a continuous rabbit hole—you'll see that there are more bits to magnitude. The equations here are just about visual magnitude. But there's more to the electromagnetic spectrum than just light. You know, cosmic and gamma rays, radio waves, infrared and ultraviolet...all that stuff we can't see that gives us everything from radar to toasted muffins to skin cancer. There are adjustments that need to be made to get the whole story (Bolometric magnitude, for instance, and adjustments for other factors like obscuring objects like dust or a companion star, and other considerations when dealing with things like planets and moons.) But this is enough to give you a basic reality check. Don't be Hollywood.
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Lifetime

One thing to keep in mind when we look at "Life" is that this refers to lifetime as a Main Sequence star. When a star is new or very old, it drifts off this convenient line of average and becomes something else. For instance, while Betelgeuse is officially classified as a Spectral Class M star, that only means that its photosphere is a certain temperature and luminosity.
We're only using "lifetime" here to help us understand how long a typical Main Sequence star is expected to be around. Betelgeuse is probably near the end of its life, so using our handy-dandy little table above isn't going to work for us there.
As you can see, bright stars aren't around for very long. Candles that burn twice as bright burn half as long, and all that. So if you're tempted to put your world on a Class A star and claim it hosts an incredibly ancient civilization that's lasted billions of years... I'm going to have to call you on that. To put it in a bit of perspective, after our Earth got popped out of the cosmic oven about 4.54 billion years ago, it took maybe about 800 million years just for single-cell bacteria to be the "in" thing, and then another 200 million to invent photosynthesis. It didn't really get interesting until sex was invented just a billion years ago. Basically, it took our planet about four billion years or so for animals to come out of the sea and become interesting things like trilobites, dinosaurs, mastodons, and Brandon Sanderson. So, we have to consider that while a hotter, sexier blue star might look nice, the chances of it producing the kind of interesting life that World Anvil articles might fill are going to take a few billion years. Plan for that.

How Many of a Spectral Type?

That being said, just how many stars of a particular Spectral Type are there? As mentioned, hot stars burn out fast while cool stars stay around for a long, long time. Because of that, the probability of a Main Sequence star being of a particular Type are as follows:

• Type O: 0.00003%
• Type B: 0.125%
• Type A: 0.625%
• Type F: 3.03%
• Type G: 7.3%
• Type K: 12.0%
• Type M: 76%

So, the chances of a star being O-type is one in three million! B-type is about 1 in 800. A-type is about 1 in 160. F-types are 1 in 33. G-types are more common at about 1 in 13. K-types are come along once every 8 stars you encounter. But about three out of every four Main Sequence stars in the sky are M-types. They are just so dim, we can't see them from Earth. And if you include all of the stars out there, including M-type giants like Betelgeuse, the chances of a star in our galaxy being M goes up to 78.6%. And this is why astronomers are rethinking about finding life. They used to think only stars like our good old G2 Sun were capable of life. But with so many M-type Main Sequence stars living for billions of years, the chances of life are actually better for our humble M's. Maybe.
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Example Time!

Example #1

Let's say you like your stars big and bright. Well, how about we start off with a main sequence star that's further up on the Hertzberg-Russell (H-R) Diagram than our little yellow-white G2 Sun? F1 sounds good. So, officially, your sun will be an F1V star, the "V" here meaning a "main sequence" star. You could choose a different one, like a supergiant IIb, but let's just stick with our nice, semi-predictable main sequence until our godlike powers fully develop.
In the hand-dandy chart above, we follow our finger past the A-list and find F1. We see that the luminosity is 7.8 times that of our Sun. Bright enough for you?
Keep following the finger horizontally and we see it has a radius that's about 43% bigger than our Sun. Yawn. Yes, I know. Not even enough to swallow the orbit of Mercury. So let's keep going.
Mass is 1.55 Sols or 1.55 times our Sun's mass. But don't tell it that it needs to lose weight. It could be a very sensitive star, and with a luminosity like that, you could find yourself sunburned if it took offense.
The temperature is 7050 Kelvin. Again, no big deal since it's the luminosity that determines how high we need to set the air conditioner and not our star's surface temperature. However, this does give us an indication of what color our star will be. Mind you, the color of stars tends to make more difference from light years away because that up close and it all looks like a blinding white. Now, it's not on the table, but I've got a hexadecimal number and an RGB value for the F0V and F2V stars. F0V is RGB of (228,232,255) or #e4e8ff in hexadecimal code. F2V is (237,238,255) or #edeeff. Slightly bluish, but pretty white to our eyes. So, sorry, no vibrant magenta this time.
Next in line is probably the most useful and interesting to building your world. The first column is the distance in astronomical units (AUs) where a planet like Venus would be feeling the same power output. Again, 1 AU is the average distance the Earth is from the Sun. In our solar system, Venus is 0.72 AU. If all we wanted to do was replace our Sun with this fabulous F1V star, but keep everything else feeling the same energy output, our new Venus would now be 2.02 AU away. That just gives us an idea of what's going on. If you wanted Mars, that would be a whole 4.26 AU.
Now, the next three columns are for what I call Hot Earth, Earth, and Cold Earth scenarios. If we kept our Earth the same and replaced our Sun with an F1 star, it would need to sit ideally at 2.79 AU away to feel just as cozy. Scientists figure that for water (and thus life) to exist, a planet needs to be in a Habitable Zone (sometimes called "Goldilocks Zone" because it won't be either too hot or too cold, just right). Those limits are labeled on the table as Hot Earth (the closest a planet can be in the HZ, Habitable Zone) and Cold Earth (the furthest a planet can be in the HZ). So, a planet somewhere between 2.27 AU and 4.03 AU from our F1 star could possibly maybe have some kind of water that could exist. It doesn't mean that it would be comfortable at those extremes, but there could be microbes or some other kind of life kicking around on the surface at those ends. Theoretically, that is.
The final columns are the Absolute Magnitude and Bolometric Magnitude, followed by the star's time spent as a Main Sequence star. Really, the magnitudes may not mean much at this stage except play attention to the difference between the Absolute and Bolometric Magnitudes. For our ideal F1, the Absolute Magnitude is about 2.8 and the Bolometric Magnitude is 2.7. What this tells us is that of all the energy coming out of that star, some of it is going to be invisible to our eyes. In the case of hotter stars, like the F- and A- class, that usually means ultraviolet. For cooler stars, that ends up being infrared, like what you'd get from a heat lamp. Compare that to our humble G2 Sun. Those numbers are about even. That's because our eyes evolved to see that energy, which we call visible light. Your F1 critters might be more sensitive to ultraviolet, and maybe the color red ends up being more invisible to them. More importantly, while a little UV light is good for Earthlings who like to tan, it tends to be pretty harmful to a lot of carbon-based tissue. Our planet better have a thick ozone layer or a lot of your life forms might have to live at the poles or under rocks to avoid getting too crispy. You think a bad sunburn after an unprotected hour hurts? Maybe that F1 star isn't looking so hot. ...er... or maybe too hot. Ultraviolet-wise.
The final column is how much time your star spends its life as a main sequence star. This is important when you consider your critters. Now, your world might have gods and goddesses running around it just blurping three-eyed nerf-shmeeps, but if science is your only Power-That-Be, evolution is going to need a bit of time to just come up with something like salmonella. You may have heard something from Shakespeare like "that which burns twice as bright burns half as long". Well, that goes for both horny teenagers in fair Verona and for stars alike. Our F1 has only been burning like this for not even a meager 2.5 billion years yet, and our Earth took that long just to invent eukaryotic cells—cells with a nucleus and more complex bits than a simple bacteria. Photosynthesis took about 1.5 billion years to develop in cynobacteria. So our planet might have life, but certainly not six-legged nerf-shmeeps. Unless your god blurped it there. Then maybe this article isn't for you. After that short 2.5 billion years, the hydrogen fuel of our star gets pretty spent and it jumps off the Main Sequence and becomes one of those supergiants that does swell and swallow planets alive! Lucky for our Earth, we've got a lot longer for humans to ruin it first.
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Example #2

Coming Soon!

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Summary

Alright, so let's bring this whole article together:

1. Stars are an important starting point for any world you create,
   • regardless of genre
   • even if you want your world's sun to be just like our real Sun
2. Stellar mass is the most important thing about a star when it comes to worldbuilding—it will be used mostly for orbits and gravity
3. Stellar luminosity is the second most important consideration—it will be used for everything from warmth and brightness to how much energy you can get in your solar panels or crops.
4. A star's class determines its basic temperature and color—the table given above shows idealized values, but it isn't exact so you can fudge a bit so long as you understand what you are fudging.

Don't worry if this doesn't all make sense now. It will later when we start to apply these things. But if you start with these considerations at the base, it will help with your system as your world comes together. It will also be easier to go back and make adjustments, too, without your system unraveling.
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