That's a star, baby! Or, well. almost.

The funny thing about stars is that they are remarkably simple. They're spherical plasma in a vacuum! Sure, they have all sorts of fun physics in the core, and stellar atmospheres are an incredibly complex environment, but as soon as you leave their immediate vicinity they become extremely easy to study.

You wanna know the best part? 90% of the time, knowing one specific thing about a star is enough to tell you everything you'd ever want to know.

It turns out that when you get a whole bunch of hydrogen all in the same place, it tends to act pretty much exactly the same as the same amount of hydrogen elsewhere. It makes sense, when you think about it - if you copied the Sun and pasted it on the other side of the galaxy, it would still look the same, right?

But let's talk about that 90% first. Baby stars are funky lil guys. They've got all sorts of weird fluid dynamics and temperature gradients and accretion and hoo boy are they neat but they are anything but simple. Turns out that about 5% of the star's lifespan is spent in this "protostar" phase. After that, they tend to settle down and just vibe for a while. When a star is vibing, we say it's "on the main sequence" (more on that later). Eventually though, it'll run out of usable hydrogen in the core and start getting funky again. These are the red giants and such that I'm sure you've heard about before, and about 5% of the star's lifespan is spent here in the retirement home before it either fizzles out or dies.

5% weird at the start, and 5% weird at the end. For that 90% in the middle, stars are very well behaved. Thriving. In their lane. Positively vibing.

So, let's look at that whole "main sequence" thing I mentioned. Like I said before, putting a given amount of hydrogen in any one place results in a star that looks pretty much exactly the same as any other star with that mass (if they're both in that middle part of their life, which is usually the case).

Turns out that more mass does a few things to a star. It makes it brighter, it makes it hotter, it makes it more blue, and all of these things are intrinsically related to each other. Every main sequence star with, say, 2 solar masses of hydrogen, will be identical in all of these ways. The same luminosity, the same temperature, the same color. All determined entirely by mass.

And now I finally get to show y'all the H-R diagram.

TADAAAA

Behold, in all its glory: a Hertzsprung-Russell diagram populated entirely by observational data. Every dot on this diagram is a real star which I think is neat.

Let's talk about the axes first.

From bottom to top, you have luminosity. Brighter stars are higher up than lower stars. Easy! Also from bottom to top, you have "absolute magnitude." I'm pretty sure I've talked about that before, but it's a weird astronomy way of saying brightness. Basically the same thing, I promise.

From left to right, we've got temperature. And color. And also spectral class. All three of these things are so related that you can plot them on the same axis. It's really neat but also kinda confusing, so bear with me. Hotter things (higher temperature) are on the left. Bluer things are also on the left, which makes sense. Hot stuff glows more blue/white than cooler stuff, we see this in candles on Earth all the time! Spectral class is gonna be another space fact, this one is already kinda long - sorry!

One of the easiest things to spot in H-R diagrams is that big streak of stars from top left to bottom right. That, as is so helpfully labelled, is the Main Sequence I've said so much about. Most stars live here on this line!

That smaller group in the bottom left are white dwarves - very hot, but not very bright. What's up with that? Well, they're the old husks left behind by medium-large stars when they die. Basically stellar embers - they're not making any more energy, but they have a ton left over from when the star was still burning. They're small, so they can't emit that much light even though they're super hot, which puts them down in the bottom (dim) left (hot) corner.

The only feature left is the blob above the main sequence, to the right - the giants. This is where the stars getting ready to die live. Due to some fun thermodynamics involving no less than 4 state equations and a whole lot of interesting fusion, these stars are cool but still pretty bright! When the hydrogen runs out of a star's core, it starts to collapse in on itself. Pressure heats things up, and eventually it gets hot enough to start fusing helium. This new fusion at higher temperatures stops the collapse, and the stellar atmosphere puffs out to cope with the hotter core. Expanding things cool down, so you end up with a much larger orb of colder hydrogen. They're not very hot, but because they're so big they end up putting out a ton of light anyway. Top (bright) right (cool) corner.

everyone look at za HR diagram again