If you really want to understand the way that stars work, he's understand what kind of stars there are and how many of different types there are, and so on.

Because if you're gonna come up with a theory of how stars form and evil, it's got a match on to those observations.

So you got to do this sort of fairly pedestrian activity of actually just classifying the stars.

So the sun, for example, is has its own classification.

The rarest different classifications schemes that have been used.

But the one that's in most common use nowadays, called the NK system, is that the sun is classified as a G to V.

Star stars are completely different.

They range in size in the amount of stuff.

They have density in what type of stuff they have.

If it's carbon or oxygen and they vary in temperature.

The real problem in classifications is that generally the classifications gets done before you understand what it is you're classifying.

That means that very often in astronomy you end up with the classifications scheme, which physically doesn't make a lot of sense because you just classify on some fairly arbitrary basis at the time.

It's sort of ah, historical nightmare as to how the stars are classified.

They were originally just you look at them, and depending on how bright they are, you give them a number.

So the brightest ones were one, and then they went fainter to about five or six.

And then it got a bit more technical technological when we were able to look at a spectrum of a star.

So if you shine the light of a star through a prism and you look at its rainbow, you can see different different characteristics of a star in particular hydrogen and so very sensible.

What seemed a sensible classifications scheme at the silent time wants to classify all the spectra according to the strength of the hydrogen lines and various other lines as well.

But maybe the hydrogen lines and what they did is they had, basically and an alphabetical sequence starting a working their way through the alphabet, where the stars were the ones that have the strongest hydrogen lines.

And so on B, C, D.

And so on.

Down through the classifications game, as the hydro lines got weaker and weaker and other lines got stronger.

So about the mid 18 hundreds is when this first started happening.

It turns out that actually, physically, this isn't a very sensible way of classifying things, because it's actually it's a It's a Goldilocks and the Three Bears problem.

And finally, Annie Jump Cannon, one of the first women who actually did astronomy, um, or is remembered for doing astronomy.

She was the one who who got very specific in the classifications of these stars.

When the way you get strong hydrogen lines in the atmosphere of a star is that the star you know, it's hot enough to kind of excite the hydrogen and then the hydrogen, then is there to kind of absorb some of the Starlight.

And if a star is too hot, then the hydrogen it'll just gets ripped apart.

It actually gets ionized, so the protons and the electrons that make up the hydrogen get ripped apart.

And there isn't any hydrogen.

There isn't any on ionized hydrogen in the atmosphere, so you don't see any of these absorption lines.

If the stars too cold, then the atoms are all in their ground state.

In other words.

The hydrogen atoms just never get excited.

It'll they never do anything.

So they just sit there.

They don't actually absorb any light.

It's only a temperature somewhere in the middle where actually, hydrogen lines get excited and you end up with these very strong tonight.

So that's why I hired you, the stars that have these strong 100 lines or somewhere intermediate temperatures.

They're not too hot and not too cold.

She kind of did away with the A through whatever letter it was because she realized they were characteristics that weren't just noticeable in the spectrum but related to physical properties of the star.

You can use the same letters, but you just need to rearrange them so that they're in temperature order rather than in the other order.

So instead of being a B C.

D, the order in which the stars come is O B a f g k m.

So it ends up being o B a f g k m.

And she just got rid of all the ones that kind of repeated their properties.

And that's not particularly memorable collection of letters.

But it's Bean and the reason where I could just rattle it off like that is because it's remembered by generations of astronomers by a demonic wave.

A little sentence remembering it will be a fine girl Kissed me is the traditional way of doing it with the other ones that people would come up with.

Well, it will be a fine guy.

Kiss me, and I think there's a few other ones you can say will be a fine girl.

Kiss me if that's your liking.

Oh, be a fine girl or guy, depending on your preference is Kiss me and so will be a fine girl.

Kiss me O B A F G K M is the order in which these things are.

Then that's then ordered from the hottest stars.

Which of the O stars down to the coolie stars?

Which of the M starts?

The temperature basically determines a lot about what's going on with the star, and the temperature is correlated very closely with its mass.

So more massive stars have general hotter temperatures and their atmospheres and Laura mass stars have core temperatures, and mass temperature has less to do with how long the star lives.

How bright is gonna be?

What kind of star.

It'll end up in eventually there, a white dwarf or a black hole?

Why didn't they just change all the letters on the stars?

Why did they keep the old letter?

It's because you get stuck.

You have a classification scheme when there's a whole bunch.

So there was a huge catalog, for example, produced in the 19th century of hundreds of thousands of stars that little being classified by this game.

And so, rather than going through and reclassifying all the stars, the easier thing to do is just to remember a pneumonic for the right order to remember the letters in.

Rather than going back to hundreds of thousands of spectra and reclassifying everything, The Sun is sort of a typical star in the sense that it's in the middle of the O.

B.

A.

G K M system to G star, and it's official classification is a G to V.

Let's start at the back end of it.

The V is actually a Roman numeral five, and all stars are classified according to the luminosity with luminosity class one thing's being the brightest all the way down to luminosity class five.

Actually, the luminosity class six which is a little fainter and occasionally white dwarf stars, which remove enter a cold luminosity class seven.

So the Viet the End is just telling you how bright the stars and the other bit of it he's telling you about.

It's effectively about its temperature.

G is actually tells you that it sze actually start somewhere in the middle of the classifications.

Schemes of the sun is not the not the hottest or not the Cory start somewhere in the middle.

Once you have a classification of a G star, you can even classify it, father and say to G zero R G 1 99 so G zeroes are the hottest and Jeanine of the coolest.

So among G stars, the sun is towards the hotter end.

So what did you two, But yeah, it's just a typical starts.

It's right in the middle of temperature range, right in middle.

The mass range.

Um, it's not the most common type of star because Thean McKay stars are much more common.

But it's a typical started, since that it's just sort of straddles all the possible ranges that we know of stars.

The neat thing comes when you actually start looking at collections of stars.

So instead of looking at the properties of individual stars, you start looking at collections off.

So the first people to do this were a couple of astronomers called Hertzberg and Russell, so it's a very famous diagram named after them.

So this is, Ah, very well known chart.

It's called the Hurt Sprung Russell diagram, named after the two people that used it.

Um, so we have going this way, brightness so brighter.

Stars are at the top and we have on this excess temperature, and it's a bit backward, so stars get hotter as you go this way, you can see at the bottom they have the classification, the O B.

A.

F G k m, and that you see this nice diagonal line here.

This is what's called the main sequence.

So most stars spend most of their life on the main sequence, and this is where they are while they're burning hydrogen while they're fusing new chemical elements in their cores, and then after some amount of time.

And for our son, it's about 10 billion years.

We're already halfway through that we have five billion more.

The star will evolve often Ming sequence and become a red giant example.

The Sun and its distant future is going to turn into a red giant star.

So it will cease being a TV star, a low luminosity, star main sequence star and will become a giant star.

So it'll be a luminous equals one or two, and it will actually get cooler.

As it does so, it will turn into a red a star, so it will move down the sequence.

So stop being a G star and probably become a case dollar, even an M star.