One thing that we noticed about lots of 
galaxies is that like the Milky Way, 
there are Spiral galaxies and so the 
issue of understanding spi, spiral 
galaxies come to be spiral is an old 
interesting one and I want to explain 
like its a problem first. 
So one could imagine that for some 
reasons stars form in a pattern like the 
one I've. 
Right, and therefore you have spirals. 
That is not the solution to how spirals 
form. 
We understand that because remember that, 
within the region where the sun is, 
rotational velocity. 
was roughly constant. 
If rotational velocity is constant, then 
periods of rotation are not constant. 
Stars on the interior moving at the same 
speeds as stars on the exterior, the 
interior stars will orbit, and with much 
shorter periods cause their orbits are 
shorter. 
What this means is if you imagine 
creating a galaxy with this pattern and 
then letting the stars rotate and 
remember that the Sun has gone through 
about 25 orbits of the Milky Way so this 
is not some theoretical construct. 
What happens is the inner stars orbit 
faster than the outer stars and the arms 
get all wound up. 
And eventually will dissolve after a few 
rotations you have no spiral left. 
You cannot, construct a spiral galaxy as 
something you're actually going to find, 
by having a spiral being a place where 
stars are. 
in general, since the sun takes 230 
million years to circumnavigate the 
galaxy, 
And remember that spiral arms are the 
position where we found O and B, hah, O 
and B type stars. 
And since O and B type stars don't live 
230 billion years, by the time the spiral 
arms continue, complete an orbit out 
where the sun is, it's a whole new 
generation of stars. 
Many new generations of stars have come 
up. 
And yet the spiral arms must be stable 
otherwise you wouldn't see so many spiral 
configurations in galaxies.So what is it 
that allows spiral arms to survive the 
model that was proposed by Lenin Shoe in 
the 1960's. 
It's called the density wave model and 
the idea is that spiral arms are not 
material objects. 
They are propagating density waves 
propagating through the disk. 
The wave propagates as you recall with a 
completely velocity that is the wave 
velocity. 
It's completely independent. 
Of the motion of the actual objects in 
the disc, when we had a wave going down 
the string. 
the string wasn't moving at all, and the 
wave propogated down the string. 
So these waves propogate through the disc 
with their own velocity and what this 
implies, is these are like sound waves. 
There are regions where the density. 
In the disk increases by say, 10 or 20%. 
And how do you generate such a situation? 
Well there's this nice image here on the 
right, what we have here is a bunch of 
elliptical or oval orbits and you could 
imagine that in each of these orbits 
there's a whole bunch of stars. 
Each following that orbit at various 
places along the orbit, and these 
ellipses or ovals are, tilted slightly 
with larger and larger radius, and, you 
see that there, this creates this, this 
twisting, creates regions in the galaxy 
Where the orbits are close together, this 
is a region of enhanced density. 
If you imagine that each oval is equally 
populated along it's length by stars. 
Then, and the regions where the ovals are 
close together there will be an increase 
population of stars and gas, and whatever 
it is that is that's orbiting along these 
orbits. 
And the fact that the stars in the inner 
ellipse orbit faster than the stars on 
the outer ellipse, has nothing to do with 
it. 
If these orbits were actually steady, 
then this pattern would not shift at all. 
It would stay completely constant because 
every star, as it hit the part where the 
orbits are bunched up, would experience 
higher density. 
Now in practice, this is not a stable 
situation. 
Stars orbiting along these orbits, 
anything by the collection of circular 
orbits leads to gravitational 
interaction. 
And so, these orbitals precess, if you 
put stars in these elliptical orbits. 
And that precession will lead to, the, 
the, different tilts of the different 
ellipsis changing at different rates. 
If you have a resonance That between the 
precession period and the orbital period, 
then you can have a situation where, as 
the, what the precession sets up is not 
the complete destruction of the spiral, 
but rather a slow rotation of the spiral, 
of, over time. 
And then What you have is a situation in 
the disk where you start with a 
population of stars and dust and gas and 
whatever but there are regions where the 
density is higher. 
Now stars as they orbit enter this region 
and then they exit it. 
Clouds of gas enter this region and exit 
it. 
When a cloud of gas encounters this 
region of higher density That's higher 
pressure. 
The cloud of gases compress. 
Some clouds may become critical in the 
gene sense. 
They will then begin to collapse and by 
the time those clouds are exiting the 
spiral at the leading edge of the spiral, 
you will find brand new O and B stars 
that have formed while the cloud was 
collapsing inside the spiral. 
Remember, the entire lifetime of an OB 
star. 
Is maybe 10, 20 million years. 
Orbital periods near where the sun is is 
200 million so the stars that were formed 
by entry into the region of increased 
density will live out their lives, the OB 
stars not far from that region. 
Now even if all stars are formed in the 
spiral arms, stars that live longer will 
have time to orbit away. 
And so you'd expect the red stars. 
To be far more uniformly scattered. 
But, the regions of star formation will 
be at the region of this density wave. 
And this we think is how spirals survive. 
And so simulation of what this looks like 
is Here is this, density we've set up and 
as the stars orbit the arms orbit with a, 
angular velocity that has nothing to do 
with the angular velocity with which the 
star is either on the inside or the 
outside of the, galaxy orbit, and in the 
simulation we see that the arms are 
stable and we've also decorated them with 
blue stars, and, Red H2 regions to make 
it look like this beautiful example on 
the right. 
And maybe one classic what is called 
grand design spiral with very coherent 
spiral arms. 
Populated by lots of blue stars and lots 
of H1 regions. 
it's H2 regions, I mean, ionized regions 
where hydrogen has been ionized by the 
ultraviolet radiation from, these new hot 
stars, and so, the density weight model 
is very successful model for describing 
these Its a little more difficult when 
you come to describe a what is called a 
flocculent spiral like as you see MGC 
4414 over here. 
When a spiral structure is. 
Lets go here. 
The model that may play part in 
describing. 
The spiral structure which is nonetheless 
present here may be a model called SSPSF 
for stochastic self-propagating star 
formation, proposed by Miller and Arnet 
in 1976. 
And the idea is that star formation 
begins in a roughly uniform disc 
randomly, at some point. 
Stars begin to form, some cloud 
collapses. 
The first stars to form as the cloud 
collapses, of course, are the biggest 
ones because they form faster and so you 
get O and B stars. 
The O and B stars live their life and end 
their main sequence existence before many 
of the smaller stars have even passed the 
proto star stage. 
And then the supernova explosions from 
these stars. 
create shock waves that propagate through 
the clouds and basically blow it apart 
inhibiting the growth of further stars, 
so whatever young stars that past the 
proto- star stage are going to get onto 
the main sequence and stay on the main 
sequence cause stars are not blown away 
by the shock wave but the gas and dust is 
going to be blown apart and therefore 
star formation ceases. 
So if you start with star formation in 
one place, it basically blows itself 
apart, like a forest fire burns itself 
out. 
But like a forest fire, the same shock 
wave can be like a spark because farther 
out when it is no longer as concentrated 
as when it blew this cloud to 
smitherines, farther out after it's 
diluted a bit, this shock wave can be 
precisely the trigger. 
Remember, we need a trigger to cause a 
cloud to suddenly become critical. 
It could be the density wave, it could be 
the shockwave from a supernova. 
Causing a nearby cloud, to now become 
critical and start collapsing. 
And so like a forest fire, this process 
of star formation will propagate in all 
directions through the disc. 
when it propogates, through the disc, the 
differential rotation will wind Whatever 
shapes these regions propagating star 
formation initially had the differential 
rotation, the fact that stars on the 
interior orbit faster, will wind it into 
this spirally shape. 
And this might be the model that 
describes 
Flocculent Spirals and maybe there is 
some combination of the 2 that is in 
action at all times because uh,we see 
that both of them seem to be reasonable 
mechanisms. 
But the question of spiral arms is at 
least intriguing.