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.