We spent the last few weeks talking about 
stars, and we had to invoke all kinds of 
great physics last week to understand the 
end of, stellar evolution. 
We're going to take it to the next level 
this week, and the next level in size, 
past stars, is galaxies. 
We live in a galaxy. 
The galaxy we live in is the Milky Way, 
so it seems fitting to start this week by 
discovering the Milky Way or following 
the discovering of the Milky Way, as this 
beautiful image is taken, obviously in 
the Southern Hemisphere shows, it's not 
hard to find the Milky Way. This grey 
path of light tilted 60 degrees to the 
celestrial equator that cuts across the 
sky has been known for antiquity. 
Understanding what it is is a different 
matter, and that's what we're trying to 
figure out in the course at most at the 
beginning of these clips. 
And I think it was Galileo who first 
realized that what this strip of light 
indicated is that we live in a 
disc-shaped universe. 
We live, we are part of disc-shaped 
collection of stars. 
What that means is that in directions 
perpendicular to the disk, we see a few 
stars nearby, and then beyond them to the 
darkness of no stars past. 
But when we look in the direction of the 
disk we see the nearby stars as always 
and then behind them we see more stars. 
And more stars, until they are very 
distant stars, to far for us to 
distinguish their light individually, but 
we can see the collective, luminosity, as 
this brightness of the sky. 
So, what Galileo realized, is that we 
live in a disc shaped collection of 
stars, 
A more systematic study of The shape of 
the universe was undertaken by Herschel 
in 1785, and what Herschel did is he 
divided the sky into a few hundred 
regions, counted the stars in each region 
assuming that the average luminosity of 
stars was same in all directions. 
He drew a map of the universe of, or if 
you want of this disc shape that we live 
in. 
and he drew this map extended in each 
direction, depending on how many stars he 
found, assuming that the farther the 
universe extended the more stars he could 
see. 
And his map is here on the right and the 
salient point, is that he put the sun, 
which is that brighter dot in the middle 
if his universe. 
The reason that he did this, as I said 
this was a scientific study, is that he 
found that in any direction, if he 
counted the stars in 1 direction in the 
sky and in the diametrically opposite 
direction, he found about the same 
numbers of stars. 
His interpretation, we are not near the 
edge of the world, else we would see few 
stars in 1 direction and more in the 
opposite direction. 
We see the same, in every direction as 
it's opposite, therefore he placed the 
sun roughly in the middle of the 
universe. 
a more refined version of this was 
carried out 150 years later by Kapteyn, 
and Kapteyn using slightly more numerical 
star counts, based on, brightness was 
able to put a size to a, the universe and 
the disc shape that he had was a 
flattened spheroid. 
It was 8 1/2 kiloparsecs in radius, and 
1.7 kiloparsecs thick. 
And the sun was not exactly in the 
middle. 
It was offset by a little bit, by 650 
kiloparsecs in the radial direction, 
and 38, sorry, 650 parsecs, in a radial 
direction, and by 38 parsecs away from 
the plane, which was the central plane of 
this disk, 
about which it extended for 1.7 
kiloparsecs. 
so this was, the universe as Kapteyn saw 
it in 1922. 
A few years earlier, Shapley in 1917, did 
a different, took a different approach to 
understanding the shape of the universe 
and our position within it. 
And, Shapley was looking at globular 
clusters. 
Globular clusters, can be found, and are 
typically found, outside the plane of the 
Milky Way, this ecliptic plane cut out by 
this, the, the plane of the center of the 
disc which when you see it in the sky is 
the plane of the circle of the Milky Way. 
In these globular clusters, Shapley found 
variable stars. They turned out to have 
been our RR-Lyrae variables. 
or he understood something about the 
period luminosity relationship for 
variable stars, so he could measure the 
periods for these stars, extract from 
this a luminosity, use that and the 
observed brightness to measure a 
distance, and so he could take a few 
globular clusters and map them in 3 
dimensions, and because he mapped them in 
3 dimensions, he could see that they are 
not scattered isotropically around earth, 
but rather, favor direction they are 
centered on direction towards the 
constellation Sagittarius, towards a 
point that he measured to be about 15 
kiloparsecs away, and they are they 
occupy a sphere of radius about 100 
kiloparsecs around this central point, so 
Shapley interpreted that point to be the 
center of the universe. 
The globular clusters formed a sphere 
around it, and, the, other stars that 
were not part of the globular clusters 
formed this disc in which the solar 
system found itself, but that too, he 
conjectured, would be centered about the 
same center as the center of the sphere 
of the globular clusters. 
He found, an interesting observation he 
found was that no globular clusters were 
visible to him within 10 degrees of the 
galactic plane. 
What we'll now call the galactic plane, 
the center plane of this, of the Milky 
Way. He called this the zone of 
avoidance, and all kinds of theories 
about how tidal forces would have broken 
apart globular clusters that strayed too 
close. 
So we have these 2 models of the 
universe. 
Kapteyn's model with a radius of 8 1/2 
kiloparsecs and Shapley's model with a 
radius of 100 kiloparsecs. 
the sun is either 650 parsecs from the 
center or 15 kiloparsecs from the center. 
A large universe and a small universe, 
who is right? 
It turns out that they were both wrong. 
The truth is somewhere in the middle, and 
they were both wrong for almost a cen, 
exactly the same reason, though in 
different ways. 
what they ignored was the fact that 
starlight, as it travels through the 
galaxy, suffers extinction, and this was 
not known. 
They knew by the 19, 20's that, there 
were interstellar gas clouds. those have 
been discovered, 
but in between the known interstellar gas 
clouds, they assumed the universe was a 
vacuum and hence completely transparent. 
it was not until 1930 that Trumpler 
proved that the interstellar medium, even 
between the known gas clouds. 
ex, absorbs and scatters star light so 
that starts undergo extinction. 
It was only in the 1930's that this 
became clear, that all stars in all 
directions suffer extinction. 
when light passes through the 
interstellar space of the galaxy. 
How did this, ignoring this, influence 
the results? Well, when Kapteyn was 
counting stars. 
He basically assumed that stars he 
couldn't see did not exist, but there 
were stars that he could not see because 
they were too dim due to extinction. 
And for this reason, he underestimated 
the size of the universe. 
He also was placing the sun much too 
close to the center, because essentially. 
If your visibility is limited by 
extinction, then you will be, unless you 
are very near the edge, in the center of 
what you can see. 
You can see about the same distance in 
all directions, so there will be as many 
stars in any given direction as in any 
other, basically because you can't see 
far enough to distinguish the, the edges 
of the world. 
And so Kepteyn's distances were 
underestimated both for the size of the 
universe and for the distance of the sun 
from it's center. 
Shapley, on the other hand, was using 
known luminosities of variable stars and 
so his his distance estimates were 
influenced by extinction in a different 
way. 
The variable stars that he saw, were 
dimmer than they actually were. 
We saw that when we did our homework, 
measuring distance using, our RR-Lyrae 
variable. 
The, he measured dim, RR-Lyrae variable 
stars, he interpreted that because he 
thought he knew their luminosities as 
them being distant, since he was 
neglecting extinction, and so he thought 
the globular clusters were farther than 
they actually were. 
Therefore, Shapley overestimated 
distances due to extinction, whereas, 
Kapteyn was underestimating distances due 
to extinction. 
The correct answer, as we now know it, is 
that the sun is about 8 kiloparsecs from 
the center of the galaxy. 
And so about half the distance estimated 
by Shapley, and about, oh, almost 10 
times, more than 10 times the distance 
estimated by Kapteyn. 
we know more about this, and we'll spend 
the rest of this week figuring out what 
it is we know about the Milky Way, what 
it is we know about other galaxies, and 
what this tells us about the universe in 
general. So that's our plan for the week. 
Study the Milky Way, understand where we 
know why I have been saying the universe, 
because this is what these astronomers 
called it. 
How we know that the Milky Way is not the 
same as the universe. 
Then study galaxies as a class, see what 
we can say about their evolution, their 
dynamics, their structure. 
This will lead us into a discussion of 
cosmic exciting topics like cosmic 
expansion and dark matter, which will set 
us up for next week's discussion of 
cosmology. 
Should be a fun week.