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We spent the last few weeks talking about 
stars, and we had to invoke all kinds of 

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great physics last week to understand the 
end of, stellar evolution. 

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We're going to take it to the next level 
this week, and the next level in size, 

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past stars, is galaxies. 
We live in a galaxy. 

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The galaxy we live in is the Milky Way, 
so it seems fitting to start this week by 

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discovering the Milky Way or following 
the discovering of the Milky Way, as this 

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beautiful image is taken, obviously in 
the Southern Hemisphere shows, it's not 

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hard to find the Milky Way. This grey 
path of light tilted 60 degrees to the 

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celestrial equator that cuts across the 
sky has been known for antiquity. 

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Understanding what it is is a different 
matter, and that's what we're trying to 

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figure out in the course at most at the 
beginning of these clips. 

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And I think it was Galileo who first 
realized that what this strip of light 

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indicated is that we live in a 
disc-shaped universe. 

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We live, we are part of disc-shaped 
collection of stars. 

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What that means is that in directions 
perpendicular to the disk, we see a few 

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stars nearby, and then beyond them to the 
darkness of no stars past. 

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But when we look in the direction of the 
disk we see the nearby stars as always 

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and then behind them we see more stars. 
And more stars, until they are very 

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distant stars, to far for us to 
distinguish their light individually, but 

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we can see the collective, luminosity, as 
this brightness of the sky. 

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So, what Galileo realized, is that we 
live in a disc shaped collection of 

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stars, 
A more systematic study of The shape of 

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the universe was undertaken by Herschel 
in 1785, and what Herschel did is he 

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divided the sky into a few hundred 
regions, counted the stars in each region 

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assuming that the average luminosity of 
stars was same in all directions. 

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He drew a map of the universe of, or if 
you want of this disc shape that we live 

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in. 
and he drew this map extended in each 

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direction, depending on how many stars he 
found, assuming that the farther the 

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universe extended the more stars he could 
see. 

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And his map is here on the right and the 
salient point, is that he put the sun, 

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which is that brighter dot in the middle 
if his universe. 

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The reason that he did this, as I said 
this was a scientific study, is that he 

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found that in any direction, if he 
counted the stars in 1 direction in the 

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sky and in the diametrically opposite 
direction, he found about the same 

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numbers of stars. 
His interpretation, we are not near the 

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edge of the world, else we would see few 
stars in 1 direction and more in the 

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opposite direction. 
We see the same, in every direction as 

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it's opposite, therefore he placed the 
sun roughly in the middle of the 

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universe. 
a more refined version of this was 

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carried out 150 years later by Kapteyn, 
and Kapteyn using slightly more numerical 

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star counts, based on, brightness was 
able to put a size to a, the universe and 

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the disc shape that he had was a 
flattened spheroid. 

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It was 8 1/2 kiloparsecs in radius, and 
1.7 kiloparsecs thick. 

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And the sun was not exactly in the 
middle. 

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It was offset by a little bit, by 650 
kiloparsecs in the radial direction, 

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and 38, sorry, 650 parsecs, in a radial 
direction, and by 38 parsecs away from 

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the plane, which was the central plane of 
this disk, 

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about which it extended for 1.7 
kiloparsecs. 

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so this was, the universe as Kapteyn saw 
it in 1922. 

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A few years earlier, Shapley in 1917, did 
a different, took a different approach to 

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understanding the shape of the universe 
and our position within it. 

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And, Shapley was looking at globular 
clusters. 

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Globular clusters, can be found, and are 
typically found, outside the plane of the 

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Milky Way, this ecliptic plane cut out by 
this, the, the plane of the center of the 

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disc which when you see it in the sky is 
the plane of the circle of the Milky Way. 

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In these globular clusters, Shapley found 
variable stars. They turned out to have 

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been our RR-Lyrae variables. 
or he understood something about the 

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period luminosity relationship for 
variable stars, so he could measure the 

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periods for these stars, extract from 
this a luminosity, use that and the 

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observed brightness to measure a 
distance, and so he could take a few 

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globular clusters and map them in 3 
dimensions, and because he mapped them in 

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3 dimensions, he could see that they are 
not scattered isotropically around earth, 

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but rather, favor direction they are 
centered on direction towards the 

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constellation Sagittarius, towards a 
point that he measured to be about 15 

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kiloparsecs away, and they are they 
occupy a sphere of radius about 100 

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kiloparsecs around this central point, so 
Shapley interpreted that point to be the 

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center of the universe. 
The globular clusters formed a sphere 

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around it, and, the, other stars that 
were not part of the globular clusters 

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formed this disc in which the solar 
system found itself, but that too, he 

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conjectured, would be centered about the 
same center as the center of the sphere 

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of the globular clusters. 
He found, an interesting observation he 

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found was that no globular clusters were 
visible to him within 10 degrees of the 

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galactic plane. 
What we'll now call the galactic plane, 

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the center plane of this, of the Milky 
Way. He called this the zone of 

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avoidance, and all kinds of theories 
about how tidal forces would have broken 

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apart globular clusters that strayed too 
close. 

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So we have these 2 models of the 
universe. 

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Kapteyn's model with a radius of 8 1/2 
kiloparsecs and Shapley's model with a 

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radius of 100 kiloparsecs. 
the sun is either 650 parsecs from the 

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center or 15 kiloparsecs from the center. 
A large universe and a small universe, 

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who is right? 
It turns out that they were both wrong. 

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The truth is somewhere in the middle, and 
they were both wrong for almost a cen, 

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exactly the same reason, though in 
different ways. 

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what they ignored was the fact that 
starlight, as it travels through the 

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galaxy, suffers extinction, and this was 
not known. 

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They knew by the 19, 20's that, there 
were interstellar gas clouds. those have 

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been discovered, 
but in between the known interstellar gas 

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clouds, they assumed the universe was a 
vacuum and hence completely transparent. 

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it was not until 1930 that Trumpler 
proved that the interstellar medium, even 

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between the known gas clouds. 
ex, absorbs and scatters star light so 

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that starts undergo extinction. 
It was only in the 1930's that this 

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became clear, that all stars in all 
directions suffer extinction. 

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when light passes through the 
interstellar space of the galaxy. 

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How did this, ignoring this, influence 
the results? Well, when Kapteyn was 

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counting stars. 
He basically assumed that stars he 

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couldn't see did not exist, but there 
were stars that he could not see because 

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they were too dim due to extinction. 
And for this reason, he underestimated 

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the size of the universe. 
He also was placing the sun much too 

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close to the center, because essentially. 
If your visibility is limited by 

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extinction, then you will be, unless you 
are very near the edge, in the center of 

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what you can see. 
You can see about the same distance in 

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all directions, so there will be as many 
stars in any given direction as in any 

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other, basically because you can't see 
far enough to distinguish the, the edges 

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of the world. 
And so Kepteyn's distances were 

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underestimated both for the size of the 
universe and for the distance of the sun 

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from it's center. 
Shapley, on the other hand, was using 

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known luminosities of variable stars and 
so his his distance estimates were 

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influenced by extinction in a different 
way. 

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The variable stars that he saw, were 
dimmer than they actually were. 

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We saw that when we did our homework, 
measuring distance using, our RR-Lyrae 

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variable. 
The, he measured dim, RR-Lyrae variable 

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stars, he interpreted that because he 
thought he knew their luminosities as 

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them being distant, since he was 
neglecting extinction, and so he thought 

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the globular clusters were farther than 
they actually were. 

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Therefore, Shapley overestimated 
distances due to extinction, whereas, 

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Kapteyn was underestimating distances due 
to extinction. 

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The correct answer, as we now know it, is 
that the sun is about 8 kiloparsecs from 

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the center of the galaxy. 
And so about half the distance estimated 

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by Shapley, and about, oh, almost 10 
times, more than 10 times the distance 

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estimated by Kapteyn. 
we know more about this, and we'll spend 

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the rest of this week figuring out what 
it is we know about the Milky Way, what 

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it is we know about other galaxies, and 
what this tells us about the universe in 

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general. So that's our plan for the week. 
Study the Milky Way, understand where we 

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know why I have been saying the universe, 
because this is what these astronomers 

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called it. 
How we know that the Milky Way is not the 

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same as the universe. 
Then study galaxies as a class, see what 

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we can say about their evolution, their 
dynamics, their structure. 

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This will lead us into a discussion of 
cosmic exciting topics like cosmic 

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expansion and dark matter, which will set 
us up for next week's discussion of 

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cosmology. 
Should be a fun week.