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.