So, here's an instance where the history is so compelling, that we have to follow
it. So, the image here is of course the great
spiral Andromeda. Over the course of the centuries
astronomers had classified stars and they'd realized found planetary nebulae,
and they knew about gaseous nebulae like the Orion Nebula.
And then, there were these spiral nebulae of which Andromeda is a spectacular
example. This thing is almost 3 degrees across in
the sky. Remember, the moon is a 1/2 degree
across, so this is huge, but very, very faint.
You need a big telescope and to get an image like this,
lots of light collection and processing. But, spectacular and beautiful though it
were, though it is, it was not at all clear to anybody what it is and the, what
this comes down to is the question that has been hounding us throughout this
class, how far? If you know how far, you can use
the small angle, its approximation to figure out how big.
But how far is this thing? How do you measure the distance? And this proved to
be a big question in early days of the twentieth century.
So in 1920 in fact, there was a public debate conducted between two of the
leading astronomers of the day, Harlow Shapley and Curtis.
And Shapley made the, the, the argument was over the following question.
These spiral nebulae, like Andromeda, are they gaseous nebulae within the Milky Way
or are they Kant's original island universes? Kant had this reasoning that
these spiral nebulae should be worlds, universes on their own just like the
Milky Way, because there should be a hierarchy of structure as I understand
it. so Shapley, in fact, argued that there is
no way, this is a scientific argument, and Shapley was arguing that these spiral
nebulae are gas clouds in the Milky Way. And his arguments were basically this if
you take M31 to be a 100 kiloparsecs in radius, because that was the radius that
Shapley had found for the radius of of the Milky Way, and he knew one galaxy he
said, okay, if you're going to make this another galaxy, then it's probably going
to be like the Milky Way. If you give Andromeda a radius of 100
kiloparsecs, that puts it two mega parsecs away, six and, more than 6
million light years away. If Andromeda were to be that far we've
already seen that there are stars embedded in these nubulae and they had
tracked most interestingly novae. So there were, presumably, we now know
white dwarfs and white dwarfs with binary partners, there were novae.
Going on in these stars and the luminosity of a nova is not really
constant enough to make it a standard candle,
but the maximum luminosity, the order of magnitude of luminosity of a novae is
known. And what Shapley noted is that if you
take M31 and put it 2 megaparsec away, then you find that its novae must be much
more luminous than those in the Milky Way.
This is unlikely, it's much more reasonable to put it 100 kiloparsec away
on the other side of the Milky Way where it can safely reside.
And he also thought that there were measurements of the rate of rotation of
another spiral and 101. And, again, if your input, assume that in 101 has the
size of of the Milky Way, then you find that it is rotating too fast to be bound.
Curtis had reasonable arguments on the other side.
His argument was that these spiral nebulae are indeed island universes far
away and outside the Milky Way. And, again he was referring to scientific
evidence. He was saying, let us assume that the
novae M31 are the same novae we find in the Milky Way.
We know the average luminosity of a nova. This puts M31 at least a 150 kiloparsec
away. if you put M31 100 kiloparsec, 150
kiloparsec away, notice a lot closer, almost by a factor of ten, than Shapley
did then, its size is about 7.5 kiloparsec, which is quite reasonable.
This is about the size of Kapteyn's measurement for the Milky Way.
Remember Shapley's measurement of the 100 kiloparsec Milky Way had not yet been
accepted. It has never been accepted, it was wrong
by a factor of two. And there was this alternative small
Milky Way, so it basically comes down to what is the size of the Milky Way will
tell us whether Andromeda is in it or out of it.
also, he noted, that we measure radial velocities for stars in these spirals.
but we measure very little proper motion that suggests that they're very far away,
unless they're, these objects are preferentially moving radially.
Something has a large radial velocity, we expect it to have a large tangential
velocity, remembering that the tangential velocity is related to proper motion by v
equals v tangential is 4.74 times mu times D.
It measure a large v and a very small mu you conclude that you have a large D.
This is how Curtis sought to reconcile these measurements.
Both of them had valid arguments, near carried the day.
All they did was illuminated the issue. the issue was actually resolved
conclusively and beautifully by Hubble in 1923.
So we have here in fact, the plate on which Hubble made his discovery.
So Hubble was investigating photographic plates taken of
M31, the Andromeda spiral nebula, and in this region of M31, this is the region
near the center is what he was taking plates of and he found a few novae.
He was marking off novae, because measuring, as we said, the luminosity of
novae was one way to estimate the distance.
How do you know that a star is a nova? Well, it's a nova if it appears as a star
in today's plate but was not there on the plate you took a week ago.
So you compare new plates to old plates, and the new stars that have popped up are
novae and there is a date on when he, he took the measurement in October of 1923.
And then he went back and he realized that one of the novae he had marked, the
one in the upper right-hand corner, actually was present in older plates and
then was absent in yet older plates, but was present and he had found a variable
star. And, this was a very exciting discovery,
because unlike novae for which you only know the maximum luminosity.
For variable stars, we have a period luminosity relationship, we can compute
the luminosity of this star. Since we can measure its brightness, we
can use that to find the distance and Hubble was clearly excited. You can see
that he vigorously crossed out the n and with a big exclamation mark, marked on
the plate, this is a variable, that's the VAR up there.
He knew he was about to measure the size to Andromeda, he made the calculation and
the period luminosity relations showed it that Andromeda was 285 kiloparsec away.
That is too big to fit, even in Shapley's large hundred kiloparsec galaxy.
Andromeda is a separate galaxy. Now, the distance measurement was wrong.
We'll discuss that in a second, but you have this pause for a minute and
appreciate the sort of Copernican magnitude of that varied why it was that
Hubble was so excited. He had just realized that the universe is
not one galaxy but two, and of course, once we have two, presumably millions,
and as we know today, hundreds of billions of galaxies.
And so he had increase the size of the universe exponentially with this one
variable star. A very exciting discovery following its
trail always gives me goosebumps. Just looking at that plate, I think.
Imagine Hubble's excitement. So, I said his distance was wrong and
this is a good exercise in following how creative and how exacting the astronomers
had to be to figure out this three-dimensional structure that we take
for granted because we read it in books. So remember, that the period-luminosity
relation was discovered by Henrietta Leavitt, in Large Magellanic Cloud,
Cepheids. there are Cepheid, these are Population I
classical Cepheids in the Large Magellanic Cloud.
Now, she did not know the distance of a Large Magellanic Cloud exactly, what she
discovered was a period brightness relation.
and she knew they were all about the same distance, because they're all in the
Magellanic Cloud. So she knew she found a period luminosity
relation, but she couldn't calibrate it. She
couldn't know exactly what luminosity corresponded to what period only that
twice the period corresponded to some factor times the luminosity.
Now, Hertzsprung and Shapley both sequentially calibrated Leavitt's
relationship using Cepheids in the solar neighborhood to which they knew the
distance from parallax measurements. What they neglected was that these were
Cepheids in the galactic plane and extinction had made them appear dimmer by
a factor of four, so there was a 75% extinction to these Cepheids.
They thought they were four times dimmer than they really were therefore, their
period-luminosity relation was completely off.
In, now, Hubble was applying this measurement to a Cepheid in Andromeda,
where he's looking outside of the plane of the galaxy.
His luminosity is off by a factor of four, because, you remember that our
calculation of distance is proportional to the square root of the luminosity,
because brightness decays looks like L / D^2.
The fact that he was fourth, a factor of four off in the luminosity of this
Cepheid, meant he was approximately a factor a two off.
In, the distance in fact, the factor he was off by, was 2.7.
Andromeda is farther then he measured by a factor of about 2.7.
It's not it's, it's about 2.5 million light years away from here.
Shapley on the other hand, had been measuring the size of the Milky Way, by
measuring distances to globular cluster variables, but globular cluster variables
are a population too. Those are W Virginis stars not Cepheids.
And, at a given period, a W Virginis star is about four times less luminous than
the Cepheid with the same period. Remember, we showed those two graphs of
Population I and Population II Cepheids. And so Shapley, unknowingly, was actually
using the right period luminosity relation,
but, remember, he was missing extinction and so he overestimated the distances.
And, this lead for awhile to a great puzzle, now that we had the distance to
Andromeda, and we could therefore estimate its size by making a small angle
approximation. You now know the distance, you have the
angle, three degrees, figure out the size of Andromeda and figure out soon the
sizes of other galaxies, the Milky Way was larger than all the galaxies they
saw. This didn't seem reasonable.
And the diffuse extinction that Trumpler
discovered in 30s helped allay some of Shapley's correct, some of Shapley's
errors, and the distinction between Population I and Population II Cepheids
was not clear until 1952 when Bodey realized that there two populations of
Cepheids and two different period luminosity relations.
Now, today in the 90s we manage to calibrate Cepheid periods directly from
parallax measurements to distant Cepheids parallax measurements give us a correct
distance. we know enough about extinction in the
Milky Way to correct them. And you can use Cepheid variables to
measure distances out to as large as 30 million, megaparsec.
So that's 10 million light years sorry, that's almost a 100 million light years
away that we can use Cepheids as distance markers, because we can still measure
their periods, that's the distance record.
Of course it's still you, that gives you a luminosity to compare to the brightness
you see. You have to figure out something about
extinction in the 100 million light years between you and the star, but we have a
better understanding of extinction at least in the Milky Way then we had
before. And so we have Cepheids more variable,
relevant, we have a more reliable distance scale.
However, this gives you again an appreciation for how difficult it is to
actually build a three-dimensional universe out of that dark sky above us.
Now, once Hubble discovers one galaxy is outside,
he immediately goes on and measures the distances to many others, because you can
find Cepheid variables now that you know what you're looking for and now you can
just look at the nebulae. And Hubble comes up with a classification
scheme, another contribution of, of Hubble's is
the classification scheme for galaxies by their apparent shape and it has this
tuning fork shape. So, to the left are elliptical galaxies
in order of increasing electricity, so some galaxies are ellipsoids and
Hubble is measuring how eccentric the ellipsoid looks to us.
Of course, this is a crazy classification, because it tells you
nothing about a galaxy because if you take a football and look at it head on,
you can't distinguish it from a sphere. And so, this the E kind of classification
tells you about the shape of the galaxy as it appears to us, but not much about
the actual shape of galaxy. And then, there's this branching off into
spirals with sort of increasingly intricate spiral arm structure on the one
hand, and barred spirals on the other hand. And
so, the Milky Way we now know recently is a barred spiral, the Milky Way is an SBb
type barred spiral. Andromeda, which has no bar, is an Sb
spiral galaxy, later other kinds of sort of less
organized spirals were added by others and a class for the degree of coherence
of the spiral arms was introduced, so, and give it, it can give a more precise
classification. And we'll talk about the properties of
various kinds of galaxies as we go on now that we know they exist.