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Lesson 2: A Short History of Stellar Astronomy – Joseph DalSanto


Joe DalSanto: In our
second lecture, we’re going to take a look
at the history of astronomy. Now necessarily this will need
to be a fairly brief history. I obviously won’t be able to
cover great details and all of the various discoveries
that were made, but I’d like to give you
a broad context of how we came to our understanding
of the universe today. I also won’t be able to
explain every discovery in this particular lecture,
but as we move along into the next
lectures, I will be able to go back and, again,
give you a much more detailed explanation. In this lecture or merely be
describing what was found. When we go back to the
most ancient cultures, we find that humans were
always looking at the sky. We think of culture
such as ancient Assyrian and Babylon, Egypt, and others. Now in these early times, we
were by no means doing science as astronomy, but rather
people were merely noticing what was in the sky. And frankly they were
somewhat superstitious and often attributed it
to supernatural beings and so forth, but they
were looking at the sky and taking note
of what they saw. In later centuries, other
cultures, perhaps in Europe, in the Far East, and even
in the Americas as well, were looking at the sky. We find that from
various records that have been left behind showing
different constellations and star almanacs and on
certain things of that nature. What were some of
the things that they were able to discover? Well, they notice of
course the daily motions of the sky, the monthly
cycles of the moon. They noticed the sun getting
higher and lower in the sky. And all these things
intrigued, them they didn’t have an
explanation for them. Even a few of the
stars, what they thought were stars,
the brighter ones, were moving, slowly
drifting, and these became known as planets. They even were able to
notice different eclipses and realize that these
recurred periodically. Well, in time they
began to think of ways that they could use this
knowledge effectively in their life. For example, they’d
begin to learn how to track time and
seasons, make calendars that they know different events
were going to take place. Now certainly, there were again
these religious ceremonial purposes perhaps to
various religions, but the point being there was
also some very practical sides to that knowledge as well. For example, most ancient
cultures are dependent heavily on agriculture to
grow their food, and they wanted to be sure
to know when was the best time to plant the crops. So that as the seasons
change, the crops would grow, they’d have food to eat. In fact, some
early civilizations even used their knowledge of the
sky to help them in navigating. We’ll talk about that
a little bit more. But for now, you might know that
as you travel north and south on the Earth’s surface,
the constellations appear to shift their position. You might even be familiar
with the North Star. As you watch that carefully
in the northern hemisphere, it gets higher in the sky,
the farther north you travel. And it gets lower in the
sky the farther south. So the point being, that you
can get a rough idea where you are simply by looking
at the North Star. Well, after some
centuries of this, one culture began to arise
that really kind of stands out in many ways, but in
particular for our discussion in astronomy, and that
was ancient Greece. As Greece rose to power, they
became known for their studies in many fields. We think of their literature
and philosophy, even their mathematics, such
as geometry and things, but the Greeks became
very good astronomers. And the reason they were
especially important is because that
was the first place we begin to see the scientific
approach to astronomy. Now the Greeks, of
course, observed the sky as earlier cultures
had, and they began to develop one of the very
first models of the universe. They certainly felt
that the earth seemed to be the center of everything. And frankly, it does
appear that way doesn’t it? Interestingly, some of
the early Greek scholars even knew that the
Earth was spherical. We often assume that
all ancient people thought the earth was flat,
that’s not necessarily true. Some of their scholars
realized it was spherical, and they placed
this spherical earth in the center of the universe,
with all the other bodies orbiting around them. Now they also made an
assumption that later on proved to be rather significant. Because they felt that
the heavens were perfect, they assumed that the orbits
of the bodies in the heavens had to be perfect circles. Because to them that was the
perfect geometrical shape. Very uniform, very
symmetrical, and so they assumed that the orbits in the
heavens were circular as well. But there were three
things in particular the Greeks did, that again,
we really point to today as showing us that
they were adopting that scientific approach. Number one, they began
to try to explain what they saw in the sky without
resorting to their mythology, or supernatural explanations. And this is what
scientists do today. Number two, as I
mentioned, they began to build models of
nature, and these were based on their observations. As they would watch the moon
move through its phases, they might think,
well, the moon appears to be going around the
Earth, and it’s probably illuminated by the Sun, and
that’s why we see the phases. Finally the Greeks became
known for reasoning on their observations
with each other. They would have
great discussions, much as they did with their
philosophy and other fields, and talk about the things
that they saw and observed and discovered. And yes, they put
their math to good use as well, using
geometry to explain what they saw in the sky. After some time, one
particular Greek astronomer began to rise above
the others so to speak. By the second century
AD, Claudius Ptolemy appeared on the scene. And Ptolemy took
upon himself to begin to take all of the known
astronomy of that time, and to compile it. He wanted to document this all. Different astronomers had
learned different things, he said I’m going
to take all of this and I’m going to
basically document it, and he did an outstanding,
comprehensive job of doing that. He wrote a total of
13 different volumes on the known
astronomy of his time. He touched on things such
as the constellations, the celestial sphere,
the motions of the Moon, the motions of the Sun,
eclipses, on and on these books went, and it really did become
quite a comprehensive work. Now this of course
was a good thing. It not only brought all
this information together in one place, and therefore
enabled others to learn, but it standardized
their learning. And in this way,
many people could be learned about astronomy. However, there was
one particular aspect that tended to be more
of a negative thing here, and that is the fact that
Ptolemy did such a good job that many people didn’t feel
it was necessary for them to maybe go on and do
so much observations. Or for them to think
about building new models, or for them to pursue
further investigations. And so this had the effect
of tending to stifle progress in astronomy. Many people felt if you wanted
to learn about astronomy, just read Ptolemy’s
volumes and you’ll be done. And unfortunately,
again, many did that. And as a result again,
progress was somewhat stifled. Well, Ptolemy’s works grew
and grew in their stature. Ptolemy began to be revered
as the primary figure in astronomy. For nearly 14 centuries
his work in astronomy was considered the
authoritative summary. While you may know
that of course the Greek civilization declined
and was overtaken by the Roman civilization, and
for several centuries the Romans then held sway. But eventually Rome fell, Europe
entered into the dark ages. There wasn’t a lot of learning
and science done at that time. But fortunate for us,
another civilization did preserve that knowledge
from the ancient Greeks. Islamic scientists were very
curious and very interested in the Greek understanding. They took those writings
and they preserved them, and they translated
them into Arabic. And for centuries, so to speak,
held that knowledge for us here in the future. Now they didn’t only preserve
and copy the information from the Greeks. No, these scholars were
quite capable themselves. They certainly had an idea of
how to do their own research. They improved timekeeping,
navigation by the stars. They developed a few
instruments such as the sextant. Which, in future times,
sailors would use to measure positions of stars. And they developed somewhat
of an interesting device called a astrolabe. That was sort of a miniature
model of the solar system. The bottom of the
astrolabe would have a metal disk with the
stars and constellations, and on top of that would
be other disks that could be turned
in a way to align with the planets, or
the Moon, or the Sun. And in a sense then you’d
have a miniature version of the solar system
in your hands. So quite a useful device. Many of these are
preserved in museums today. So again, for a number
of centuries here, these scientists
preserved, extended, improved our knowledge
of astronomy. Now like the ancient
Greeks, they still believed in an Earth
centered universe. And they refined
that to the point that it was pretty much as
good as it was going to get. But we have them
to thank, again, for preserving that information. At some point of
course, as Europe now began to come out
of the dark ages, European scholars were
interested in their writings. They took those Arabic works,
translated them into Latin. And Latin, of course, led
to many of our modern day languages. So here matter stood toward
the 14th, 15th century. And yet, a few
astronomers had noticed that there were a few nagging
problems with Ptolemy’s conception of the universe. In other words, as you would
look up and watch perhaps the planet Mars as it’s
moving week by week by week, it never quite
corresponded exactly to Ptolemy’s best model. Or maybe I should say that the
other way around, Ptolemy’s model never quite
corresponded exactly to what was really happening. So at some point,
people began to say, maybe we need to look
at something else. Well, you may have heard the
name Nicholas Copernicus. Copernicus was the first to
really stand up and suggest a different way to
look at the universe. Now this was quite a change. For centuries humanity
had just assumed the Earth was the center of everything. Few if any questioned it as
it seemed so very obvious that the Earth was the center,
and everything moved around us. But Copernicus
revived an idea that had been mentioned centuries
earlier in ancient Greece. That perhaps the Sun was
the center of the universe. And perhaps the Earth
was moving around it. Copernicus was very cautious
in publicizing this. He knew he’d be ridiculed. In fact, even the book that
he wrote on this topic, he delayed its publication
until much later in his life. Much of the
resistance was because the intellectual climate
simply stated that Earth was the center of everything. That humanity was the
center of everything. So it was really quite
a revolutionary idea. A whole new paradigm for us
to try to absorb and accept. And frankly, in
the beginning there wasn’t a lot of proof
for that particular view of the universe. But about this time,
shortly thereafter, another important figure
appears on the scene. His name is Tycho Brahe. Now Tycho Brahe was
a Danish nobleman, and he very much wanted
to observe the sky. By this point in time, not so
many people were doing that. But he wanted to go back and
do a very expert, first rate job of observing the sky. So he talks to the king,
and he obtained some funds. He builds really one of the
world’s first observatories. Now he doesn’t have
a telescope, but he does invent some
instruments that allow him to carefully
measure positions of stars, the Moon, the planets,
to better accuracy than ever before. Brahe does a tremendous
job for over 20 years. He makes these very exacting,
very precise observations. He does what we would today say
collect data, or observations that, again, could be used. What does he find? He finds that the
existing predictions of Ptolemy’s Earth-centered
model are simply wrong. You can not recognize
or reconcile that model with what’s happening. So here was a turning point now. It becomes very obvious through
his careful observations that our understanding
of the universe is going to have to change. It simply doesn’t work anymore. Well, Brahe’s done an
excellent job of observing, but another man appears
on the scene now, who is very good mathematically. And he recognizes the
value of what Brahe’s done. And he would like to get the
access to Brahe’s records and use them to try
to establish, again, this new Sun-centered
model of Copernicus. This man’s name is
Johannes Kepler. Kepler eventually gets some
access to Brahe’s data. Brahe’s a little cautious about
giving him all the information, but as time goes on and he
obtains the information, he begins to try to fit orbits
to Mars and other bodies. And he as well realizes that
you simply cannot fit a circular orbit to Mars. Well, he has a choice now. Either he trusts that
Brahe did a good job, or he goes back
to still sticking with the old model of Ptolemy. Which again, is becoming
increasingly inaccurate. Well thankfully for us,
Kepler very bravely says no, I do trust Brahe;s measurements,
and this is telling me that the planets do not move
in perfect circles. Rather, they move in ellipses. Now that may not seem all
that important to you and I, but it really opened the door
to breakthroughs in astronomy. Because Kepler was then able to
go on and determine that when the planet is closer to the
Sun in its elliptical orbit, it moves faster. And when it’s farther from
the Sun, it moves slower. This finally explains
why Mars’ motion doesn’t match the Ptolemy model. So this is a
significant advance. Kepler goes one step
further and discovers yet another law that relates the
period, or time for the planet to orbit, and its distance. And this third law
of planetary motion is a law that astronomers
have used to this day, and continue to use,
to help us understand the motion of planets. About this time,
another important figure arises on the scene. Perhaps. You’ve heard the name Galileo
Galileo dramatically changed our view of the heavens. Because you might know, he was
the first to use a telescope to observe the sky. We don’t think that he
necessarily invented it, but when he saw one, he went
home and made himself one and improved it, and
turned it on the sky and saw things no one
had ever seen before. He saw craters on the Moon
as we’ll see in a minute. He saw moons orbiting
Jupiter, phases of Venus. So many wondrous things that
have never been seen before. Now Galileo it was very excited
to share this with others, but many were quite skeptical. Because again, the intellectual
and even the religious climate at that time was somewhat
rigid, and believed in a certain view
of the universe that Galileo’s observations
were beginning to challenge. But the key for us is that
his observations provided the proof, or the evidence,
for the Sun-centered model of Copernicus. So this led to great
advances as time went on. Others began to get telescopes,
look at the sky and begin to see some of these things . And gradually these
ideas took hold. Some decades later, yet
another important figure comes on the scene. I’m sure you’ve heard
the name Isaac Newton. Now up at this point in time,
most astronomers, again, are merely describing
what they see. They really can’t explain why
different things would occur, but they could tell
you what they did see. For example, Johannes
Kepler could tell you how the planets move, but
he really couldn’t tell you why they move that way. Well Isaac Newton is
now going to explain further why we see what we see. Newton begins to
learn about motion, establishes three
laws of motion, and realizes that these
apply in the heavens. He also correctly learns
about, as we mentioned earlier, the law of gravitation. And he applies this,
not just here on Earth, but again in the heavens. He correctly realizes,
according to his laws of motion, that the Moon should be
moving on a straight line, and yet it’s not. It’s curving around the Earth. And when he thinks
about why that would be, he realizes the Earth must
be exerting a force that keeps the Moon in its orbit
instead of allowing it to travel at a straight line. So again, you see the
equation in the diagram of the law of gravity. And we emphasize
that Newton went beyond describing
what we saw up there, but he began that first
steps at explaining what we saw up there. Why does the Moon
move around the Earth? It was because of gravity. So in the succeeding
decades astronomers, again slowly, gradually
improved their telescopes. They began to see,
for example again, moons around Jupiter,
rings around Saturn. But at first their telescopes
really only allow them to explore the solar system. It wasn’t until the late 1700s
that a very important observer appears on the scene. And this man’s name
was William Herschel. William Herschel really
proved to be pretty much the most important observer
of the 18th century. Why is that? Well Herschel was dissatisfied
with the poor quality of telescopes in his day,
and so he built better ones. Instead of using lenses,
which were somewhat imperfect at that time, Herschel
switched over to a design by Isaac Newton
that used mirrors. He was able to make
the mirrors bigger, and therefore the
telescopes bigger. And he was able to see things
beyond the solar system that others had
never seen as well. So Herschel built
these huge telescopes. He spent many, many countless
evenings under the night sky, essentially sweeping the
sky looking for objects that he could observe. And he made our first
systematic catalog of these different objects
at the end of the 1700s. He had found hundreds of them. And frankly, this
was the first time that humanity was
able to see these. So this later on would
lead to great advances as well, after we had done
a first survey of the sky. Well, as we moved
into the 19th century, there were certain questions
that of course, humans had wondered about for ages. What are the stars? How far away are they? How big are they? What are they made of? All these questions
remained, and astronomy began to provide answers. The first important discovery
was the distance to the stars, and again we’ll see more
in a future lecture, but for now what
we essentially want to discuss is that
nearby stars could have their distance determined. If we observed their apparent
position at one point in time when the Earth was on one
side of the Sun in its orbit, and six months later
when the Earth was on the other side
of its orbit, we would get a slightly different
view, or slightly different perspective of nearby star. In effect it would
appear to shift position relative to the
background stars. Well, some simple geometry
would allow us then to determine the distance. The angle that that star
appeared to move in the sky could be used, again, to
determine its distance. So we’ll come back and
talk about this again. The actual term was parallax. And this enabled
astronomers to first get an idea, in the mid-1800s, of
the distances of the stars. They had to invent a new
unit called the light year. One light year being
approximately six trillion miles. And the nearest stars
turned out to be perhaps 10 or 20 light years. So quite a great distance
that humans had never really considered before. But we now had some idea
the distance of the stars. Well, as time went
on, astronomers continued to observe the stars. And they focused in in
particular on occasions when they would notice two stars
appearing very close together in the sky. They suspected,
and were correct, that these stars were
orbiting around each other. And this first of all
proved that Newton’s concept of gravity operated
among the stars. Well, as we’ll see in our
future lecture, observations of these two stars orbits,
the distance between them, and the amount of time
it took them to orbit, could tell us the
masses of the stars. In other words, how much
material does each star have. And this would open the
door and allow us then to discover other
characteristics of the stars in the future. So this was a very important
task for 19th century astronomy, was to
carefully measure the orbits of these
binary or double stars. And that led us to
an understanding of the masses of those stars. That really became, as
I said, the foundation for other important discoveries. Towards the end of
the 19th century, astronomers began to make
use of a discovery to tell us the composition of the stars. It had been discovered
that if here on Earth you took certain elements
and burned them, why the light that they
gave off if you broke that light into a
rainbow or a spectrum, would have certain lines in
it, some dark some light. And those lines were a
very unique pattern that corresponded to that element. In other words, if we heated
up a certain substance, looked at its light, broke
it into that spectrum, those lines indicated,
again, the identification of that particular element. Well I think you can see that
we could do something similar with the stars couldn’t we? And again towards the
end of the century astronomers began to do this. We take the light from
a star, we carefully break it into a spectrum,
and we look for those lines. And those lines tell us the
elements present in the star. So quite a ingenious
way for us to be able to determine the
composition of stars, without ever going there or
trying to get a sample of them. So we’ll talk more about
that in a future lecture, but this was a
major breakthrough to understand what the
stars were made out of. Some of the objects that
astronomers had cataloged, such as William
Herschel, appeared to be glowing clouds
of gas out in space. And sure enough, when
astronomers took the light from those clouds and again,
broke it into a spectrum, and looked at those
spectral lines, they were able to confirm
that yes indeed, these were clouds of primarily
hydrogen gas with some helium and a few others mixed in. But this told us that the bulk
constitution of the universe was hydrogen. Later
on we’re going to see that some of these
beautiful objects, some of these clouds, often glowing,
are where stars are forming. And so this led to another
whole subfield of astronomy, to talk about the formation
or the birth of stars. So these nebulae,
or clouds, began become a subject
of intense interest in the late 19th,
early 20th centuries. There were a few
different types. Some again appeared
as just clouds. Others were smaller and
rounder, planetary nebulae. But as astronomers began
to do their research, they began to identify the
composition of these objects, and learn what they
really meant for us. As we moved into
the 20th century, astronomers began to see
the differences in stars. That they had different
characteristics and properties. But at first we merely
wanted to sort them into various
categories so to speak. And this was somewhat
tedious laborious work. But again, it was important
for us to lay that foundation. And as astronomers
began to examine those early
classifications, they began to realize
that stars don’t come in all combinations of
brightness and temperature. You might think that, strictly
speaking, the hottest stars would be the brightest, and
that was generally true, but there were important
exceptions to that. So early in the 20th
century, two astronomers devised a chart, which
you’ll see in the slide here, and this became known as the
Hertzsprung-Russell diagram. This diagram, the
HR diagram, proved to be a key to understanding
the lives of the stars. So we’ll see much more of
it in a later lecture where we’ll put it to use, and
we’ll use it to understand the lives of the stars. Again, many stars are
related by their brightness and temperature, but
there are exceptions. And as a result
astronomers began to learn that they were
not all the same size. Some were much larger,
giant and super giant stars. Some were much smaller, and
became known as dwarf stars. So we’ll be back
later in the course to examine that in
much more detail. How we can use that diagram
to understand the stars. As we moved into the middle
decades of the 20th century, a real outstanding
discovery took place. You’ve probably heard
the name Edwin Hubble, and Hubble was the first to use
the large telescopes of his day to look out at some
of those nebulae and find that not all of
them were clouds of gas. Instead, Hubble was able
to make a discovery that changed our view of
the entire universe. Earlier a female astronomer,
Henrietta Leavitt had found the
distance to an object just somewhat outside
of our Milky Way galaxy, noticing the
variations in stars, and using those to get
an idea of the distance. Well Hubble took that
technique and he applied it to other objects
that he suspected were outside our galaxy. And sure enough, that’s
exactly what he determined. At the time, there was
known the great spiral and the constellation Andromeda. Hubble very patiently and
tediously observed that object, found those stars
changing their brightness, and was able to
determine its distance at over a million light years. Today we’ve actually
revised that number upward. We know that that particular
object is approximately 2 1/2 million light years away. But this was the first evidence,
the first proof, of a galaxy beyond the Milky Way. So this really
expanded humanity’s perception of the universe. To know that even our Milky Way
galaxy was not all there was. Well Hubble wasn’t finished yet. He made a second huge discovery. As he was starting to look at
a few other of these objects and determine that in fact, they
were galaxies outside of ours, he noticed something
very interesting. By carefully measuring the
galaxies spectral lines that we talked about, we
take the galaxies light break into that spectrum,
look at those lines, he could get a real good
idea of the speed or velocity of the object towards
or away from us. And he noticed that nearly every
galaxy was moving away from us. This seemed very odd. Why weren’t they just
all randomly moving? Well he began to realize
what this implied. There was a direct correlation
between the distance of the galaxy and its velocity. In other words, the
farther away the galaxy was, the faster it was
moving away from us. Well this implied something
very profound didn’t it? If you think about
that for a minute, if it appears at least to
us that all the galaxies are moving away, that would imply
the universe is expanding. And so this was the first
evidence that told us the universe was expanding. Well, you take that even
one step further and say if the universe is
expanding, surely that means that in the
past it was smaller. And as a result, if you go
back far enough into the past, the universe would
be very, very small, and had to originate
single point. Which today we
call the Big Bang. And this is really
the prevailing theory about the history of
our universe today. The Big Bang Theory began
with Hubble’s discovery of an expanding universe. So what a change in our
view of the universe. Well, we’ve sure covered a
lot of ground haven’t we? Let’s do a summary and
review some of the things we discussed. We started way back centuries
ago, millennia ago really, looking at the very earliest
cultures looking at the sky. They began to get a
knowledge of the sky. The ancient Greeks developed
this Earth-centered model that seemed to work for a while,
but some centuries later Copernicus, knowing that
model was imperfect, proposed a
Sun-centered universe. Today we know that the Sun is
the center of our solar system, not the entire universe. And it was Galileo’s telescope
that really provided the proof. When he saw moons
orbiting Jupiter, when he saw the
phases of Venus, that indicated that Venus had
to be orbiting the Sun. It could not be
orbiting the Earth. We then moved through
more recent centuries where astronomers determined
the distances of stars, then their masses,
than their compositions and their temperatures. And finally, in
the 20th century, we saw that Edwin
Hubble not only showed that there were
galaxies outside the Milky Way, but he showed that they
were expanding away from us. And this implied an
expanding universe, which began in a big bang. So quite an exciting story. What a grand sweep
astronomy has showed us now. And discoveries
continue to come today, as we’re going to see
in future lectures. So I look forward
to sharing more with you in our next lecture.

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