Hello, in this first lecture, we will take
a look at our place in the universe, and try
to wrap our minds around the sizes of our
planet, our solar system, our galaxy, and
our universe.
The backdrop here is an image that was taken
with the Hubble Space Telescope.
It's a very small piece of sky.
Imagine taking a picture through a 1 millimeter
by 1 millimeter square held at a meter away,
and that's what we have here.
But even with such a small field of view,
look at all those galaxies!
The whole image contains about 10,000 galaxies,
and each individual galaxy contains billions
of stars.
The smaller smudges are galaxies so far away
that the light takes billions of years to
reach Earth.
The only objects that aren't galaxies are
a few stars in the foreground.
These are part of our own Milky Way galaxy.
The study of astronomy can be likened to a
grand journey, and like most journeys, we'll
begin from home.
Home for us is planet Earth, the third of
eight planets in our solar system.
Earth is on average 93 million miles (or about
150 million kilometers) from the Sun.
At highway speeds it would take about 152
years to travel from the Earth to the Sun.
Our solar system consists of the Sun, the
planets and their satellites, and many other
smaller objects such as asteroids and comets.
The eight planets are: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and Neptune.
There are many mnemonics for remembering the
planets.
The one I learned when I was a young sprout
is: "Mary's Velvet Eyes Make John Stay Up
Nights".
You've probably heard that Pluto is no longer
considered a planet.
And this is why- the definition of planet
was set in Prague in 2006 by the International
Astronomical Union.
They say that in the Solar System, a planet
is a celestial body which: is in orbit around
the Sun, has sufficient mass to assume a nearly
round shape, and has "cleared the neighborhood"
around its orbit.
Pluto fails on number three.
It's just too small.
There are objects besides the Sun and planets
like the millions of asteroids, or minor planets.
Most are thought to be the shattered remnants
of planetesimals, bodies within the young
Sun's solar nebula that never grew large enough
to become planets.
Comets are essentially dirty snowballs made
of rock, dust, water ice, and frozen gases
like carbon dioxide, carbon monoxide, methane,
and ammonia.
Determining their size is difficult, but comet
nuclei with radii of up to 20 kilometers have
been observed.
The tail of a comet can be much larger- thousands
or even millions of kilometers across.
The orbits of comets are highly elliptical-
that is their orbits look like really squished
ovals.
Periodically Earth will pass through a comet's
orbital path.
Because comets leave all sorts of rocky debris
behind them, when Earth passes through a comet's
orbit, the debris hit our planet.
You may have seen "shooting stars" in the
night sky.
These are meteors- little comet pieces burning
up in our atmosphere.
Therefore when we pass through a comet's orbit,
we get a "meteor shower".
The Sun is our parent star, and the source
of light and life on planet Earth.
As far as stars go, the Sun is nothing exceptional.
It formed approximately 4.5 billion years
ago, and will continue to provide energy via
nuclear fusion in its core for another 5 billion
years or so.
All of the other stars we see not really different
from the Sun.
Our parent star is just one of several hundred
billion stars in our galaxy, the Milky Way
Galaxy.
The sky is divided up into 88 official constellations
defined by the International Astronomical
Union.
Every star in the sky belongs to a constellation.
From a dark sky site, you may notice a cloud-like
band off light arching overhead.
It may be difficult to tell with just your
eye, but binoculars or a small telescope will
show that this band of light is a result of
many stars that appear close together.
This is the view we have of our galaxy, the
Milky Way.
The band of the Milky Way is nearly impossible
to see from Rockville or DC unless all the
power goes out.
It may seem strange that we see a band of
light like this, but once you think about
how we're situated inside the galaxy, it should
make a little more sense.
The Milky Way resembles a thin circular disk,
like a pancake.
Our solar system is inside the disk of the
galaxy, about two-thirds of the way out from
the center.
If we look straight up or straight down, we
don't see as much pancake compared to when
we look into the pancake.
That is, when we look in the direction of
our galaxy's disk, we see many more stars-
the band of the Milky way- than when we look
in directions above or below the disk.
Every star you see in the night sky is part
of our Milky Way galaxy.
There are billions of other galaxies with
billions of stars all their own.
We can see a few of our neighboring galaxies
from a dark sky sight with the naked eye (they
look like fuzzy cotton balls), but we cannot
distinguish the individual stars in any galaxy
with our eyes except our own.
Each galaxy can be thought of as an island
of stars.
If you lived in another galaxy, you would
see a whole new set of stars in your sky.
Within the disk of the Milky Way are clouds
of dust and gas in space called nebulae, like
the beautiful Orion Nebula.
Nebulae can be enormous compared to the size
of our solar system, but small relative to
the total size of our galaxy.
There are billions of galaxies in the universe
and galaxies tend to group together.
Our Milky Way, for example, is one of two
largest galaxies among about 40 galaxies in
what is known as the Local Group.
Groups of galaxies with more than a dozen
members are known as galaxy clusters, like
the Coma Cluster shown here.
This is an image of our neighboring galaxy,
the Andromeda Galaxy, also part of the local
group.
On a very large scale the universe has structure.
Galaxies and galaxy clusters appear to be
arranged in sheets and chains with large voids
of emptiness between them.
The superclusters are where the galaxies are
clumped together.
Everywhere we look in the universe we see
this overall structure.
Later in the semester we'll talk more about
the large-scale structure of the universe.
This is cosmology!
Now, let's take a moment to talk about units
of distance.
There are a variety units that are used for
measuring distance, but there are two that
are often used in astronomy.
The first is the astronomical unit, or AU.
The astronomical unit is Earth's average distance
from the Sun and it's about 150 million kilometers
or 93 million miles.
The AU is useful for smaller measurements.
For larger scales we generally use the light
year.
A light year is the distance that light can
travel in one year.
Light is pretty darn fast so it can cover
a lot of space in a year.
A light year amounts to 10 trillion kilometers
or about 6 trillion miles.
It's important to remember that a light year
is a measure of distance, not time.
Also, nothing travels faster than light.
It's the universal speed limit and it's a
constant.
The speed of light is 300,000 km/s.
Because it's a special number, it gets its
own a special letter.
The speed of light is denoted by the letter
'c'.
The 'c' stands for celeritas which is Latin
for 'rapid' or 'swift'.
Light has a finite speed, so it means that
it takes a certain amount of time for light
to travel from an object to us.
Even for close objects, you never see anything
in real time.
Consider the Moon.
It takes light from the Moon one second to
reach us on Earth.
It takes light from the Sun about eight minutes
to reach us.
This means if something really strange happened
right at this moment on the Sun, we would
not know about it for eight minutes.
Sirius is a nearby star.
It takes light 8.6 years for the light to
reach us from Sirius.
Light from the Andromeda Galaxy spends two
and a half million years traveling to reach
Earth.
This means we see objects as they were in
the past, and the farther away we look in
distance, the further back we look in time.
Here is the lovely Andromeda Galaxy again.
The Andromeda Galaxy is actually bright enough
that you can see it with your naked eye from
a dark sky site.
It looks a little bit like a fuzzy cotton
ball.
From Rockville or anywhere with city lights,
we need binoculars or a telescope to see it.
If you could go out tonight and see the Andromeda
Galaxy, the light that would hit your eye
left that galaxy two and a half million years
ago.
When will be able to see what the Andromeda
Galaxy looks like right now at this very moment?
Well, we have to wait two and a half million
years.
We estimate that the universe is about 14
billion years old, and we'll talk all about
how we know this later in the semester.
Because of the finite speed of light, the
farther away something is, the longer it took
the light to reach us.
At great distances we see objects as they
were when the universe was much younger.
The observable universe is the portion of
the entire universe that we could potentially
observe.
If we try to look beyond 14 billion light
years, we'd be looking to a time more than
14 billion years ago, and this is before the
universe existed, so there's nothing for us
to see.
The distance of 14 billion light years therefore
marks the boundary or horizon of the observable
universe, and the observable universe is the
portion of the entire universe that we could
potentially observe.
Finally, let's take a brief look at the history
of our universe.
Observations of distant galaxies outside of
our Local Group reveal that they are moving
away from us.
We also find that the farther away a galaxy
is, the faster it is moving away from us.
These observations imply that the space between
galaxies is increasing with time.
The universe is expanding!
Following this line of thought, we can figure
that galaxies must have been closer together
in the past.
If we go back far enough, we must reach the
point at which this expansion began.
We call the beginning of the universe the
Big Bang.
Astronomers use the observed rate of expansion
to calculate that the Big Bang occurred about
14 billion years ago.
The universe as a whole has continued to expand
ever since the Big Bang, but on smaller scales,
gravity holds things together.
The stars within galaxies and galaxy clusters
do not expand.
Things on Earth (people, rulers, coffee cups)
are not expanding.
It's only the space between galaxies that
is expanding.
Let's go back to our galaxy for a moment.
The disk of the galaxy is where all star formation
takes place, simply because that's where the
material is to make new stars.
We'll talk much more about this later in the
semester, but for now know that a star goes
through most of its life generating energy
via nuclear fusion reactions in its core.
Eventually every star will run out of nuclear
material.
The largest stars use their nuclear fuel the
quickest and die in massive explosions called
supernovae.
When any star dies it blows its atmosphere
back into space.
Eventually this material will form new stars.
Our Sun and solar system were created from
the remains of other stars.
Every atom on Earth was once part of a star.
From the iron in your blood, to the calcium
in your bones- every bit of you was once inside
of a star.
The late astronomer Carl Sagan was fond of
saying "We are all stardust".
It's true!
We are made of the universe.
We are a cosmic consciousness.
We are truly a way for the universe to know
itself!
That's all for the first lecture.
Over the course of the semester we'll explore
the details of the topics I touched on today,
as well as many other fascinating things like
light and telescopes and exoplanets and dark
matter.
But for now take care, and I'll talk you again
soon.
