In this lecture we'll continue to look at
our place in the universe.
And we'll start by looking at what we know
about the history of our cosmos.
Observations show the universe is expanding.
The space between galaxies is actually increasing
with time. This implies that galaxies must
have been closer together in the past, and
if we go back far enough, we must reach the
point at
which this expansion began. We call this beginning
the Big Bang, and astronomers use the observed
rate of expansion to calculate that it 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.
We as people do not expand.
Within galaxies, clouds of gas and dust collapse
to form stars. These stars shine so brightly
because of nuclear fusion in their cores.
The nuclear fusion produces new elements-
hydrogen gets converted into helium. It's
true alchemy!
Stars eventually run out of nuclear fuel,
and when they do, they explode, spreading
their atmospheres with the elements they produced
at their cores into the galaxy. New stars
will be formed from this material, and the
cycle
continues.
Everything on Earth, all of us, are made of
elements that were produced in stars. The
material from which Earth is made was created
inside stars that lived and died before the
birth of our Sun. My favorite astronomer,
Carl Sagan was fond of saying "We are all
star stuff". Because it's true! The calcium
in your bones, the iron in your blood, the
carbon in your body- it was all formed inside
of a star.
You know Earth is moving, it doesn't feel
like it, but it is. And this motion fast and
perhaps even crazier than you might imagine.
Earth rotates one each day around its axis
from west to east. This is counterclockwise
if you were viewing Earth from above the North
Pole. This is why the Sun and stars appear
to rise in the east and set in the west. We
are whirling around Earth's axis at a speed
of more than 600 miles per hour!
We are also revolving around the Sun, one
orbit each year. We don't feel it, but it's
about 60,000 miles per hour! The Earth orbits
the sun in the same direction that it rotates
on its axis- counterclockwise as viewed from
above the North Pole.
Our entire solar system is also traveling
through the Milky Way. Stars in our neighborhood
move essentially random relative to one another,
but it's fast. On average our Sun is moving
relative to nearby stars at a speed of about
40,000 miles per hour.
We don't see this motion because stars are
so incredibly far away. Over thousands of
years, this motion will be evident, though.
In about 10,000 years, the constellations
we see today will look quite different. In
500,000 years, we
won't be able to recognize them at all.
The seemingly random motions of stars in the
Milky Way give way to a simpler and even faster
motion- the rotation of our galaxy. Our solar
system makes one orbit around the galaxy every
230 million years. We're moving around the
galaxy at an incredible 500,000 miles per
hour.
Within our local group, some galaxies move
towards us, some move away from us. At least
two small galaxies, the Large Magellanic Cloud
and the Small Magellanic Cloud are orbiting
our Milky May. The speeds are enormous! The
Milky Way, for example, is moving toward the
Andromeda Galaxy at about 180,000 miles per
hour.
In about 4 billion years the galaxies- the
Andromeda Galaxy and the Milky Way Galaxy-
will plow into one another, but don't worry,
the stars inside each galaxy are so far apart
that they will not collide. Nevertheless,
the stars will
be thrown into different orbits around a new
galactic center. Simulations of the collision
show that our solar system will probably be
tossed much farther from the galactic core
than it is today.
When we look outside the Local Group, we find
two things: First, virtually every galaxy
outside our Local Group is moving away from
us, and second, the more distant the galaxy,
the faster it appears to be racing away from
us.
This tells us the universe is expanding! But
remember it's space that is expanding, not
us, or the stars within galaxies.
The fact that we see all other galaxies moving
away from us doesn't imply that we are the
center of the universe. A viewer on any other
galaxy will also see all the other galaxies
moving away from them in an expanding universe.
A rising loaf of raisin bread is a good visual
model. If you imagine putting raisin bread
into your oven, as the raisin bread rises,
each raisin sees every other raisin moving
away from it as the loaf expands.
In this model, each galaxy is a raisin. The
difference is that unlike raisin bread, the
universe doesn't have a center and it doesn't
have an edge.
Okay, this slide summarizes our motion through
the universe.
The speeds are enormous. The next time you
have the opportunity, go out at night, lie
down, look at the stars, and start thinking
about this motion. I guarantee once you start
thinking about it, you'll start reaching for
the ground because you'll feel like you need
to hold on.
We'll end this chapter talking about the human
adventure of astronomy and how it continues
to be an adventure.
Humans have always looked at the sky- in awe,
in fear, in wonder. We are the universe, we
are essentially a cosmic consciousness, it
makes sense that we look out into the darkness
of night and want to know.
Astronomy offers us the best chance to answer
our biggest questions. How did we come to
be? Are we alone in the universe? Why are
we here?
Our species has invested enormous amounts
of time and treasure to search for answers.
We've built giant eyes to see farther into
space, we've sent probes to the outer reaches
of the solar system, and we continue to do
so. We are
so small, so tiny on the cosmic scale, but
our curiosity about the universe is vast.
I hope this course will begin to answer some
of your questions about the universe. But
I know it's likely that you will end up with
more questions than you began with. And I
don't think that's not necessarily a bad thing.
I'll leave you with this quote by my personal
hero, Carl Sagan. "For small creatures such
as we, the vastness is bearable only through
love."
