There is something in our universe that is an invaluable tool to astronomers,
and you encounter it every day. It's how we learn about the life cycles of stars, the movements of
planets, and even the gravitational pull of black holes. In fact, it makes it possible
to observe anything at all. But, even beyond that, it allows us to measure immense distances,
to prove that galaxies are moving, and even to see into the past. Sounds like a pretty
incredible scientific tool, right? It is, and lucky for astronomers, it's all around us: light!
Today, we're going to take a closer look at some of the
details of the universe that light helps us understand. By the end of this lesson,
you'll be able to explain how galaxies are moving apart in the universe and explain
how studying galaxies helps astronomers understand the past. Let's get started.
From
moons and planets to solar systems and galaxies, everything in space is always
moving. But our universe isn't just in motion, it's also expanding. All the galaxies in the
universe are gradually moving farther and farther apart. But galaxies are so massive and far away
from us that learning about them isn't as simple as just watching them move through the universe.
Instead, astronomers have to study how galaxies move in comparison to each other.
To understand how they did this, we first need to take a closer look at
what makes it possible for us to observe anything at all: light.
Light travels in waves. These waves can have different distances between waves, or wavelengths.
How that light looks to us, or whether we can see it at all, depends on its wavelength. For example,
light that we see as red has a long wavelength. Orange is a little shorter. Yellow is shorter than
that, and so on through the color spectrum, with blue and violet having the shortest wavelengths.
If the light source isn't moving, then no matter where you look at it from,
the wavelength is the same, so the light looks the same. The thing is, if the light source is moving,
the wavelength of light changes depending on where you're looking at it from. If this light source
were moving away from you, see how the wavelengths would start looking farther and farther apart?
On the other hand, if the light source were moving toward you, the wavelengths would
start looking closer together. This is called the Doppler effect, which is a change in wavelength
as a light source moves toward or away from an observer. Now think about how wavelength impacts
what we see and how this change in wavelength would change what astronomers observe when they
look at moving galaxies. Pause the video here and record your thoughts in your guided notes.
Remember, lights' wavelength determines its color, so the Doppler effect means that astronomers can
observe the movement of galaxies by looking at their color. If a galaxy is moving away from us,
the wavelength of the light it emits becomes longer, so the galaxy appears red.
This is called redshift. On the other hand, if a galaxy is moving toward us, the wavelength
of the light it emits becomes shorter, so the galaxy appears blue. This is called blueshift.
The patterns of redshift and blueshift tell us that the universe is expanding, and,
as a result, all the galaxies in it are moving apart.
So by studying these patterns of light wavelengths and colors in the galaxies,
astronomers can learn all about how galaxies are moving, or rather, how they were moving.
Every time an astronomer studies a galaxy, or even when we look at the stars, the night sky,
we're basically looking into the past. But how is that possible? Think about what you know already
about how light moves, then pause the video here and record your prediction in your guided notes.
Light is incredibly fast. In fact, it's the fastest thing in the universe. But the universe
is truly huge. At a speed of over 670 million miles per hour, it takes light from the sun
about eight minutes to reach Earth, and about four hours for it to reach the farthest planet
in our solar system, Neptune. In cosmic terms, Earth and Neptune are practically neighbors,
so imagine how long it takes for light to cover the vast expanses of space.
As you might remember, a light year is a measure of distance equal to the amount of time light
can travel in a year. Well, the nearest galaxy to the Milky Way, the Andromeda galaxy, is two
and a half million light years away. So, let's put this another way. When astronomers observe
the Andromeda galaxy, they're seeing light that traveled for over two and a half million years
before reaching their telescopes. In other words, some scientists think we're seeing the Andromeda
galaxy as it might have been two and a half million years ago. Studying astronomy is as close
as we can get to time travel, and Andromeda might sound far away, but remember, it's the closest
galaxy to ours. Astronomers have found galaxies over ten billion light years away from ours.
So, if light from these distant moving objects takes so long to reach us,
aren't we observing a really outdated version of everything in space?
Couldn't it have changed? And the short answer is yes, there could be stars we see in the night
sky that have gone supernova and collapsed a long time ago. In fact, entirely new galaxies
could have formed, but we won't find out until their light reaches us in a few million years.
Seeing galaxies as they were at different points in the past also allows astronomers
to study processes that they think must take a long time, like how stars and galaxies form.
So, if you've ever wondered what it would be like to time travel,
I have some great news - all you need to do to find out is look up at the stars.
Let's recap what we've learned today. You now know that the Doppler effect is responsible
for the redshift and blueshift astronomers observe that show them how galaxies move,
and since light from moving galaxies takes so long to reach us, we are always observing long
outdated versions of the universe. Pretty cool, huh? Be sure to check out the practice questions
and extension activities that go with this lesson to learn even more about how light
helps us study the universe. And always remember, you are out of this world! See you next time.