Video Transcript
In this video, we will learn about
the Milky Way and how the light-year can be used as a measure of the distance
between stars in the galaxy.
When we look up at the night sky on
a dark, clear night, we can see thousands of stars. To us, they appear as faint specks
of light, but that’s only because they’re very far away. If we were able to travel to one of
these stars and see it up close, we would see that it is a very hot, very bright
sphere, much like our Sun. In fact, the Sun is a star, just
like all the ones we can see in our night sky. It only looks so much bigger and
brighter than the others because it’s so much closer to Earth.
In fact, the Sun is quite an
average star. If this drawing was Sirius, the
brightest star in our night sky, and we could put the Sun right next to it, this is
roughly how they would compare. And if we could travel far enough
away that we could see all of our neighboring stars together, each as a tiny,
indistinguishable speck of light, we would see that they make a shape something like
this.
This is the Milky Way galaxy. It contains hundreds of billions of
stars, so many that we can’t see them individually. The Sun is located approximately
here, just one of the hundreds of billions of specks of light that make up the Milky
Way galaxy. The Milky Way has a spiral disc
shape. It’s somewhat similar to a fried
egg with a bulge in the middle and a large flat disc surrounding it. If we could see it from the side,
it would look something like this, with a bulge in the center and a flat disc
surrounding it and the Sun and all of its planets, including Earth, located
somewhere in the middle of this disc.
And this is why if we look at the
sky on a very dark, very clear night, we can sometimes see a band of light across
the sky. It looks a bit like a cloud. But it’s actually the combined
light of billions of stars too far away to see individually. This is our view of the edge-on
Milky Way galaxy as seen from Earth. From parts of the Southern
Hemisphere, places like parts of South America, Southern Africa, and Australia, we
can even see the bright light of the bulge of the galaxy. This is our view from inside the
Milky Way, looking out through the disc.
Because we’re inside the Milky Way,
we can’t see the spiral shape. But we can measure the positions of
all of the stars and work out what it would look like if we could see it from
outside. But if we look deep into space
using telescopes, we can see other shapes like this. These are other galaxies. Once known as “island universes,”
they each contain hundreds of billions of stars.
Galaxies come in lots of shapes,
sizes, and colors. Most of them have spiral arms
similar to our Milky Way, but not all of them. There are some that are spherical
or shaped like rugby balls. What all galaxies have in common is
that they contain billions of stars. And there are at least a hundred
billion galaxies. That’s a one with 11 zeroes. And since each one of those
galaxies can contain hundreds of billions of stars, they’re also very large. The Milky Way, for example, is
approximately 10 to the 18 kilometers across. The Sun, located down here, orbits
the center of the Milky Way, much as the Earth orbits the Sun. But because the Milky Way is so
large, it takes the Sun approximately 250 million years to complete one orbit. To do this, it has to travel at
around 220 kilometers per second, taking all of the planets, such as Earth, with
it.
As we can see, when we’re dealing
with very large distances, like the size of galaxies, it doesn’t make much sense to
measure things in kilometers because the numbers get so large and unwieldy. A better alternative is a unit
called the light-year. The light-year is defined as the
distance light travels in one year. And one light-year is equal to 9.46
times 10 to the 12 kilometers. Because the light-year has the word
year in it, a common mistake is to think it refers to a period of time. But remember that a light-year is a
unit of distance, the distance that light travels in one year, or 9.46 times 10 to
the 12 kilometers.
What makes the light-year useful is
that it makes the numbers involved a lot more manageable. We saw earlier that the Milky Way
is around 10 to the 18 kilometers across. This is about 100,000
light-years. The light-year is an especially
useful unit when we’re considering the distances between stars inside the
galaxy. Consider the distance between our
nearest star, the Sun, and our next closest neighbor, Proxima Centauri. If we were to write this distance
in kilometers, we would find that it’s 4.02 times 10 to the 13 kilometers. But this is just 4.2
light-years. And if we measure the distance
between the Sun and Vega, a bright blue star, we find that it’s 2.37 times 10 to the
14 kilometers, or just 25 light-years. As we can see, when we’re dealing
with these very large distances, it’s much easier to work in light-years.
Most of what we have discussed in
this video about stars and galaxies has been discovered using telescopes. There are lots of different types
of telescopes, but they come in two main categories. A refracting telescope uses a lens,
whereas a reflecting telescope contains mirrors. A refracting telescope is what
Galileo used in the 1600s to make a lot of important discoveries, including
separating the Milky Way into stars. And refracting telescopes are still
very popular today. But most modern-day research
telescopes, including those on the ground and the Hubble Space Telescope, which
makes observations from space, are all reflecting telescopes.
What all telescopes have in common
is that they make distant objects appear larger, whether they use lenses or mirrors
to do this. A telescope doesn’t make an object
brighter. It just makes it larger so that
it’s easier to see. Telescopes also do not change an
object’s color.
Now that we’ve talked about the
nature of stars, galaxies, and the telescopes used to observe them, let’s get some
practice with some questions.
Which of the following correctly
describes what stars are? (a) Stars are small, hot objects
located just above the atmosphere. (b) Stars are small, hot objects at
the edge of the solar system. Or (c) stars are very large, very
hot objects that are very far away from the solar system.
When we think of stars, we might
think of them as small, faint specks of light. But recall that this is just
because they’re so far away. In reality, stars are very similar
to the Sun and in many cases even larger. And they only appear small and
faint to us because of how far away they are. If stars were located just above
the atmosphere, they would be even closer than the Sun and so appear even
larger. This is clearly not the case, so we
can eliminate option (a).
Even if stars were at the edge of
the solar system, they would not be a great deal further away from us than the
Sun. And also we know that stars are not
small. So we can eliminate option (b).
The correct answer is option
(c). Stars are very large, very hot
objects that are very far away from the solar system.
Complete the following sentence: A
light-year is a unit of blank. Is it (a) time, (b) distance, or
(c) speed?
Recall that a light-year is the
distance light travels in one year. And one light-year has a value of
9.46 times 10 to the 12 kilometers. It’s important not to be misled by
the word year. Just like the kilometer, a
light-year is a unit of distance. And so the correct answer is that a
light-year is a unit of (b) distance.
Let’s now summarize the key points
covered in this video. Stars are large, hot spheres that
are very far away. We saw that galaxies contain
billions of stars, often arranged in spiral shapes. We saw that the Sun and the solar
system are located in a spiral arm of the Milky Way galaxy. We discussed the unit of the
light-year and that a light-year is the distance traveled by light in one year and
that one light-year is equal to 9.46 times 10 to the 12 kilometers. We saw the two main types of
telescope, which can be reflecting or refracting, and that both make objects appear
larger.
