Lesson Video: Stars and Galaxies Science

In this video, we will learn about the Milky Way and how the light-year can be used as a measure of the distances between stars in the galaxy.

09:27

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.

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