Lesson Explainer: Stars and Galaxies Science

In this explainer, 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.

When we look up at the night sky on a clear night, we can see thousands upon thousands of stars. The image below is a photo of the night sky taken in Yellowstone National Park in America.

Although stars look like very small, very faint points of light, this is actually just because they are very far away. If we were able to travel to a star and look at it up close, we would see that it is a very large, very hot, very bright sphere, just like our Sun. In fact, the Sun is a star. The only reason why the Sun looks so much bigger and so much brighter than the other stars is that it is much closer to Earth.

When we look up at the night sky, we might also see a faint band of light across part of it. This is easier to see from the southern hemisphere, in places such as South America, South Africa, and Australia. This band of light is shown in the photo below that was taken in the Atacama Desert in Chile.

This band of light is known as the Milky Way.

For many hundreds of years, the Milky Way was thought to be like a bright “cloud.” But in the seventeenth century, an Italian scientist called Galileo Galilei used a telescope—an instrument that can be used to view things that are very far away—to discern that the Milky Way is actually made up of very many stars, all very close together.

In fact, the Sun and the solar system, which includes Earth, the other seven planets, and many moons, asteroids, and comets, are part of the Milky Way. The Milky Way is actually a collection of billions of stars, many of which also have planets orbiting them.

This collection of stars has the shape of a disk. The Milky Way does not look like a disk from Earth, however, because Earth, the Sun, and the rest of the solar system are inside the disk.

The image below shows what this disk looks like if we were able to look down on it.

The stars of the Milky Way form a spiral.

The green dot on the above image shows the approximate position of our solar system in the Milky Way. Notice that our solar system is not at the center of the Milky Way, nor is it at the very edge. This means that no matter which direction we look into space from Earth, we should be able to see part of the Milky Way.

In reality, it is not possible to take a photo of the Milky Way from the view in the image above. In order to take a photo from this view, we would have to travel billions upon billions of kilometres away from Earth, and this is not possible. However, we are able to measure the positions of the stars in the Milky Way and then work out the overall shape of it.

A collection of billions of stars like this is known as a galaxy. The Milky Way is just one of billions upon billions of galaxies spread out across space.

Example 3: Describing the Shape of the Milky Way Galaxy

The picture shows a top-down view of the Milky Way.

Which of the following best describes the shape of the Milky Way?

1. A circle
2. A spiral
3. A sphere

Answer

Looking at the picture given in the question, we can see that the Milky Way has a clear spiral shape. In the diagram below, the spiral arms of the Milky Way have been highlighted in green.

The galaxy is also mostly flat. If we were to look at the Milky Way from the side, we would see something like what we see in the image below.

So, the correct answer is B.

The Sun and all of the other stars in the Milky Way orbit the center of the Milky Way. They move very fast along their orbits; the Sun, along with the rest of the solar system, moves at a speed of approximately 220 kilometres per second along its orbit around the center of the galaxy. However, because the distances involved are very large, it takes the solar system hundreds of millions of years to make one full orbit.

When measuring distances between objects in the galaxy, units such as metres and kilometres are not very convenient. These units are very small compared to the distances between objects in the galaxy, which means that distances have to be expressed as billions, trillions, quadrillions, or even larger multiples of metres or kilometres.

So, a unit that is often used instead is the light-year, which has the symbol ly. The light-year is a very large unit of distance that is equal to approximately kilometres, which we can write using scientific notation as km.

People often think that the light-year is a unit of time because it has the word “year” in it, but it is important to remember that the light-year is a unit of distance. The name “light-year” comes from the fact that 1 light-year is equal to the distance that a beam of light would travel in 1 year, if there were nothing in its way.

To see how useful the light-year is, we can consider the distances to some nearby stars. The closest star to the Sun is a star called Proxima Centauri. Expressed in kilometres, the distance between the Sun and Proxima Centauri is km, but expressed in light-years, this distance is 4.2 ly. Similarly, Vega, which is a very bright star, is located km away from the Sun, which is equal to 25 ly. As we can see, it is a lot easier to express these distances in light-years.

Earlier, we noted that it was an Italian scientist, Galileo Galilei, who first discovered, in the 1600s, that the Milky Way was comprised of many individual stars. He did this using a telescope, which is an instrument that can be used to view objects that are very far away. The design of telescopes was improving a lot around the time that Galileo was alive.

There are many different kinds of telescopes, but two of the simplest ones are refracting telescopes and reflecting telescopes. Refracting telescopes use lenses to make distant objects appear larger, while reflecting telescopes use mirrors to do this.