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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.

Blue dark night sky with many stars above field of trees

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.

Wonderful Milky Way, seen from the desert of northern 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.

Our galaxy milky way

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 1: Describing What Stars Are

Which of the following statements correctly describes what stars are?

  1. Stars are small, hot objects located just above the atmosphere.
  2. Stars are small, hot objects at the edge of the solar system.
  3. Stars are very large, very hot objects that are so far away from the solar system.

Answer

When we look up at the night sky, we see thousands upon thousands of stars that look like very small, very faint points of light.

However, this is just because they are very, very far away. Stars are actually very large, very hot, very bright spheres. They are located a very long way from Earth and a very long way from our solar system, which makes them appear very small. So, the answer is C.

Example 2: Describing What Galaxies Are

Complete the following sentence: A galaxy contains of stars.

  1. hundreds
  2. billions
  3. thousands
  4. hundreds of thousands
  5. millions

Answer

A galaxy is a collection of billions of stars; a billion is a 1 with nine zeros after it, or 1‎ ‎000‎ ‎000‎ ‎000, so this is an extremely large number of stars. Our own galaxy, the Milky Way, is thought to have between 100 billion and 400 billion stars in it, some of them like our own Sun, many bigger, and many smaller.

So, the correct choice is B.

Example 3: Describing the Shape of the Milky Way Galaxy

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

Our galaxy 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.

Our galaxy milky way

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.

Colorful Nebula in Space Background

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 9.46trillion kilometres, which we can write using scientific notation as 9.46×10 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.

Definition: Light-Year

A light-year is a unit of distance equal to the distance that a beam of light would travel in 1 year. Expressed numerically, 1=9.46×10.light-yearkilometres

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 4.02×10 km, but expressed in light-years, this distance is 4.2 ly. Similarly, Vega, which is a very bright star, is located 2.37×10 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.

Example 4: Describing What a Light-Year Is

Complete the following sentence: A light-year is a unit of .

  1. time
  2. distance
  3. speed

Answer

It is important to remember that even though the word “light-year” has the word “year” in it, a light-year is not a unit of time.

A light-year is a unit of distance equal to the distance that a beam of light would travel in 1 year, if there were nothing in its way. It has a value of approximately 9.46×10 km.

So, the correct answer is B.

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.

Example 5: Describing the Function of Telescopes

Complete the following sentence: A telescope makes distant objects appear .

  1. bluer
  2. dimmer
  3. smaller
  4. larger
  5. brighter

Answer

A telescope is an instrument that can be used to view objects that are very far away. It does this by making the objects appear larger—we say that it magnifies the objects.

Note that a telescope does not change the brightness of the image we see through it—the object does not appear brighter or dimmer—it simply appears larger. It also does not change the color of the object, so the object will not appear bluer or redder.

So, the correct answer is D.

Key Points

  • Stars are very large, very hot, very bright spheres similar to the Sun. The reason why stars appear to be so much smaller and so much fainter than the Sun, however, is that they are very far away.
  • The Sun, along with billions of other stars, is part of the Milky Way, which is a galaxy.
  • The Milky Way has a spiral shape. All of the stars in the Milky Way orbit the center of the Milky Way. Our solar system is located on one of the spiral arms of the Milky Way.
  • A light-year is a unit of distance equal to the distance that a beam of light would travel in 1 year if there were nothing in its way, and it has a value of 9.46×10 km.
  • Scientists have, for centuries, used telescopes to observe distant stars. Two of the simplest kinds of telescopes are refracting telescopes and reflecting telescopes.

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