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8.8D Light Years Earth and Space Light Years Lexile 960L 1 A beam of sunlight streams through your bedroom window, disturbing the darkness. You blink the brightness out of your sleepy eyes. It is hard to imagine, but the beam of light has traveled for over eight minutes to reach your room. Although this seems like a long time for light to travel to reach us, it is incredibly fast. The sunlight had to cross the vast distance between Earth and the Sun, a total of 149 million kilometers. It moves through space at 300,000 kilometers per second. This is faster than any known object in the universe can travel. The speed at which light moves is called the speed of light. The distance light can travel in a year is called a light year. Even though a light year may remind you of a unit of time, remember that it is actually measuring huge distances between objects in space. Light years also help astronomers study the early universe. 2 Astronomers use light years as a way of measuring distances that are too vast to use standard units such as miles or kilometers. The Sun, the closest star to Earth, is 149 million kilometers away. Scientists could also say that the Sun is eight light minutes away, since its light takes eight minutes to reach Earth. If something were to suddenly happen to the Sun, we would not know about it for eight minutes because that is how long it would take to reach Earth. The next closest star to Earth, Proxima Centauri, is 4.2 light years away. This means that it takes this star’s light over four years to reach Earth. As astronomers observe Proxima Centauri, they are seeing how it appeared 4.2 years ago. 1 8.8D Light Years Earth and Space 3 4 Light years also help scientists observe conditions at an earlier point in the universe. As you gaze up into the night sky, the twinkling little lights that you see may not even be present anymore. If a star is three million light years away, then its light has traveled for three million years to reach your eyes. If the star exploded into a red giant one million years ago, we would not know it. The explosion would not be observed on Earth for another two million years! What we see now is how that star appeared three million years ago. Studying the stars is like peering back through time. When scientists use powerful telescopes, like the Hubble Space Telescope, they can see as far away as the farthest galaxies. These galaxies are over 13 billion light years away. In a sense, astronomers are looking back into the beginning of the universe. Light years are an important unit of distance for astronomers or other scientists, who study the objects in the sky. Light years make enormous distances easily manageable. They also provide a window into the past for many objects in the universe. 2 8.8D Light Years Earth and Space 1 2 3 What is a light year? A A unit of measure that shows how much light is in one year B The distance that light can travel in one year C The amount of time it takes for light to reach Earth D The distance that a star travels in one year How do scientists use light years? A B 4 To report distances between objects in spaces and study the early universe To measure how fast an object is traveling and determine the origins of the universe C To measure the amount of light that a star produces and determine how hot it is burning D To determine the age of an object in space and study how it was formed 3 Why is staring at a star that is three million light years away like seeing back into time? A Ancient astronomers looked at the stars, too. B The star’s distance is far away from Earth. C The age of the star is older than Earth. D The star’s light has been traveling for three million years and the light shows how the star looked three million years ago. Why is it possible for a star that is no longer in space to still appear to shine? A A new star took its place B Stars never burn out C It is a reflection from the other stars in space D The light may have been traveling for a very long time and by the time the light reaches Earth, the star is no longer there 8.8D Light Years Earth and Space 5 The sun is _____ light minutes from earth. A two B eight C thirty-two D one 4