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Scientists use the light-year to describe the relationships between objects in space. What does a light-year measure?

• the distance that light can travel in one year

• the brightness of light that travels through space

• the number of years it takes light to travel to Earth

• the time it takes light to travel 1 million km

Astronomers use the light-year to describe the relationship between objects in space. For example, the Andromeda galaxy is 2.5 million ly from Earth. Which of the following units is most similar to the light-year?

• gram

• second

• kilometer

• degrees Celsius

Distances between objects in space can be enormous. Which of the following distances is the longest?

• 2.1 ly

• 950,000 km

• 9.5 trillion km

• 1 million ly

A light-year (ly) is the distance that light travels in one year. If a star is 3 ly from Earth, how long does it take light from that star to reach Earth?

• 3 min

• 3 h

• 3 y

• 3 ly

The sun is much larger than the moon. However, as viewed from Earth, the sun and moon appear to be the same size. Why do the sun and moon appear to be the same size when viewed from Earth?

• The moon is much hotter than the sun.

• The moon is much denser than the sun.

• The moon is much brighter than the sun.

• The moon is much closer to Earth than the sun.

Which two objects in space are located farthest apart?

• Earth and Mars

• Earth and the sun

• Earth and the moon

• the Milky Way galaxy and the Andromeda galaxy

Space exploration has advanced our knowledge of the universe. Which space journey would take the longest?

• a journey from Earth to the sun

• a journey from Earth to the moon

• a journey from Earth to a star in the constellation Centaurus

• a journey from Earth to the farthest planet in our solar system, Neptune

Which object is closest to Earth?

• Barnard’s Star

• Planet Neptune

• Andromeda galaxy

• Triangulum galaxy