Practice Questions
Use the following information to answer the first practice question.Some of the stars you see in the night sky are so far away from Earth that the light from them has taken millions of years to reach you. So what you are really seeing is what those stars looked like millions of years ago. You are actually seeing into the past. To help us handle these huge distances, we use a special unit of measurement called a light-year to describe them. As its name suggests, a light-year is equal to the distance that light travels in one year. Light travels at 300,000 km per second in the vacuum of space.
- Sirius is the name of a star that is approximately 86 trillion kilometers (8.6 light-years) away from the Earth. How long does the light from Sirius take to reach the Earth?
A. 86 trillion seconds B. 300,000 seconds C. 8.6 seconds D. 8.6 years
- What can you deduce about a star if the light from it has been blue-shifted?
A. The star is cooler than most other stars. B. The light from the star is traveling faster than light normally travels. C. The star is moving away from Earth. D. The star is moving toward Earth.
Answers and Explanations
- The correct answer is D.
Because Sirius is 8.6 light-years away from Earth, the light from the star takes 8.6 years to reach the Earth.
- The correct answer is D.
According to the Doppler effect, the frequency of the light emitted from moving objects will shift toward either the red end of the spectrum or the blue end (depending on the direction of motion). Objects that are moving away from Earth are red-shifted. Objects that are traveling toward Earth are blue-shifted, making Choice (D) the correct answer.
Blue-shifted light doesn’t tell you how hot the star is, so you can reject Choice (A). Light always travels at 300,000 km per second in a vacuum, so Choice (B) is incorrect.
