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Difficulty: EasyStars, Galaxies, and the Universe

When calculating the separation between distant galaxies, astronomers express their measurements in light-years. What physical quantity is measured by a light-year?

  1. A
    The time required for starlight to cross the Milky Way galaxy
  2. The total distance that light travels through a vacuum in one Earth yearAnswer
  3. C
    The constant speed at which electromagnetic radiation travels through space
  4. D
    The total radiant energy output produced by a star over a twelve-month period

Answer

The total distance that light travels through a vacuum in one Earth year
The option stating that a light-year is the total distance light travels through a vacuum in one Earth year is correct because a light-year is an astronomical unit of length defined by the product of the speed of light (cc) and one Earth year.

Step-by-Step Solution

1
Identify the fundamental definition of the astronomical term 'light-year'.
A light-year is defined as the product of the speed of light and the duration of one year (d=c×td = c \times t).
Because cosmic distances are immense, standard units like meters or kilometers are impractically small.
2
Determine the physical quantity represented by this unit.
The unit represents distance (length), equating to approximately 9.46×1012 km9.46 \times 10^{12}\text{ km}.
Multiplying speed by time yields a distance measure rather than a unit of time, speed, or energy.

Key Concept

Definition of a light-year as a unit of astronomical distance
Estimated Time:45s
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Topics

Stars, Galaxies, and the Universe

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