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Difficulty: MediumStars, Galaxies, and the Universe

An astronomical survey records that a newly cataloged planetary nebula is situated 24002{}400 light-years from Earth. What does this measurement of 24002{}400 light-years represent?

  1. A
    The elapsed time of 24002{}400 years that has passed since the nebula formed
  2. The total distance that electromagnetic radiation travels through a vacuum over a span of 24002{}400 Earth yearsAnswer
  3. C
    The constant propagation velocity of visible light traveling through interstellar space
  4. D
    The time duration required for a spacecraft traveling at maximum sub-light speed to complete a journey to the nebula

Answer

The total distance that electromagnetic radiation travels through a vacuum over a span of 24002{}400 Earth years.
A light-year is strictly an astronomical unit of length and distance. It is defined as the distance that light travels through a vacuum in one Julian year (approximately 9.46×1012 km9.46 \times 10^{12}\text{ km}). Therefore, stating an object is 24002{}400 light-years away indicates the total distance traversed by light over 24002{}400 Earth years.

Step-by-Step Solution

1
Identify the physical quantity measured by a light-year.
A light-year is a unit of astronomical distance, not a measure of time or speed.
Although the term includes the word 'year', it quantifies the linear distance traveled by light moving at approximately 300000 km/s300{}000\text{ km/s} during one full Earth year.
2
Apply the definition to a quantity of 24002{}400 light-years.
The measurement corresponds to the total distance traversed by light in a vacuum across 24002{}400 Earth years.
Multiplying the one-year light distance (9.46×1012 km9.46 \times 10^{12}\text{ km}) by 24002{}400 yields the spatial distance between Earth and the nebula.

Key Concept

Astronomical Units of Distance and the Definition of a Light-Year
Estimated Time:1m 0s
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Topics

Stars, Galaxies, and the Universe

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