A team of astrophysicists detects the spectral signature of a supernova in a spiral galaxy… — Armed Services Vocational Aptitude Battery (ASVAB)

A team of astrophysicists detects the spectral signature of a supernova in a spiral galaxy located $2.5 \times 10^6$ light-years from Earth. Which statement correctly interprets the physical meaning of this measurement in the context of astronomical observations?

The light captured by Earth's telescopes today was emitted $2.5 \times 10^6$ years ago because a light-year is a unit of distance representing how far light travels in one Earth year.Answer
B
The explosive phase of the supernova lasted for a total duration of $2.5 \times 10^6$ years because light-years measure elapsed cosmic time.
C
The emitted photons traveled across interstellar space at a speed of $2.5 \times 10^6\text{ km/s}$ before striking Earth's sensors.
D
The supernova is located $2.5 \times 10^6$ astronomical units away, which is based on the average distance between Earth and the Sun.

Answer

The light captured by Earth's telescopes today was emitted

2.5 \times 10^6

years ago because a light-year is a unit of distance representing how far light travels in one Earth year.

A light-year is defined as the distance that light travels in a vacuum in one Julian year. Because light travels at a constant finite speed (

c

), when astronomers observe an object located

2.5 \times 10^6

light-years away, they are viewing photons that departed the source

2.5 \times 10^6

years ago. Thus, looking across vast cosmic distances is equivalent to observing the universe's past.

Step-by-Step Solution

Define the astronomical unit 'light-year'

A light-year is strictly a unit of length/distance, defined as the distance an electromagnetic wave travels in a vacuum over the course of one Julian year (

9.46 \times 10^{12}\text{ km}

Clarifies the fundamental physical quantity being measured.

Relate distance, speed of light, and lookback time

Since light travels at constant speed

c

, covering a distance of

2.5 \times 10^6

light-years requires exactly

2.5 \times 10^6

years of travel time.

Establishes that looking deeper into space corresponds to observing events further in the cosmic past.

Evaluate observational implications

The observation on Earth reflects the physical state of the galaxy and star as it existed

2.5 \times 10^6

years ago.

Connects astronomical distance measurement with lookback time.

Key Concept

Astronomical distance units and lookback time

Estimated Time:1m 15s

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