Question 15

The soonest an astronomer on Earth could observe the explosion is in 15 years.

Given that the star is located 15 light-years away from Earth, the light from the supernova explosion would take 15 years to reach us. Therefore, the soonest an astronomer could observe the event would indeed be in 15 years.

A) immediately

This option is incorrect because light from the supernova would not reach Earth instantly. It takes time for light to travel across the vast distance of space, especially the 15 light-years separating the star from Earth.

B) in 15 seconds

This choice is incorrect as well. While 15 seconds is a very short time frame, it does not account for the fact that the light from the supernova explosion needs 15 years to travel the distance from the star to Earth.

C) in 15 minutes

This option is also incorrect. Similar to the previous options, 15 minutes is far too short a time for light to travel 15 light-years. The distance is vast, and it would take significantly longer than just a few minutes.

D) in 15 years

This is the correct answer. The distance of 15 light-years means that the light from the supernova explosion will take exactly 15 years to reach Earth. Therefore, astronomers would observe the explosion only after this duration.

Conclusion

The correct answer, 15 years, is definitive because it accurately represents the time it takes for light to travel from a star 15 light-years away to Earth. All other options fail to account for the physics of light travel over astronomical distances, making them incorrect.