The light from a star 100 light years away takes about 100 years to reach Earth. How long will the star appear to us after it explodes (if it explodes now)?

Correct option is B

The correct answer is (B) In 100 years

Explanation:
• A light-year is a unit of astronomical distance defined as the total distance that a beam of light travels through a vacuum in one Julian year (365.25 days).
• Light propagates through space at an absolute, constant cosmic speed limit of approximately 300,000 kilometers per second ($3 imes 10^8 ext{ m/s}$).
• When an astronomical object, such as a star, is located exactly 100 light-years away from Earth, it means the electromagnetic waves carrying visual information from that star take precisely 100 years to cross the interstellar void and reach our eyes or telescopes.
• Therefore, if that star undergoes a cataclysmic supernova explosion right now, the initial burst of bright light, photons, and radiation generated by the explosion will just begin their journey across space at the speed of light.
• This traveling visual information will take exactly 100 Earth years to traverse the physical distance, meaning observers on Earth will only perceive and witness the star's explosion 100 years after the actual event took place.

Information Booster:
• Looking out into the deep universe is effectively looking back in time. For example, because of light-travel time, we see our Sun as it existed 8.3 minutes ago, the nearest star system (Alpha Centauri) as it looked 4.3 years ago, and distant galaxies as they looked billions of years ago, allowing astronomers to study the history of the cosmos.

Additional Knowledge:
• Immediately (Option A): It is physically impossible to see the explosion instantly because information cannot travel faster than the speed of light, as established by Albert Einstein’s Theory of Special Relativity.
• In 1 year (Option C): In one Earth year, the light emitted from the explosion will have only traveled a distance of 1 light-year, covering just 1% of the total distance required to reach Earth.
• Never (Option D): The light will eventually reach Earth unless it is completely blocked or absorbed by an opaque celestial structure, like a dense dark nebula, or trapped behind the event horizon of a black hole along its path of travel.