To determine how long ago a supernova occurred when we see it from Earth, we need to account for the time it takes for light to travel from the supernova to our planet. Since light travels at a speed of approximately 299,792 kilometers per second (or about 186,282 miles per second) in a vacuum, we can calculate the time it takes for light to travel a distance of 8,000 light years.
One light year is the distance light travels in one year, which is equal to the speed of light multiplied by the number of seconds in a year. Therefore, we can calculate the time it takes for light to travel 8,000 light years as follows:
Time = Distance / Speed of Light = (8,000 light years) / (speed of light)
To convert the result into Earth years, we divide the time by the number of seconds in a year. Since there are approximately 31,536,000 seconds in a year, the calculation becomes:
Time (in Earth years) = Time / (31,536,000 seconds/year)
Let's calculate it:
Time = (8,000 light years) / (speed of light) = (8,000 * (9.461 × 10^12 km)) / (299,792 km/s) ≈ 2.535 * 10^5 seconds
Time (in Earth years) ≈ (2.535 * 10^5 seconds) / (31,536,000 seconds/year) ≈ 8.04 years
Therefore, when we observe a supernova that is 8,000 light years away, it occurred roughly 8.04 years ago in Earth years for us to see it now.