Q:
An article I read says a pulsar is 1,615 years old - the Chinese saw something 1,600 years ago...and astronomers say the pulsar was born in the year (Earth year) 386. But then the article states that the "1,600 year old pulsar is 15,000 light years away from Earth!" Doesn't that mean that what the Chinese saw in the sky was an event that took place at least 15,000 years ago? So if you add 1,600 to 15,000 you get a pulsar about 16,600 years old?
A:
The age of 1615 years for the pulsar refers to the age of the outburst in the source. Assuming a distance of 15,000 light year, the light took 15,000 years to get here, so the Chinese astronomers observed the outburst 15,000 years after it occurred. Since it's the evolution of the source that is of primary interest, astronomers talk about the age SINCE the outburst was observed. Our information for this source is always 15,000 years out of date. It's similar to finding a series of photographs of a child in a 100 year-old time capsule. We could see how the child was developing back then, even though he/she may no longer be alive.
Revised: September 05, 2008