Expert: Jayendra Upadhye - 1/22/2008

Question
Problem: In 1054, Chinese astronomers abserved the light from a supernova in the night sky. This supernova was the incredibly powerful origin of what is now the Crab Nebula.
Using the speed of light, devise and execute a strategy for finding out when the supernova actually accurred, relative to when the Chinese astronomers saw the explosion.

Im working in a Strategies for Problem Solving Workbook
Third edition
Baseline Project 7 "Speed of Light"
Can you help me understand and Devise a Plan to solve this problem?

Answer
Hi,
[please refer:- http://en.wikipedia.org/wiki/Crab_Nebula#Distance & http://en.wikipedia.org/wiki/Crab_Nebla].
To do this using the sped of light, we use the principle that for each light year crossed by light, as it travels towards us, 1 actual year passes in physical time.
Also we do need to know how far the Crab Nebula actually is.

And this is where the problem lies.
Accurate distance estimates are difficult to make. Chiefly due to the variable rates (over time) that field theory predicts.
The doppler shifting (like the red shift) of gases as they expand within the nebula, tells us that they are moving at 1500 km/sec!
If we back calculate to zero time, from the nebula's current size and observed variation in size(considering the rate of expansion),  
to the time when the star went supernova, that date falls after 1054 when it was observed on earth!
This led people to believe that the rate has not been static over time but actually increased due to magnetic interactions of the pulsar with the emitted gas.

Even then people agree it is "roughly" 6300 light years away.
Current major axis dimensions :- 13 + - 3 lightyears.

This means that it actually exploded (2008 - 1054) years + 6300 years ago.
Light took 6300 years to first reahc us, and we all "know about it" since 1054, for 2008 - 1054 years!
thus time elapsed since zero time is 2008-1054+6300=7254 years of physical time!
hope that suffices.
Please do rate the answer.
Jayen