The age of a supernova remnant has dropped from 10,000 years to less than 2,000 years.
According to a news item on Space.com, the object RCW 86 in Centaurus has been linked to sightings by the Chinese in 185 AD, making it the oldest supernova recorded by man, taking place 1821 years ago.
But astronomers thought this supernova remnant was 10,000 years old. How could the earlier age estimates be so far off? The article explains:
The new age estimate matches the supernova spotted in 185 AD. But this calculation means the remnant is 8,000 years younger than previously thought. The astronomers said the difference can be attributed to the irregular shape of the remnantĂ˘â‚¬â„˘s expanding bubble. Stellar wind from the progenitor star pushed some of the remnantĂ˘â‚¬â„˘s gases in a certain direction, forming a dense pile.
Ă˘â‚¬Ĺ“The idea for RCW 86 is that in some regions the shock has hit this piled-up material. In those regions the shock will start moving slower,Ă˘â‚¬Âť [Jacco] Vink [U of Utrecht] said. And in other regions, the shock wave is much speedier.
X-ray measurements from the Chandra X-ray Observatory were used in making the new age determination based on outflow speeds of the gas. The new estimate was about 2,000 years, within the range of the event in 185 AD.
One of the captions in the article was Ă˘â‚¬Ĺ“Shell Shocked,Ă˘â‚¬Âť but it was not clear if this referred to the supernova remnant or to the astronomers finding out how wrong they had been. In this case, we had an observation to calibrate a dating method, and the result was drastically lower than predicted from theory. There are many other things in space and time that cannot be so calibrated. The parameter to watch in dating methods is the observation-to-assumption ratio.