View Full Version : Clarification Re: RCW 86

2006-Sep-27, 03:17 PM
I'm a little confused about the age calculation for RCW 86. It has recently been suggested that the remnant is 2000 years old - much less than the 10,000 years previously estimated. However, the Chandra site indicates that the remnant is more than 8000 light years away. This leads me to two related questions:

1. How can we see a remnant only 2,000 years old if it is more than 8,000 light years away?

2. If Chinese astronomers first saw the explosion prior to AD 200 and it is 8,600 light years away, doesn't that put the minimum age at around 10,000 years as previously estimated?

Apologies if, as I suspect, this is a dumb question.


2006-Sep-27, 03:33 PM
1. How can we see a remnant only 2,000 years old if it is more than 8,000 light years away?

Hi RichmondPhil, welcome to the BAUT forum.

We get this question quite often.

Astronomers and Science Journalists often quote the age as we see it here. So the image we see can look like it is 1850 years old. That is all we can really measure. Yes, the object IS 8000 light years away, and so the actual event happened 10,000 years ago, but the timeline for the stream of light passing the Earth is what is being referred to here.

Some Journalists are careful to make the distinction, and you'll see it worded more carefully to represent what was seen, and when it was seen, vs. when the event occurred.

2006-Sep-27, 04:15 PM
That other Phil, the BA, recently wrote about age and distance in the BA Blog: Mi Cas A es su Cas A (http://www.badastronomy.com/bablog/2006/08/29/mi-cas-a-es-su-cas-a/)

Yes, I know– some of you will think that it didn’t blow up in 1680, that’s just when the light reached us, and it really blew up 10,000 years ago. I disagree. We cannot say anything about that event until the light reaches us, and in a real sense that event has not happened until the light reaches us. Time flows like light, I sometimes say, meaning that the event itself happens when the light reaches us. So it is acceptable to say that the explosion actually happened in 1680.

Just be aware of what kind of ages you have been presented with, when distant-object ages are compared or used in calculations.