help understanding how light travels...

[trauts14]

I read an article that I did not understand. Here is part of the article: "Capture by NASA's Chandra X-Ray Observatory and the European Space Agency's XMM-Newton, "the image shows the remains of a supernova that would have been witnessed on Earth about 3,700 years ago. "

   My question, if this event could be viewed on Earth 3,700 years ago then how can we see this with a telescope? It seems the light has already traveled to Earth, so how can we on Earth see it? In term of travel I ( a complete dummy in astronomy) compare light travel to sound traveling at a sports event. The other team screams from across the stadium and it takes one second for the sound to reach me on the other side. Light leaves an event (the side of the stadium many light years away)  and it takes many many years for me to see the light. Once I see the light, then the event is over. So if we on Earth could see something 3,700 years ago, how can we still see that event today with a telescope? The light arrived to Earth 3,700 years ago. Please help me understand this. Thank you.

[Helen]

I assume that they are talking about the supernova remnant rather than the supernova.  The star exploded and threw off loads of material which carries on expanding into space, we see it as it moves through space (but given the distances we don't see it move in real time but observation over many years might show differences).  To use an analogy...  you throw a stone into a pond, the first thing you see is the splash (the supernova explosion) but then there are ripples which move outwards (the supernova remnants) and so you see those later.  We can keep on seeing things as long as they continue to emit sufficient electromagnetic radiation for us to detect - to use your analogy, we keep hearing the sound as long as the crowd keeps cheering plus the time it takes for their last cheer to reach us.  

HTH

Helen

[Coastliner]

I agree with the above - the article is saying that the initial "bang" would have been seen on Earth 3700 years ago, what we are seeing now are the remnants of the explosion, and in another 3700 years observers will see the same remnants but the shape will have altered as the remnants continue to expand in to space.

[Owmuchonomy]

That's right. Just read up on any of the Planetary Nebula visible today in the night sky. For example: http://en.m.wikipedia.org/wiki/Crab_Nebula

[jambouk]

Yeah, the explosion happened 3700 years ago (as seen from earth), and the fire is still burning but not as ferociously. Remember all these observations are "as viewed from earth" so you would need to factor in how far away the object is in terms of time...

James

[Coastliner]

And, just to bend your mind a little more: Put yourself on Earth 3700 years ago, you look up into the night sky and you see the first signs of the explosion. The star that exploded would have actually done so many years before you saw the explosion. For example, our nearest star, Proxima Centauri, is 4.25 light years away, so we are seeing it now as it was 4.25 years ago. Similarly, the Andromeda galaxy is 2.5 MILLION light years away, so we are seeing it as it was 2.5 million years ago. We see the Sun as it was about 8 minutes ago.

[RT65CB-SWL]

Hi 'trauts14' and welcome to SGL.

And, just to bend your mind a little more: Put yourself on Earth 3700 years ago, you look up into the night sky and you see the first signs of the explosion. The star that exploded would have actually done so many years before you saw the explosion. For example, our nearest star, Proxima Centauri, is 4.25 light years away, so we are seeing it now as it was 4.25 years ago. Similarly, the Andromeda galaxy is 2.5 MILLION light years away, so we are seeing it as it was 2.5 million years ago. We see the Sun as it was about 8 minutes ago.

...and if you really want to bend you mind even further: When you look at/see your reflection in a mirror, you are seeing you as you were 1000th's of a second ago! - I cannot remember where I heard this tit-bit of 'useless' info, but the same person/scientist also reckons there are more stars than grains of sand on planet Earth.

[trauts14]

Thank you all for your assistance. I have a much better understanding now. All of the analogies helped.