APOD: NGC 2736: The Pencil Nebula (2013 Mar 21)

[Chris Peterson]

Yesterday I posed the question "what forces cause this reported shock wave in interstellar space to slow down from millions of kph to .5million kph?"
OK, let's say an explosion of particles occurs, and the density of all those particles decreases as a function of the radial distance traveled. In a relatively short time the ratio between the particle diameter and the distance between particles becomes "astronomical", which is also the case with interstellar space. The odds of collision of particles becomes so low that the great majority of particles travel without encounter, therefore no loss of momentum. So what am I missing here?

The probability of a collision isn't as low as you think, for two reasons. First, the material was ejected as a shell, so the density is falling off at about the square of the distance, not the cube (as you might expect for an expanding sphere). Secondly, you aren't factoring in the supersonic speed of the particles. The probability of a collision depends not just on the density, but also the speed. We are seeing the interaction between very fast (> Mach 100) particles with the hot ISM (thousands of kelvins). That results in a mean free path that is short enough that many collisions occur. Only a tiny fraction of the particles actually need to be involved in collisions for us to see the ionized structures we do; remember, we're looking at a large volume of space.

[will hunt]

One of the most interesting assertions I have ever seen, vis-a-vis humanity's interaction with the cosmos, was today's "... the expanding debris cloud of a star that was seen to explode about 11,000 years ago." (emphasis added)

I don't doubt it could have and may have been seen, but what historical evidence do we have that the explosion was observed by human beings? Surely that proof is one of archeology's triumphant moments.

I want to believe it. Help, please.

[ta152h0]

you believe in science ? Like the Antikitera mechanisn, the evidence could be under the sea, or up in smoke from the Alexandria library burnout, or even inside a cave not yet found.

[Chris Peterson]

One of the most interesting assertions I have ever seen, vis-a-vis humanity's interaction with the cosmos, was today's "... the expanding debris cloud of a star that was seen to explode about 11,000 years ago." (emphasis added)

I don't doubt it could have and may have been seen, but what historical evidence do we have that the explosion was observed by human beings? Surely that proof is one of archeology's triumphant moments.

I want to believe it. Help, please.

The proper way to interpret "was seen to explode about 11,000 years ago" is "the light from the supernova reached Earth 11,000 years ago", or "we are looking at this event 11,000 years after the supernova". This is to avoid the ongoing confusion in APOD captions about when something "really" happened, where people try to compensate for the distance... which does not, in fact, typically matter.

The expression should not be literally taken to imply "seen by humans", although it is quite certain that, at only 800 ly distance, this event must have been witnessed by people.

[Richard astro]

magnifique