They never tell me the stuff I want to know:

Quote:

Cosmic Cannonball snapped blazing a bloody trail of star guts The so-called Cosmic Cannonball, a neutron star moving at over three million miles an hour, has been captured in this new satellite image - or at least the red rose of supernova remnant that encases it. NASA's Wide-field Infrared Survey Explorer (WISE) snapped this shot of Puppis A, the remains of a supernova explosion that happened around 3,700 years ago. The expanding shockwaves from the explosion are heating up dust and gas clouds, causing them to glow red in the infrared camera's sights. The star-ending supernova also left behind a dense mess of material called a neutron star, the part of Puppis A that astronomers call the Cosmic Cannonball. Star-gazing boffins are mystified as to why the neutron star, which is too faint to be seen in the photograph, is travelling so fast - hence the nickname. The green field around the red rose of Puppis A is the remnant of an even older supernova event, Vela, which happened about 12,000 years ago and was four times closer to Earth than Puppis A. "If you had X-ray vision like Superman, both of these remnants would be among the largest and brightest objects you would see in the sky," NASA said. ®

Cosmic Cannonball snapped blazing a bloody trail of star guts ? The Register

It's moving? Towards us? Should we worry? Or away? Or round in a circle? If it's moving so fast how come it hasn't left its shockwaves and gas clouds and things behind? What does "four times closer to Earth" mean? 1/4 of the distance away? Did the green stuff just blow over there, or has it spread out that far? In that case surely we'd be surrounded in it too? Who came up with the name "Puppis"?

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Well, I kinda agree, but most of those answers probably require explaining more than would be worthwhile. It's presumably moving in a straight line, by omission as any kind of curve would require acceleration be continually applied, which would be the most remarkable thing they could possibly have observed. It hasn't left the cloud behind, because it's much more massive and therfore moving slower than the gas particles thrown out by the same explosion. I can't say much more as they don't mention how far away they are (addition: other site says about 6000ly). Presumably its not coming towards us, as then the observation would be based on blushift, nor away from us, as it would then be in redshift. You could probably draw a ling on a map of the galaxy to illustrate it's trajectory, but as at the moment no point in the galactic disk has any property like "north" it wouldn't be very meaningful. Puppis is apprently part of a constellation, and means "stern" (of Jason's ship). The green cloud looks bigger because it is closer; they can be very big, but we are not in any such clouds.

I don't think that telling us what direction it's moving in would be too much of a stretch. Omitting it is rather like reporting that 17/20 cars finished a race and not bothering to say who came first.

And surely if objects that are further away from the centre of that green thingy than us are enveloped in it, then we must be too? Unless the clouds only blew out in one direction, or it's like a smoke ring and it already passed us by years ago.

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Well I don't see anything in the above article saying that other things are enveloped in the green cloud. I don't know off hand how big supernova remnants get, certainly much more than the orbits of our outer planets but probably not that much in terms of lightyears. The Vela cloud is presumably something like 1000ly away from us, and to put that in perspective, the galaxy is about 100,000-120,000ly across, so it's 1% of the galactic diameter away from us. The image above is just looking through the Vela cloud.

As for direction, all you'd end up with is a diagram of an ellipse with a dot representing us and a line representing the object, but that wouldn't be very informative becuase the galactic disk has no context.

I was more thinking of its direction in terms of where it started out. Never read a single well-written description of how an accretion disk works either.

Personally, I suspect that if no one is capable of describing a thing in simple language it's probably made up anyway. Like those half-arsed fantasy/scifi stories that start with about three chapters of tedious and unhelpful exposition. Most GCSE textbooks can't even explain the difference between mass and weight coherently. Something should be done about this.

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Well there is this:


I'm not sure we have that good an idea about accretion disks. I mean the basic idea is that stuff bumps into stuff and sticks together through gravity, but we only have one solar system to look at so far.

Obviously I haven't seen the textbooks you have, but mine certainly handled the distinction between mass and weight adequately. Short version: weight only exists in a gravity field.

Some things can't be easily explained. Our brains are evolved to live on an apparently flat surface and dealing with abstractions is not in the original design spec. I don't really expect the universe to be simple, in fact that would be rather disappointing.

Originally Posted by contracycle

Obviously I haven't seen the textbooks you have, but mine certainly handled the distinction between mass and weight adequately. Short version: weight only exists in a gravity field.

To which any kid who isn't a total dullard, replies "Why?"

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Basically because weight is a property an object appears to have, while mass is a property an object really has. Weight varies by local conditions, i.e. how much it is being accelerated, while mass is constant. An object on the moon weighs less, but masses just as much as it always did.

Yeah, but why?

If I had to do it I think I'd say something like "mass is all the atoms added together, and weight is how hard gravity pushes them". But that depends on having done atoms before Newtonian physiscs, which actually I don't think is a bad idea.

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Well you can ask "why" to anything. Why atoms? Why gravity? they are just things we observe.

More specifically, weight is the concept we came up with when looking at the world as we encountered it, but it was realised that it was a bad or at least incomplete, concept. When we came up with concepts like inertia and momentum, mass was created as a label for the property that an object always has regardless of any forces acting on it or the way in which other objects experience it.

Also, atoms were invented before Newton, by the ancient Greeks. But you don't really need to know about atoms first; all you need to know is that an object has a certain mass but it is experienced differently in different situations. The fact is they describe different things; you might be able to pick up a car on the moon, because it has low weight, but if you hit by one that was moving you'd still get hurt, because it still has all its mass.