# Difference between blast & shock wave

1. Jan 10, 2012

### Jain_Mor

Hi guys and gals,

with respect to SNRs (or anything for that matter)

What is the difference between a shock wave and a blast wave?
Is it just that a blast wave is a sharp peak in the change in variables, where as a shock wave has a region behind it in which the variables gradually decrease?

Also read that the Sedov-Taylor or Self-Similar phase of an SNR is a blast wave, does this mean the other phases aren't?

Many thanks in advance, its confusing quite a lot of us.

2. Jan 10, 2012

### Chronos

A blast wave is the initial pulse from an energetic explosion. It is simply a strongly peaked shock wave, so you're on the right track. Propogation of a shock or blast wave is dependent on the mean free path between particles in the medium through which it travels. The mean free path is another way of saying how far apart particles are on average. In the vacuum of space, the mean free path can be quite huge - not unlike an enormous billiard table with only a few balls. The time between collisions can be lengthy. The individual collisions can result in highly energetic photon emissions, but, lack the systemic kinematical effects of shock waves in a denser medium. Plasma effects tend to dominate under these circumstances.

3. Jan 10, 2012

### Jain_Mor

Are you speaking relative to SNRs?

Is it possible that a Blast Wave is a Shock Wave whose rarefaction has caught up with the shock front? Thereby giving it a distinct peak?

4. Jan 12, 2012

### twofish-quant

I didn't make that much of a distinction between the two when I did my dissertation, but a blast wave is when you put all of the energy at one point and then you get a very nice easy to solve equation.

Yes. Suppose you have a shock wave that goes out for 0.1 light years. At that point you can model it as a smooth uniform gas in which someone dumped a massive amount of energy at one point, and the numbers work out since the size of the original star is much, much smaller than the area you are looking at. Since the star is a "point" the math becomes really simple.

Now if you try to model the shock wave when it is deep inside the star you can't do that.

5. Jan 12, 2012

### twofish-quant

I don't think that works. There really isn't much in the way of rarefaction.

6. Jan 12, 2012

### Jain_Mor

Thanks TwoFish, thats great :)