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A supernova remnant is the structure resulting from the explosion of a giant star . The supernova remnant is surrounded by an expanding shock wave that is formed from material ejected by the explosion and interstellar material swept away during the process.

When this material collides with the circumstellar or interstellar gas, forms a shock wave that can heat the gas at high temperatures up to 10 million K, forming a plasma, this is true.

The first question is: exactly what is the material ejected by the supernova?

other issue is: if we consider for a moment the idea that a "human" might be witnessing the exact moment of the explosion of a supernova, the "superhuman" would hear the explosion? ie, does the explosion would produce some kind of sound?

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2 Answers 2

Supernovae explosions are more violent than novae explosion. In a typical novae explosion star sheds just the outermost layer of it's mass.

Well, supernovae are the explosive events caused in case of high mass stars, after the iron core is formed at the centre. As iron is most stable element, no further fusion takes place and stars central engine stops for final time, this disturbs the equilibrium, and gravity starts dominating. The star collapses where "photodisintegration" and "neutronisation" takes place. At this point of time the density reaches of the order of $10^{17}$ $kg/m^3$, the collapse is eventually halted by neutron degeneracy pressure, and star explodes, where it's huge matter is thrown in space. Hence, materials ejected by supernovae are many higher elements formed during the different fusion cycles. It is also believed that during this explosive event elements higher that the iron are formed.

Well, coming to the second question, obviously sound is produced. You can hear it if you are connected with star by interstellar matter. I mean to say, if there is vacuum between you and star (i.e you are extremely far) then you will not hear the sound, unless until the shock waves reaches you.

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Should we really call a supersonic shockwave a sound? Or should sound be reserved for wave amplitudes that are still in the linear regime? –  Omega Centauri Oct 11 '11 at 20:31

The article in wikipedia gives complete answers, to both questions.

Various types are dominated by different elements, and also the explosion gives rise to the elements heavier than Fe.

With velocities of wave expansion up to 35000km/sec no "human" could consider it as sound if he/she were in the expanding mass, would burn anyway. We are built to hear 342m/sec or so, and need the density of air and moderate temperatures.

As @mehulphy said, no sound in vacuum anyway, so if far away no sound would be heard.

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We don't hear speeds, we hear frequencies, so the sound would depend upon the spectrum of pressure variations superimposed over the bulk flow. If there were a means of transducing this pressure variation we could certainly hear it as sound, although it would likely have as much to do with random particle collisions as stellar dynamics. –  Colin K Oct 11 '11 at 18:57
    
@Colin A human hears modulations of pressure that come at a speed of 342m/second. That is sound for humans and the question asks about a human. Sure the modulations could be transcribed, brought to the scale of human sound. –  anna v Oct 12 '11 at 4:38
    
The speed of propagation has nothing to do with human hearing. I'm not trying to be mean here, but I honestly can't tell if you're making a joke that I don't get? Humans can hear in many situations where the speed of sound is abnormal. Underwater, for example. The ear measures the frequency at which the eardrum is displaced by pressure variations. The velocity has literally nothing to do with it. –  Colin K Oct 12 '11 at 10:59
    
@Colin I would be interested to see how the velocity of sound in the medium has nothing to do with the way the ear hears. It is the carrier after all, you cannot hear until it reaches your ear, and must limit the physiology but if you have a link that says the opposite, I will read it with interest. Abnormal speed of sound ends up in the ear which listens in air, and in any case will be of the same order of magnitude as in ambient air, not thousands of meters/second more. –  anna v Oct 12 '11 at 13:47

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