Thank you very much guys, always happy to help. Physics is a hobby of mine now, and i once studied it, so i know a thing or two and i am happy to share said knowledge ^^

Its both, but more the compression then the friction. You should also know the counterpart of this effect, that gas cools of when it expands, for example when you refill a lighter. If you let the gas flow out it becomes ice cold very quick. Ice spray works also like this (the one dentists use to test if your tooth is still alive for example)
btw a fridge works with the same 2 principles. Inside the fridge the cooling agent expands and thus cools down, and at the back of the fridge its compressed again, and thus gets warm. Ever felt the backside of a fridge ? It usually gets quite warm ^^

btw what i forgot to mention, all the heavier elements stem from such novas. After the big bang ONLY helium and hydrogen existed (about 24% helium, the rest hydrogen). All the other elements came from the first generation of stars. I am always fascinated that our whole planet and all the stuff we are made of was born in the death of a star, that shined for a while brighter then a whole galaxy and gave of more energy in a few days then the sun in its whole existence. When the beatles sang:"we are star dust" you can really take that literally. That really blows ones mind, doesn´t it ? Since i am at it :
Most of the time this reaction only gets up to iron and then stops, only in rare cases the nova is strong enough to also produce heavier elements then iron. That´s the reason all the elements heavier then iron (like gold) are so much more seldom then anything below iron.
In the year 1054 astronomers around the planet saw a new star at the sky, that was so bright you could even see it by day. Modern day astronomers searched for the source, and found it, its today known as the crab nebula. That thing is 6500 light years away, and was bright enough to be seen by day, and yet it was just a small supernova compared to what else the sky has to offer ^^ Here the wikiarticle on the crab nebula, i find it quite interesting :

and again, any questions feel free to ask, no matter what it is. Remember, there are no stupid question, the only thing stupid is not to ask at all