As this isn't a specialist astronomy forum it might be useful to put the image above in context. The Andromeda Galaxy is in many ways similar to our own Milky Way although it's a bit larger. If we were to transpose the position of our own Sun into Andromeda then think somewhere like the outer ring of reddish nebulosity. That explains why at some times of the year the Milky Way is much less impressive than at others.
Many of us, certainly those who spend time away from the light pollution typically generated by large towns and cities, will be familiar with the constellation of Orion
and, in particular, the faint misty patch that forms part of Orion's sword - check out the forum thread Your best shots of M42 - the Orion Nebula!
. That is a region of gas and dust which plays host to star birth and the heavier newly born stars emit copious amount of energetic photons. That can give them a blue tinge but it also has the effect of lighting up the gas and dust cloud which gave them birth and one of the strongest components of that light is due to hydrogen, the most abundant element in the universe, which, in the visible spectrum, emits both a deep red and a less intense bluish component. That red emission is specifically selected by a Hα (hydrogen-alpha) filter so if you look at the spirals and clumps of red in the image above and think that each of those is a site of star birth then you won't be far wrong. I think I am also right in suggesting that something the size of our own Orion nebula, which is fairly close to the Sun in galactic terms, would be pretty small in the image above.
It's very easy to look at a spiral galaxy and make the analogy with a Catherine wheel. That's not what is going on but maybe a discussion of compression waves is beyond where I ought to go here. Suffice to say that the structure is part of a dynamic process which includes star birth so the actual composition of the disk isn't as chemically differentiated circumferentially as one might be tempted to believe. That's not so true when one travels out from the center towards the edges however. To see why it's helpful to remember that stars are factories which release energy by fusing light elements such as hydrogen into heavier ones. The really heavy elements (heavier than iron) actually reverse that energy flow as they are synthesised so they only get made in any quantity in violent explosions (supernovae) but the lighter elements can also be returned to interstellar space in a less spectacular fashion (think "solar wind"). The net result is that as star formation continues over the lifetime of a galaxy larger and larger proportions of "star dust" (elements heavier than hydrogen) get incorporated into the new stars and, of course, the planets that orbit them. Without that star dust we wouldn't be here!
There are even clues to this in the image above. The stars of the central bulge tend to be older and cooler and the region is not so conducive to fresh star formation. Star formation is much more easily triggered in the outlying disk of the galaxy by those compression waves I referred to and those spirals of Hα emission are the smoking gun. They travel around and through the disk and as each new generation of stars is formed, the heavier stars living short but intense lives which end as supernovae, the material of the disk is enriched with the heavier elements such as those which make up you, me and the planet we live on. No surprise then that we live on the outskirts of our own galaxy.