irregular galaxy
None. A nebula is a cloud of gas and dust that can contain young stars, but a nebula is not a planet by any means.
Hydrogen (and a little Helium).
Milky way
There is photographic evidence of other young stars where such disks exist.
Milky Way: Arms extend from a core full of black holes Elliptical Galaxy: Contains mostly older stars with a red color Irregular Galaxy: Contains dust, gas, and young stars Apex :)
A stellar nursery.
Gas and dust were stripped away in collisions that also stripped away many young stars.
Spiral Galaxies
No. They are made mainly of stars and planets. The stars and planets came from gas and dust.
That means matter between stars - mainly dust and gas.
Almost all of the galaxies have dust in them, most have new stars, but I think you are thinking of the Horse head nebula which is cited as a stella nursery.
Stars come in a variety of types. Blue stars, which are very hot, tend to have shorter lifetimes than red stars, which are cooler. Regions of galaxies where stars are currently forming are therefore bluer than regions where there has been no recent star formation. Spiral galaxies seem to have a lot of gas and dust, while elliptical galaxies have very little gas or dust.