Structure:
The disk of the Milky Way exhibits a spiral structure, which shows up in the distribution of the objects populating the disk component. These objects include, in particular, young stars, diffuse star-forming nebulae and open star clusters. The origin of the spiral structure is thought to be in density waves which are triggered by gravitational disturbances, in particular during encounters with neighbor galaxies. The density waves first concern interstellar matter, which is compressed, forms diffuse nebulae which become star forming regions, and later form clusters and associations of young stars, the most luminous and conspicuous of which are massive, hot, blue and short-lived. The stars which are even older have dissipated into a more diffuse background; this includes intermediate population I stars, older star clusters and the younger representatives of the planetary nebulae.
The disk of the Milky Way exhibits a spiral structure, which shows up in the distribution of the objects populating the disk component. These objects include, in particular, young stars, diffuse star-forming nebulae and open star clusters. The origin of the spiral structure is thought to be in density waves which are triggered by gravitational disturbances, in particular during encounters with neighbor galaxies. The density waves first concern interstellar matter, which is compressed, forms diffuse nebulae which become star forming regions, and later form clusters and associations of young stars, the most luminous and conspicuous of which are massive, hot, blue and short-lived. The stars which are even older have dissipated into a more diffuse background; this includes intermediate population I stars, older star clusters and the younger representatives of the planetary nebulae.
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