.....................

edgeonphotographofMilkyWayseenfromNevadashowinglocalterrainatbottomofimage

The diagram identifies the primary features of the Milky Way galaxy not counting the massive nearly spherical dark matter halo that has three times the diameter and nearly 30 times the volume of the stellar halo shown in the diagram. The galaxy includes a distribution of stars and intragalactic medium. Population II stars are found in a nearly spherical stellar halo, a central bulge and a thin disk. Population I stars are concentrated in the younger thin disk, but are also found in the bulge. The galactic center is a supermassive black hole. Much smaller black holes are scattered throughout the galaxy. The spiral arms are regions of high density of interstellar gas and dust where giant stars continue to form and live very brief lives before recycling their metal-enriched gas back into the “empty space” of the interstellar medium.

The table shows the mass, dimensions, density, and density form function of the structures of the Milky Way galaxy. Nearly 90% of the mass of the galaxy is in the dark matter halo. Nearly 70% of the ordinary baryonic matter is in the young stars in the thin disk. Most of the rest are distributed fairly uniformly among the thick disk, bulge, and interstellar medium. About 1% of the stellar mass is in the stellar halo. The supermassive black hole has about four million solar masses, a huge number, but only a small fraction of the stellar or galactic mass. Density form factors are models that approximate the distribution of stars and dark matter in the galaxy.

Table of composition of the interstellar medium from Wikipedia. The interstellar medium is home to the cool dense molecular clouds that breed stars including massive short-lived stars that enrich the metal content of the medium when they explode. Despite their evolutionary importance, the clouds occupy less than 1% of the volume of the medium. Most of the volume is occupied by warm or hot gases that range from 6000 K to several million degrees, too hot to condense into clouds. Metals by definition include any element other than hydrogen or helium, even oxygen, nitrogen, and carbon.

Populations of stars co-evolve with the interstellar medium in which they form. This highly simplified diagram shows the flow of material as giant stars evolve into supernovas which enrich molecular clouds with metals that they synthesized which are then inherited by the next generation of stars. The remains of supernovas may be neutron stars whereas dwarf stars remain dwarf and may not share their synthesized metals with the interstellar medium that breeds the next generation of stars. The diagram shows three populations of stars. The first generation of stars (known as Population III) were metal free and many had masses hundreds of times greater than the sun and lived very brief lives before exploding to enrich interstellar space with the first metals in the universe. Higher metal content is essential for the formation of planets and life.