HII regions are bright nebulae produced when ultraviolet radiation from hot stars ionizes the surrounding interstellar gas. An HII region develops when hot stars form inside molecular clouds. The HII region grows in size, eventually breaking out of the cloud. When the central star of an HII region stops emitting large numbers of ultraviolet photons, the HII region fades out.
The presence of diffuse interstellar gas is revealed by interstellar absorption and emission lines. Between clouds, the interstellar gas is warm except where it has been heated by supernovae. Superbubbles are large regions of hot interstellar gas produced when many supernovae occur within a short time in a cluster of stars.
Interstellar dust clouds appear either as dark nebulae, which obscure the stars behind them, or as reflection nebulae, which scatter the light of nearby stars toward us.
Interstellar dust blocks starlight both by scattering it away and by absorbing it. The scattered light contributes to the general glow of the galaxy, whereas absorbed starlight heats the particles and causes them to radiate in the infrared.
Interstellar dust particles are very tiny. They form in the gas that flows into interstellar space from cool giant and supergiant stars. While in interstellar space, the dust particles may develop coatings of icy material.
Initially, counts of stars in various directions led astronomers to conclude that the Milky Way is a small galaxy centered near the Sun. This result turned out to be wrong, however, because it failed to take interstellar absorption into account.
The center of the galaxy is located 8 kpc from the Sun. The galaxy consists of a flat disk a few kpc thick and 40 kpc across, a spheroidal bulge, and a spherical halo that may extend to more than 40 kpc from the galactic center.
The orbital period of the Sun is about 220 million years. According to Kepler's third law, the mass of material inside the Sun's orbit must be about 100 billion solar mass.
The rotational speed of the Milky Way is constant between 1 and 20 kpc from the center, and may be constant to a much greater distance. This shows that the mass of the galaxy increases with distance from the center and may total as much as 600 billion solar mass. Most of the mass of the galaxy consists of dark matter of an unknown nature.
The spiral structure of the Milky Way has been found by mapping the locations of young stars and interstellar clouds. However, the overall structure of the galaxy's spiral pattern is still not well known.
The spiral arms of the Milky Way are density waves, which contain different material at different times. The gravitational force of the increased density of matter in spiral arms modifies the orbits of stars and interstellar dust. Interstellar clouds that enter spiral arms are compressed and form stars. Hot stars are found mainly in spiral arms because they don't live long enough to emerge from the spiral arm in which they form.
Radio and infrared observations have revealed the nature of the center of the Milky Way. Many stars are concentrated near the center, which also contains a ring of rapidly rotating gas and streamers of gas that may be falling into a central black hole of several million solar masses.
Since the galaxy formed, about 11 billion years ago, collisions between gas clouds have steadily flattened it. Nuclear reactions in stars have steadily enriched the galaxy in heavy elements. The orbits and compositions of the different populations of stars show the shape and composition of the galaxy at the time that the stars formed.
