Summary of Key Ideas

Discovering the Milky Way

• A century ago, astronomers were divided on whether or not the Milky Way Galaxy and the universe were the same thing.
• The inconclusive Shapley–Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe.
• Cepheid variable stars are important in determining the distance to other galaxies.
• Edwin Hubble proved that there are other galaxies far outside of the Milky Way.

The Structure of Our Galaxy

• Our Galaxy has a disk about 100,000 ly in diameter and about 2000 ly thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars.
• Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds, as well as by other nonvisible emissions.
• The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained.
• A supermassive black hole of about 4.3 × 10⁶ M_⊙ exists in the galactic nucleus.
• The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called the central bulge, through which a bar of stars and gas extends.
• A disk with two bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic central bulge.
• Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming.
• The Sun is located about 26,000 ly from the galactic nucleus, between the spiral arms. The Sun moves in its orbit at a speed of about 878,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy.
• The entire Galaxy is surrounded by two halos of matter. The inner halo includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter. It orbits in the same general direction as the disk. The outer halo is composed of dark matter and very old stars, which have retrograde orbits.