Summary of Key Ideas

Types of Galaxies

• The Hubble classification system groups galaxies by their shapes into five major types: spiral, barred spiral, lenticular, elliptical, and irregular.
• The arms of spiral and barred spiral galaxies are sites of active star formation.
• According to the theory of self-propagating star formation, spiral arms of flocculent galaxies are caused by the sequential births and deaths of stars over extended regions of a galaxy. Differential rotation of a galaxy stretches these star-forming regions into elongated arches of stars and nebulae that we see as spiral arms.
• According to the spiral density wave theory, spiral arms of grand-design galaxies are caused by density waves. The gravitational field of a spiral density wave compresses the interstellar clouds that pass through it, thereby triggering the formation of stars, including OB associations, which highlight the arms.
• Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in elliptical galaxies.
• Irregular galaxies are rich in gas and dust, and star formation occurs in them.
• Lenticular galaxies are disk galaxies without spiral arms.

Clusters and Superclusters

• Galaxies group into clusters rather than being randomly scattered through the universe.
• A rich cluster contains at least a thousand galaxies; a poor cluster may contain only a few dozen up to a thousand galaxies. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is asymmetrical.
• Our Galaxy is a member of a poor, irregular cluster, called the Local Group.
• Rich, regular clusters contain mostly elliptical and lenticular galaxies; irregular clusters contain more spiral and irregular galaxies. Giant elliptical galaxies are often found near the centers of rich clusters.
• Each galaxy is held together with the aid of dark matter.
• No cluster of galaxies has an observable mass large enough to account for the observed motions of its galaxies; a large amount of dark matter must be present between the galaxies.
• Hot intergalactic gases emit X-rays in rich clusters.
• When two galaxies collide, their stars initially pass each other, but their interstellar gas and dust collide violently, either causing gas and dust to be stripped from the galaxies or triggering prolific star formation. The gravitational effects of a galactic collision can cast stars out of their galaxies into intergalactic space.
• Galactic mergers occur. A large galaxy in a rich cluster may also grow steadily through galactic cannibalism.

Superclusters in Motion

• A simple linear relationship exists between the distance from Earth to galaxies in other superclusters and the redshifts of those galaxies (a measure of the speed at which they are receding from us). This relationship is the Hubble law: Recessional velocity = H₀ × distance, where H₀ is the Hubble constant.
• Astronomers use standard candles—Cepheid variables, the brightest supergiants, globular clusters, H II regions, supernovae in a galaxy, and the Tully–Fisher relation—to calculate intergalactic distances. Because of difficulties in measuring the distances to remote galaxies, the value of the Hubble constant, H₀, is not known with complete certainty.