SUMMARY OF KEY IDEAS

• 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.

• 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 two spiral arms. The Sun moves in its orbit at a speed of about 878,000 km/h (550,000 mi/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.

• 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.

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• 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.

• 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.

• 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.

• The development of radio astronomy in the late 1940s led to the discovery of very powerful and extremely distant energy sources.

• An active galaxy is an extremely luminous galaxy that has one or more unusual features: an unusually bright, starlike nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects.

• A quasar, or quasi-stellar radio source, is an object that looks like a star but has a huge redshift. This redshift corresponds to a distance of billions of light-years from Earth, according to the Hubble law.

• To be seen from Earth, a quasar must be very luminous, typically about 100 times brighter than an ordinary galaxy. Relatively rapid fluctuations in the brightness levels of some quasars indicate that they cannot be much larger than the diameter of our solar system.

• An active spiral galaxy with a bright, starlike nucleus and strong emission lines in its spectrum is categorized as a Seyfert galaxy.

• An active elliptical galaxy is called a radio galaxy. It has a bright nucleus and a pair of radio-bright jets that stream out in opposite directions.

• BL Lacertae (BL Lac) objects (some of which are called blazars) have bright nuclei whose cores show relatively rapid variations in luminosity.

• Double-radio sources contain active galactic nuclei located between two characteristic radio lobes. A head-tail radio source shows evidence of jets of high-speed particles that emerge from an active galaxy.

• Many galaxies contain huge concentrations of matter at their centers.

• Some matter that spirals in toward a supermassive black hole is squeezed into two oppositely directed beams that carry particles and energy into intergalactic space.

• The energy sources from quasars, Seyfert galaxies, BL Lac objects, radio galaxies, and double-radio sources are believed to be matter ejected from the accretion disks that surround supermassive black holes at the centers of galaxies.