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SUMMARY

Theoretically, stars are born in clouds of hydrogen gas and dust in the mature stars fall on a narrow band called the main sequence of stars.
space between other stars. Gravity pulls huge masses of hydrogen gas Where a star falls on the main sequence is determined by its brightness
together into a protostar, a mass of gases that will become a star. The and temperature, which in turn are determined by the mass of the star.
protostar contracts, becoming increasingly hot at the center, eventually Other groups of stars on the H-R diagram have different sets of proper-
reaching a temperature high enough to start nuclear fusion reactions ties that are determined by where they are in their evolution.
between hydrogen atoms. Pressure from hot gases balances the gravita- The life of a star consists of several stages, the longest of which is
tional contraction, and the average newborn star will shine quietly for the main sequence stage after a relatively short time as a protostar. After
billions of years. The average star has a dense, hot core where nuclear using up the hydrogen in the core, a star with an average mass expands
fusion releases radiation, a less dense radiation zone where radiation to a red giant, then blows off the outer shell to become a white dwarf
moves outward, and a thin convection zone that is heated by the radia- star, which slowly cools to a black dwarf. The blown-off outer shell
tion at the bottom, then moves to the surface to emit light to space. forms a planetary nebula, which disperses over time to become the gas
The brightness of a star is related to the amount of energy and and dust of interstellar space. More massive stars collapse into neutron
light it is producing, the size of the star, and the distance to the star. The stars or black holes after a violent supernova explosion.
apparent magnitude is the brightness of a star as it appears to you. To Galaxies are the basic units of the universe. The Milky Way galaxy
compensate for differences in brightness due to distance, astronomers has three distinct parts: (1) the galactic nucleus, (2) a rotating galactic
calculate the brightness that stars would have at a standard distance disk, and (3) a galactic halo. The galactic disk contains subgroups of stars
called the absolute magnitude. Absolute magnitude is an expression of that move together as galactic clusters. The halo contains symmetrical
luminosity, the total amount of energy radiated into space each second and tightly packed clusters of millions of stars called globular clusters.
from the surface of a star. All the billions of galaxies can be classified into groups of four struc-
Stars appear to have different colors because they have different tures: elliptical, spiral, barred, and irregular. Evidence from astronomical
surface temperatures. A graph of temperature by spectral types and and physical “clocks” indicates that the galaxies formed some 13.7 billion
brightness by absolute magnitude is called the Hertzsprung-Russell years ago, expanding ever since from a common origin in a big bang. The
diagram, or H-R diagram for short. Such a graph shows that normal, big bang theory describes how the universe began by expanding.

KEY TERMS APPLYING THE CONCEPTS

absolute magnitude (p. 356) 1. A referent system that can be used to locate objects in the
apparent magnitude (p. 355) sky is the
big bang theory (p. 365) a. celestial globe.
black hole (p. 361) b. celestial coordinates.
c. celestial sphere.
celestial equator (p. 352)
d. celestial maps.
celestial meridian (p. 353)
2. Our Sun is
Cepheid variable (p. 358)
a. one of several stars in the solar system.
galaxy (p. 362)
b. the brightest star in the night sky.
Hertzsprung-Russell diagram (p. 358)
c. an average star, considering mass and age.
light-year (p. 352)
d. a protostar.
luminosity (p. 356)
3. The brightness of a star at a defined standard distance is
main sequence stars (p. 358)
a. apparent magnitude.
nebulae (p. 354) b. apparent longitude.
neutron star (p. 361) c. absolute magnitude.
north celestial pole (p. 352) d. absolute latitude.
protostar (p. 354) 4. The total amount of energy radiated from the surface of a star
pulsar (p. 361) into space each second is
red giant stars (p. 358) a. stellar radiation.
south celestial pole (p. 352) b. luminosity.
supernova (p. 361) c. convection.
white dwarf stars (p. 358) d. dispersion.
5. Which of the following colors of starlight is from the hottest star?
a. Blue-white
b. Red
c. Orange-red
d. Yellow

370 CHAPTER 14 The Universe 14-20