The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- One method for determining the distance to a star is to use a measurement called
stellar parallax, the extremely slight back-and-forth shifting in a nearby
star's position due to the orbital motion of Earth. The farther away a star is, the
less its parallax. A unit used to express stellar distance is the light-year, which
is the distance light travels in a year, about 9.5 trillion kilometers (5.8 trillion miles).
- The intrinsic properties of stars include brightness, color, temperature,
mass, and size. Three factors control the brightness of a star as
seen from Earth: how big it is, how hot it is, and how far away it is.
Magnitude is the measure of a star's brightness. Apparent
magnitude is how bright a star appears when viewed from Earth. Absolute
magnitude is the "true" brightness of a star if it were at a standard
distance of about 32.6 light-years. The difference between the two magnitudes is directly
related to a star's distance. Color is a manifestation of a star's temperature. Very hot
stars (surface temperatures above 30,000 K) appear blue; red stars are much cooler (surface
temperatures generally less than 3000 K). Stars with surface temperatures between 5000 and
6000 K appear yellow, like our Sun. The center of mass of orbiting binary
stars (two stars revolving around a common center of mass under their mutual
gravitational attraction) is used to determine the mass of the individual stars in a binary
system.
- A Hertzsprung-Russell diagram is constructed by plotting the absolute
magnitudes and temperatures of stars on a graph. A great deal about the sizes of stars can
be learned from H-R diagrams. Stars located in the upper-right position of an H-R diagram
are called giants, luminous stars of large radius. Supergiants
are very large. Very small white dwarf stars are located in the lower-central
portion of an H-R diagram. Ninety percent of all stars, called main-sequence
stars, are in a band that runs from the upper-left corner to the lower-right corner
of an H-R diagram.
- Variable stars fluctuate in brightness. Some, called pulsating
variables, fluctuate regularly in brightness by expanding and contracting in size.
When a star explosively brightens, it is called a nova. During the outburst,
the outer layer of the star is ejected at high speed. After reaching maximum brightness in a
few days, the nova slowly returns in a year or so to its original brightness.
- New stars are born out of enormous accumulations of dust and gases, called a
nebula, that are scattered between existing stars. A bright
nebula glows because the matter is close to a very hot (blue) star. The two main
types of bright nebulae are emission nebulae (which derive their visible light
from the fluorescence of the ultraviolet light from a star in or near the nebula) and
reflection nebulae (relatively dense dust clouds in interstellar space that
are illuminated by reflecting the light of nearby stars). When a nebula is not close enough
to a bright star to be illuminated, it is referred to as a dark nebula.
- Stars are born when their nuclear furnaces are ignited by the unimaginable pressures
and temperatures in collapsing nebulae. New stars not yet hot enough for nuclear fusion are
called protostars. When collapse causes the core of a protostar to reach a
temperature of at least 10 million K, the fusion of hydrogen nuclei into helium nuclei
begins in a process called hydrogen burning. The opposing forces acting on a
star are gravity trying to contract it and gas pressure
(thermal nuclear energy) trying to expand it. When the two forces are
balanced, the star becomes a stable main-sequence star. When the hydrogen in a
star's core is consumed, its outer envelope expands enormously and a red giant
star, hundreds to thousands of times larger than its main-sequence size, forms. When
all the usable nuclear fuel in these giants is exhausted and gravity takes over, the stellar
remnant collapses into a small dense body.
- The final fate of a star is determined by its mass. Stars with less
than one half the mass of the Sun collapse into hot, dense white dwarf stars.
Medium-mass stars (between 0.5 and 3.0 times the mass of the Sun) become red giants,
collapse, and end up as white dwarf stars, often surrounded by expanding spherical clouds of
glowing gas called planetary nebulae. Stars more than three times the mass of
the Sun terminate in a brilliant explosion called a supernova. Supernovae
events can produce small, extremely dense neutron stars, composed entirely of
subatomic particles called neutrons; or even smaller and more dense black
holes, objects that have such immense gravity that light cannot escape their
surface.
- The Milky Way Galaxy is a large, disk-shaped spiral
galaxy about 100,000 light-years wide and about 10,000 light-years thick at the
center. There are three distinct spiral arms of stars, with some showing
splintering. The Sun is positioned in one of these arms about two-thirds of the way from the
galactic center, at a distance of about 30,000 light-years. Surrounding the galactic disk is
a nearly spherical halo made of very tenuous gas and numerous globular
clusters (nearly spherically shaped groups of densely packed stars).
- The various types of galaxies include (1) irregular galaxies, which
lack symmetry and account for only 10 percent of the known galaxies; (2) spiral
galaxies, which are typically disk-shaped with a somewhat greater concentration of
stars near their centers, often containing arms of stars extending from their central
nucleus; and (3) elliptical galaxies, the most abundant type, which have an
ellipsoidal shape that ranges to nearly spherical and that lack spiral arms.
- Galaxies are not randomly distributed throughout the universe. They are grouped in
galactic clusters, some containing thousands of galaxies. Our own, called the
Local Group, contains at least 28 galaxies.
- By applying the Doppler effect (the apparent change in wavelength of
radiation caused by the motions of the source and the observer) to the light of galaxies,
galactic motion can be determined. Most galaxies have Doppler shifts toward the red end of
the spectrum, indicating increasing distance. The amount of Doppler shift is dependent on
the velocity at which the object is moving. Because the most distant galaxies have the
greatest red shifts, Edwin Hubble concluded in the early 1900s that they were retreating
from us with greater recessional velocities than were more nearby galaxies. It was soon
realized that an expanding universe can adequately account for the observed
red shifts.
- The belief in the expanding universe led to the widely accepted Big Bang
Theory. According to this theory, the entire universe was at one time confined in a
dense, hot, supermassive concentration. Almost 14 billion years ago, a cataclysmic explosion
hurled this material in all directions, creating all matter and space. Eventually the
ejected masses of gas cooled and condensed, forming the stellar systems we now observe
fleeing from their place of origin.
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