Working through this chapter of the study guide will enable you to:

1. List the physical characteristics of the Sun and explain its inner workings.
2. Show how the celestial sphere can be used to find the positions of objects in the sky and how declination and right ascension define these positions.
3. Understand how stars are formed and how they proceed through well defined life cycles as they burn up their nuclear fuel.
4. Describe the groupings of stars into gigantic "island universes" called galaxies.
5. Explain why quasars are believed to be the most distant visible objects in the universe.
6. Trace the history of our universe from its conception in the Big Bang to its present day structure, using our current knowledge of cosmology.

Discussion

Observation of the night sky has fascinated mankind for centuries and has led to the development of many concepts and devices to aid man in this endeavor. The celestial sphere and the development of the ideas of declination and right ascension have provided a method of charting the skies in much the same way that locations on Earth can be charted using longitude and latitude. Telescopes of all kinds have enhanced our knowledge of the heavens by allowing us to collect and analyze extremely weak electromagnetic signals from stars and other luminous objects that we observe in the sky.
The Sun is the key to our understanding of other stars and many of the celestial processes that make up our modern theories of astronomy. The evolution of a star through its main sequence lifetime, and on into its eventual collapse and death, provides us with an understanding of the processes that create the many elements making up our world and the remainder of the universe. The interaction between gravity and nuclear fusion that produces a main sequence star that is stable for millions or even billions of years is a truly amazing process. This long-term stability of our own Sun is, of course, the very reason that life such as ours has been able to develop on a small, insignificant planet such as Earth.
The overall structure of the universe depends on the formation of stars in galaxies and the grouping of galaxies into clusters. Some of the most profound mysteries of the universe may eventually be resolved as we learn more about the behavior of matter in the cores of gigantic galaxies. It is remarkable that we have learned so much about the structure and history of the universe from the limited amount of information that has come to us in the form of electromagnetic radiation from distant stars and other luminous objects. This chapter contains a tremendous number of facts about the universe that have been pieced together from this meager data into an overall theory of surprising detail and beauty.

Section  18.1 The Sun

Like all stars, our Sun is a gigantic, rotating ball of luminous gas. It is about 864,000 miles in diameter, has an average density of 1.4 g/cm³ (a little greater than that of water), and moves through space at the astonishing speed of nearly 150 mi/s carrying our entire solar system, including Earth, along with it.
The yellowish surface of the Sun as seen from Earth is called the photosphere. This surface has a temperature of about 6000 K and shows an overall granular appearance, with areas of sunspot activity, flares, and prominences clearly visible if proper filters are used to protect the eyes of the observer. Above the photosphere are the two outermost layers of the Sun, the thin reddish chromosphere and the gorgeous white expanse of heated gases called the corona. These outer layers can be seen only during total solar eclipses or with specially equipped telescopes.
The core of the Sun is heated by a nuclear reaction called the proton-proton process, which converts hydrogen into helium, releasing large amounts of heat in the process. This reaction keeps the core temperature at about 15 million K. As heat works its way outward toward the photosphere, the outbound radiant energy balances the pull of gravity and keeps the Sun in a state of equilibrium, thus preventing it from collapsing under its own weight. When heat from the core finally reaches the Sun's surface, this energy is released into space as light and other types of electromagnetic radiation. Only a small fraction of this energy reaches Earth, but it is the basis for our understanding of the Sun's overall structure and energy-producing fusion reactions.

Section  18.2 The Celestial Sphere

The direction to all objects seen in the sky can be recorded on a great, imaginary, transparent globe known as the celestial sphere. If we envision the celestial sphere to be rotating around Earth, this motion accounts for the daily movement of the "fixed" stars across the sky. (Remember that this apparent motion is really caused by the rotation of Earth on its axis.) The exact location of each object on the celestial sphere can be specified using a system of right ascension and declination. This system is similar but not identical to longitude and latitude used to locate places on Earth's surface.
The imaginary celestial sphere is useful in finding the relative position of objects in the sky, but since it has a uniform diameter, it tells us nothing about the distance from Earth to these objects. Such distances can be measured in units such as astronomical units (AU), parsecs (pc), or light-years (ly). Astronomers spend a great deal of time trying to determine these distances with as much accuracy as possible.
Even though the Sun appears to move with respect to the fixed positions of the distance stars, its path among these stars can be plotted on the celestial sphere. This path is called the ecliptic, and the constellations along this path make up the signs of the zodiac, a group of 12 star patterns, one representing each month of the year. Since the planets move around the Sun in nearly the same plane, they also move among these zodiacal constellations from day to day, and we see them against this background of stars where they are visible from Earth.

Section  18.3 Stars

All stars, like our Sun, are luminous balls of hot gases. As seen from Earth, each has its own brightness or apparent magnitude that was originally measured on a scale from 1 to 6. The brightest stars had the designation of first magnitude, the next brightest are second magnitude, and so forth. Today this scale has been extended in both directions to better represent the magnitudes of stars and other celestial objects. The actual energy emitted by a star determines its true brightness or absolute magnitude. Absolute magnitude is calculated to be a star's apparent magnitude if it were located (in our imagination) at exactly 10 pc distance from Earth.
Stars progress through a life cycle much like a person, only the time frame is much longer. A star begins its life as a gas and dust cloud that collapses under its own self-gravitation into a main-sequence star. It next becomes a red giant, followed by a pulsating phase, and ending up in one of three final states depending on its mass. The lightest stars like our Sun become white dwarf stars, intermediate-mass stars form neutron stars, and the heaviest stars end their lives as black holes. An H-R diagram is a convenient way of plotting the properties of stars and showing their present placement in their life cycles. The diagram plots the absolute brightness of a star versus its surface temperature or its spectrum type. (See Figures 18.11 and 18.15 in the textbook.)

Section  18.4 Gravitational Collapse and Black Holes

As stated in the last Section, the most massive stars end their lives by collapsing into a strange structure called a black hole. A black hole is defined as a singularity in space showing no discernable extent, in which is concentrated the mass of an entire star. This singularity is surrounded by an event horizon that defines a spherical boundary inside which any matter or radiation is permanently and irreversibly trapped. Although it is not possible to see the black hole itself, X-rays may be emitted from a rapidly swirling disk of incoming gases that are being pulled into a black hole from a nearby companion star. Such a swirling cloud is called an accretion disk. The radiation emitted from the accretion disk can be observed because it is still outside of the event horizon and so is not subject to the limitations on radiation escape imposed on the black hole itself.

Section  18.5 Galaxies

Individual stars are grouped into large collections of stars known as galaxies. Galaxies are classified by their appearance as seen from Earth as elliptical, spiral, or irregular, and each can have from a few million to many billion individual stars held together by mutual gravitational attraction. Some galaxies have very interesting properties in addition to their overall appearance. For example, some tend to emit large quantities of radio signals, and others produce X-rays or have gas jets protruding from their centers.
Galaxies often form into large aggregate systems called galactic clusters. The number of members in a cluster can vary from a few dozen, like our own Local Group, to many thousands or even millions of individual galaxies. Even larger collections are sometimes produced when galactic clusters pull together under mutual gravitational attraction to form superclusters.
Hubble's law shows, for example, a definite relationship between the distance to faraway galaxies and the speed at which these galaxies are moving away from us. This relationship implies that these distant groups of stars that we can see are galaxies that have been moving away from us at high speed since the time of the formation of the universe. This universal expansion is the foundation on which the most generally accepted theory of cosmology, the Big Bang theory, is based.

Section  18.6 Quasars

Another interesting group of distant objects in our universe is made up of extremely powerful radio sources that otherwise, at first glance, resemble stars. Because they resemble normal stars, these objects have been named "quasistellar radio sources," or quasars for short. Careful study of quasars has revealed that they are about the size of our solar system but produce more light than most large galaxies. This description is based on the belief that quasars are very far away, which is itself based on the fact that they show extremely large cosmic redshifts. Redshifts indicate large recessional velocities that, based on Hubble's law, mean that quasars are very far away. If quasars are really that far away, they must be exceptionally bright and give off vast amounts of energy. Although the source of such large energy output is not totally understood, our best theories suggest that these luminous objects may be powered by super-massive black holes and their accretion disks that are located at the quasars' centers.

Section  18.7 Cosmology

Cosmology deals with the origin, evolution, and structure of the entire universe. Today we believe that our universe began with a tremendous explosion called the Big Bang. The Big Bang theory was suggested by cosmological red shifting of the light observed from distant galaxies, and it is also supported by the existence of a very uniform isotropic radiation, called the three-degree radiation, that is thought to be residual energy from the Big Bang explosion itself. Three-degree radiation has been greatly red-shifted and cooled over the billions of years since the beginning of the universe, until it has achieved the form in which we observe it today.
It is not presently known whether our expanding universe will continue to expand forever or will someday stop and reverse its expansion because of gravitational effects. Such a reversal or collapse back on itself is referred to as the Big Crunch. Only the acquisition of more detailed astronomical data can answer our many remaining questions about the origin, history, and fate of our universe. During the next few years, it is hoped that larger and better telescopes such as the two Keck ground-based telescopes in Hawaii and the Hubble Space Telescope will be able to provide such data. The new century promises to be a very interesting and enlightening time for astronomers and the general public alike.

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