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 classified and 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. 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 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  18.1The Sun

Like all stars, our Sun is a gigantic, rotating ball of luminous gas. It is about 865,000 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 chain, 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  18.2The 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 because 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 only appears to move with respect to the fixed positions of the distant 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  18.3Classifying 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 were 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. 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 18.12 and 18.15 in the textbook.)


Section  18.4The Life Cycle of Low-Mass Stars

Astronomers know that stars are born, radiate energy, expand, possibly explode, and then die. The exact details depend on a star's mass, and in a minor way, on its composition. The birth of a star begins with the accretion (gathering) of interstellar material (mostly hydrogren) within an enormous, interstellar molecular cloud, or nebula, to form a protostar.

As the protostar continues to decrease in size, the temperature continues to increase, and fusion reactions begin in which hydrogren is converted to helium. The newly formed star moves onto the main sequence of Hertzsprung-Russell (H-R) diagram and stays there for billions of years. The larger the star's mass, the smaller its lifetime on the main sequence. If a protostar does not have enough mass to sustain fusion, a "failed" star called a brown dwarf is the result.

When the star's supply of hydrogren gets low, the star expands and moves into the red giant phase of its life. Eventually, the core of the red giant gets so hot that helium begins to fuse into carbon. The star becomes a variable star for a relatively short time and then gets so unstable that its outer layers are blown off, forming a planetary nebula. The remaining core of the planetary nebula is a white dwarf star, which is about the size of Earth but is very dense. A white dwarf is the final phase of low-mass stars—those about the mass of the Sun or less.


Section  18.5The Life Cycle of High-Mass Stars

A nova is the result of a nuclear explosion on the surface of a white dwarf, an explosion caused by relatively small amounts of matter falling onto its surface from the atmosphere of a larger binary companion star. The explosion causes the star to temporarily increase in brightness by a factor of 100 to millions and thus become more noticeable. Here the name nova, or "new," star.

A Type I supernova, on the other hand, results from the catastrophic explosion of a massive white dwarf that has accumulated enough additional mass to put it over the limit of 1.4 times the Sun's mass. A Type II supernova results from the collapse of the iron core of a massive red supergiant. Large amounts of material and radiation are emitted in all supernova explosions, and the neutrons present form the elements with atomic numbers higher than iron.

A Type II supernova usually leaves behind a neutron star, or pulsar. Neutron stars are about the size of a small city and very, very dense. However, if the mass of the remaining core of the supernova is greater than about three times the mass of the Sun, the star's matter continues to collapse to form a black hole. The center of a black hole is a point called singularity. The singularity is surrounded by an invisible, spherical boundary known as the event horizon. The event horizon defines the size of the black hole (about 10 to 20 miles in radius) and is a one-way boundary; matter and radiation can enter, but they cannot leave.

Thus all stars are born in the same manner, but their mass determines their ultimate fate—either a white dwarf, neutron star, or black hole.


Section  18.6Galaxies

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 larger aggregate systems called 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.

Almost everywhere astronomers look, whether in the Milky Way, in other galaxies, or in clusters of galaxies, 90% of the matter needed to cause their gravitational behavior emits no electromagnetic radiation. This "unseen" matter is called dark matter, and astronomers are diligently searching for its identity.


Section  18.7Cosmology

Cosmology deals with the origin, evolution, and structure of the entire universe. Hubble's Law shows 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 Band theory, is based.

Today we believe that our universe began with a tremendous explosion called the Big Bang. The Big Bang theory is supported by the existence of a very uniform isotropic radiation, called the cosmic microwave background, that is thought to be residual energy from the Big Bang explosion itself. The 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.

Recent investigations indicate that our universe will continue to expand forever. Only the acquisition of more detailed astronomical data can answer our many remaining questions about the origin, history, and fate of our universe. The new century promises to be a very interesting and enlightening time for astronomers and the general public alike.

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