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OVERVIEW

Astronomy is an exciting field of science that has fascinated people since the beginnings of recorded history. Ancient
civilizations searched the heavens in wonder, some recording on clay tablets what they observed. Many religious
and philosophical beliefs were originally based on interpretations of these ancient observations. Today, we are still
awed by space, but now we are fascinated with ideas of space travel, black holes, and the search for extraterrestrial
life. Throughout history, people have speculated about the universe and their place in it and watched the sky and
wondered (Figure 14.1). What is out there and what does it all mean? Are there other people on other planets, looking
at the star in their sky that is our Sun, wondering if we exist?
Until about thirty years ago, progress in astronomy was limited to what could be observed and photographed.
Today, our understanding has expanded by the use of technology and spacecraft. It is now understood that our Sun
is but one of billions of stars that move in large groups of stars called galaxies. Where did the Sun come from? Will it
always be here? Our understanding has now developed to the point where we can answer such questions, but we can
also calculate the conditions in the center of our Sun and other stars. We can describe what the universe looked like
billions of years ago, and we can predict what will probably happen to it billions of years in the future. How stars and
galaxies form and what eventually will happen are two big ideas in this chapter.
Theoretical ideas about stars, galaxies, and how the universe formed will lead to some understanding of how stars
are arranged in space and how our Sun fits into the big picture. This will provide a framework of understanding for
moving closer to home, the local part of our galaxy—the solar system—which will be considered in chapter 15. This
will lead to greater understanding of our Earth and Moon.

moving air of different densities does not affect the image as
14.1 THE NIGHT SKY
much. Sufficient air movement can cause planets to appear to
Early civilizations had a much better view of the night sky before shimmer, however, just as a road appears to shimmer on a hot
city lights, dust, and pollution obscured much of it. Today, you summer day.
must travel far from the cities, perhaps to a remote mountain top, How far away is a star? When you look at the sky, it appears
to see a clear night sky as early people observed it. Back then, that all the stars are at the same distance. It seems impossible to
people could clearly see the motion of the Moon and stars night know anything about the actual distance to any given star. Stan-
after night, observing recurring cycles of motion. These cycles dard referent units of length such as kilometers or miles have
became important as people associated them with certain events. little meaning in astronomy since there are no referent points of
Thus, watching the Sun, Moon, and star movements became a comparison. Distance without a referent point can be measured
way to identify when to plant crops, when to harvest, and when in terms of angles or time. The unit of astronomical distance that
it was time to plan for other events. Observing the sky was an im- uses time is the light-year (ly). A light-year is the distance that
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portant activity, and many early civilizations built observatories light travels in one year, about 9.5 × 10 km (about 6 × 10 mi).
with sighting devices to track and record astronomical events. To locate planets, or anything else in the sky, you need
Stonehenge, for example, was an ancient observatory built in something to refer to, a referent system. A referent system is eas-
England around 2600 b.c. by Neolithic people (Figure 14.2). ily established by first imagining the sky to be a celestial sphere
Light from the stars and planets must pass through Earth’s (Figure 14.3). A coordinate system of lines can be visualized on
atmosphere to reach you, and this affects the light. Stars appear this celestial sphere just as you think of the coordinate system
as point sources of light, and each star generates its own light. The of latitude and longitude lines on Earth’s surface (see p. 412 ).
stars seem to twinkle because density differences in the atmo- Imagine that you could inflate Earth until its surface touched
sphere refract the point of starlight one way, then the other, as the celestial sphere. If you now transfer the latitude and longi-
the air moves. The result is the slight dancing about and change tude lines to the celestial sphere, you will have a system of sky
in intensity called twinkling. The points of starlight are much coordinates. The line of the equator of Earth on the celestial
steadier when viewed on a calm night or when viewed from high sphere is called the celestial equator. The North Pole of Earth
in the mountains where there is less atmosphere for the starlight touches the celestial sphere at a point called the north celestial
to pass through. Astronauts outside the atmo sphere see no twin- pole. From the surface of Earth, you can see that the celestial
kling, and the stars appear as steady point sources of light. equator is a line on the celestial sphere directly above Earth’s
Back at ground level, within the atmosphere, the reflected equator, and the north celestial pole is a point directly above the
light from a planet does not seem to twinkle. A planet appears North Pole of Earth. Likewise, the south celestial pole is a point
as a disk of light rather than a point source, so refraction from directly above the South Pole of Earth.

352 CHAPTER 14 The Universe 14-2