Chapter Outline

Astronomy

Astronomy: the traditional name given to the general study of all celestial bodies in the universe—the planets and their moons, stars, comets, meteoroids, asteroids, and interstellar clouds of material

The Universe as We See It

• The Sun is a star and is at the center point in the solar system.
• Earth and the eight other major planets travel in orbits around the Sun (Copernican theory).
• In addition to the Sun and nine planets, the solar system contains a field of asteroids between Mars and Jupiter and many comets orbiting the Sun.
• Within the universe are millions of widely separated galaxies. An average-size galaxy contains 100 million stars.
• The solar system is located in the Milky Way galaxy.
• The distance of the Milky Way galaxy to the nearest galaxy is about 2 million light-years.
• Gravity holds together every object in the universe.

Life and Death of Stars

The lifetime of a star may be billions of years.

• Stage 1: A star is formed by the contraction and rising temperature of a cloud of gas.
• Stage 2: The star’s energy comes from its own nuclear fuel until that fuel (hydrogen) is totally consumed.
• Stage 3: Depending on its original size, the star changes into a specific type of new star.
• Stage 4: The star explodes and turns into a different structure.

New stars become one of the following types:

• blue star—a massive, hot star that uses up its hydrogen quickly, expands, and turns into a giant or supergiant
• giant star—a blue star that has run out of hydrogen, resulting in a shrinking inner core and an expanding outer core
• white dwarf star—a small, hot star that is the leftover core of a giant or supergiant
• red dwarf star—the smallest and coolest of stars, having the longest lifetime of all stars

Old stars become one of the following structures:

• supernova—a flash of light caused by the explosion of a massive blue star
• neutron star—a very dense small star made up entirely of neutrons from the leftover materials near the center of a supernova
• pulsar—a neutron star that spins, sending out beams of pulsing radiation
• black hole—the collapsed leftovers of a supernova

Galaxies

Galaxies are made up of clusters of stars and have different shapes:

• spiral galaxy—a huge core of stars surrounded by spiral arms
• elliptical galaxy—an elongated sphere of stars
• irregular galaxy—a random-shaped galaxy that is neither spiral or elliptical

Besides stars, galaxies contain other structures:

• gas clouds—vast clouds of gas and dust
• globular clusters—ball-shaped clusters of older stars
• open clusters—groupings of a few thousand stars found in some spiral galaxies

Origin and Evolution of the Solar System

• The solar system began as a solar nebula (a huge cloud of interstellar gas and dust).
• The heaviest elements in the cloud moved toward the center while the lighter elements moved away from the center.
• The planets and moons formed from these elements.
• The solar system will continue to exist until the Sun runs out of hydrogen and begins to expand, engulfing the inner planets.

Origin and Evolution of the Universe

There is one main theory as to how the universe came into being:

• big bang theory—the universe began with the explosion of a dense, hot, compact mass under extreme pressure

There are three theories as to how the universe will end:

• open universe theory—the universe will continue to slowly expand forever
• flat universe theory—the universe will eventually reach a size and then remain that size forever
• closed universe theory—the universe will eventually begin to contract and eventually collapse back into a dense, hot, compact mass