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CONCEPTS Applied is just an ordinary star with an average brightness. Like the
other stars, the Sun is a massive, dense ball of gases with a sur-
The Night Sky face heated to incandescence by energy released from fusion
reactions deep within. Since the Sun is an average star, it can be
Become acquainted with the night sky by first locating the used as a reference for understanding all the other stars.
Big Dipper. In autumn, the Big Dipper is close to the north-
ern horizon. During fall the Big Dipper cup opens upward,
straight up at midnight on September 8. During spring the cup ORIGIN OF STARS
opens downward, straight down at midnight on March 8. The
Theoretically, stars are born from swirling clouds of hydrogen gas
stars of the far end of the cup are called the pointer stars.
in the deep space between other stars. Such interstellar (between
Trace an imaginary line through the pointer stars to find the
stars) clouds are called nebulae. These clouds consist of random,
Little Dipper. The bright star at the end of the Little Dipper
handle is Polaris, also called the North Star (Figure 14.5). swirling atoms of gases that have little gravitational attraction for
Observe the stars around Polaris until you can describe their one another because they have little mass. Complex motions of
apparent movement. Compare the brightness and color of stars, however, can produce a shock wave that causes particles
different stars. Can you find stars of different colors? to move closer together and collide, making local compres-
If you have a camera that has a “time” or “bulb” sions. Their mutual gravitational attraction then begins to pull
exposure setting and can be attached to a tripod, try a them together into a cluster. The cluster grows as more atoms
20-minute exposure of Polaris and nearby stars. To find
are pulled into it, which increases the mass and thus the gravi-
the direction of apparent movement, try a 15-minute
tational attraction, and still more atoms are pulled in from far-
exposure, cover the lens for 5 minutes, then remove the
ther away. Theoretical calculations indicate that on the order of
cover for an additional 2 minutes. 57
1 × 10 atoms are necessary, all within a distance of 3 trillion km
Pointer stars (about 1.9 trillion mi). When these conditions occur, the cloud
of gas atoms begins to condense by gravitational attraction to a
protostar, an accumulation of gases that will become a star.

Polaris Big Dipper EXAMPLE 14.1
3
19
Compared to the 10 molecules/cm of air on Earth, an average con-
3
centration of 1,000 hydrogen atoms/cm in the Orion Nebula does not
seem very dense. However, considering that the Orion Nebula is about
20 light-years (20 × 10 cm) across, a sphere with a volume of 4.19 ×
18
Little Dipper 10 cm would enclose the Orion Nebula, and it would contain
57
3
FIGURE 14.5 Polaris, or the North Star, is found by _
1,000 atoms
×

57
60
3
imagining a line running through the pointer stars at the far 3 (4.19 × 10 cm ) = 4.19 × 10 atoms
end of the Big Dipper and then extending it for five times the cm
distance between the two pointers. This is a sufficient number of hydrogen atoms to produce
__ 4,190 stars
60
4.19 × 10 atoms
=

57
1 × 10 atoms/star
Myths, Mistakes, & Misunderstandings Thus, there is a sufficient number of hydrogen atoms in the Orion
Nebula to produce 4,190 average stars like the Sun.
Stars Visible from the Bottom of a Well?
Perhaps you have heard that stars are visible from the bottom of
Gravitational attraction pulls the average protostar from a
a well, even in full daylight. This is a myth, as you cannot see any
cloud with a diameter of trillions of kilometers (trillions of miles)
stars from the bottom of a well in full daylight. This idea is based
on the belief that the brightness of the Sun, which obliterates the down to a dense sphere with a diameter of 2.5 million km
light from stars during the day, is somehow less in the bottom of (1.6 million mi) or so. As gravitational attraction accelerates the
a well. This idea might be based on the observation that looking atoms toward the center, they gain kinetic energy, and the interior
at a star that you cannot see well is aided by looking at it through temperature increases. Over a period of some 10 million years of
a long, dark tube (a telescope). This overlooks the role of lenses contracting and heating, the temperature and density conditions
or mirrors in a telescope. at the center of the protostar are sufficient to start nuclear fusion
reactions. Pressure from hot gases and energy from increasing fu-
sion reactions begin to balance the gravitational attraction over
the next 17 million years, and the newborn, average star begins its
14.2 STARS
stable life, which will continue for the next 10 billion years.
If you could travel by spaceship a few hundred light-years from The interior of an average star, such as the Sun, is modeled
Earth, you would observe the Sun shrink to a bright point of after the theoretical pressure, temperature, and density condi-
light among the billions and billions of other stars. The Sun tions that would be necessary to produce the observed energy
354 CHAPTER 14 The Universe 14-4