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Convection zone
100%
Convection
carries energy
70% outward.
Energy produced in
the core is carried
Nuclear 25% outward by
burning core photons.
0
(Core) Nuclear reactions
produce energy in the
Sun’s core.

Radiative zone

FIGURE 14.6 Energy-producing nuclear reactions occur only within the inner 25 percent of the Sun’s radius. The energy produced by these
reactions is carried outward by photons to 70 percent of the Sun’s radius. From that distance outward, convection carries most of the Sun’s energy.

and light from the surface. This model describes the interior as temperatures mean increased kinetic energy, which results in
a set of three shells: (1) the core, (2) a radiation zone, and (3) the increased numbers of collisions between hydrogen nuclei with
convection zone (Figure 14.6). the end result being an increased number of fusion reactions.
Our model describes the core as a dense, very hot re- Thus, a more massive star uses up its hydrogen more rapidly
gion where nuclear fusion reactions release gamma and X-ray than a less massive star. On the other hand, stars that are less
radiation. The density of the core is about 12 times that of massive than the Sun use their hydrogen at a slower rate, so
solid lead. Because of the plasma conditions, however, the core they have longer life spans. The life spans of the stars range
remains in a gaseous state even at this density. from a few million years for large, massive stars, to 10 billion
Our model describes the radiation zone as less dense than years for average stars like the Sun, to trillions of years for
the core, having a density about the same as that of water. Energy small, less massive stars.
in the form of gamma and X rays from the core is absorbed and
reemitted by collisions with atoms in this zone. The radiation
slowly diffuses outward because of the countless collisions over BRIGHTNESS OF STARS
a distance comparable to the distance between Earth and the Stars generate their own light, but some stars appear brighter
Moon. It could take millions of years before this radiation fi- than others in the night sky. As you can imagine, this difference
nally escapes the radiation zone. in brightness could be related to (1) the amount of light pro-
The model convection zone begins about seven-tenths of duced by the stars, (2) the size of each star, or (3) the distance to
the way to the surface, where the density of the gases is about a particular star. A combination of these factors is responsible
1 percent of the density of water. Gases at the bottom of this for the brightness of a star as it appears to you in the night sky. A
zone are heated by radiation from the radiation zone below, ex- classification scheme for different levels of brightness that you
pand from the heating, and rise to the surface by convection. At see is called the apparent magnitude scale (Table 14.1). The
the surface, the gases emit energy in the form of visible light, apparent magnitude scale is based on a system established by
ultraviolet radiation, and infrared radiation, which moves out a Greek astronomer over two thousand years ago. Hipparchus
into space. As they lose energy, the gases contract in volume and made a catalog of the stars he could see and assigned a numeri-
sink back to the radiation zone to become heated again, con- cal value to each to identify its relative brightness. The bright-
tinuously carrying energy from the radiation zone to the surface ness values ranged from 1 to 6, with the number 1 assigned
in convection cells. The surface is continuously heated by the to the brightest star and the number 6 assigned to the faintest
convection cells as it gives off energy to space, maintaining a star that could be seen. Stars assigned the number 1 came to be
temperature of about 5,800 K (about 5,500°C). known as first- magnitude stars, those a little dimmer as second-
17
As an average star, the Sun converts about 1.4 × 10 kg magnitude stars, and so on to the faintest stars visible, the sixth-
of matter to energy every year as hydrogen nuclei are fused to magnitude stars.
produce helium. The Sun was born about 5 billion years ago When technological developments in the nineteenth
and has sufficient hydrogen in the core to continue shining for century made it possible to measure the brightness of a star,
another 4 or 5 billion years. Other stars, however, have masses Hipparchus’s system of brightness values acquired a precise,
that are much greater or much less than the mass of the Sun, quantitative meaning. Today, a first-magnitude star is defined
so they have different life spans. More massive stars generate as one that is 100 times brighter than a sixth-magnitude star,
higher temperatures in the core because they have a greater with five uniform multiples of decreasing brightness on a scale
gravitational contraction from their greater masses. Higher from the first magnitude to the sixth magnitude.

14-5 CHAPTER 14 The Universe 355