Chapter 15--Our Sun - Geological Sciences

Table 15.1 Basic Properties of the Sun
Radius (R Sun
)
Mass (M Sun
)
Luminosity (L Sun
)
Composition (by
percentage of mass)
Rotation rate
Surface temperature
Core temperature
696,000 km (about 109 times the
radius of Earth)
2 10 30 kg (about 300,000 times
the mass of Earth)
3.8 10 26 watts
70% hydrogen, 28% helium,
2% heavier elements
25 days (equator) to 30 days (poles)
5,800 K (average); 4,000 K (sunspots)
15 million K
Figure 15.3 This photo of the
visible surface of the Sun shows
several dark sunspots.
nosity, it would be enough to meet current human energy
demands for roughly the next 500,000 years!
Of course, only a tiny fraction of the Sun’s total energy
output reaches Earth, with the rest dispersing in all directions
into space. Most of this energy is radiated in the form
of visible light, but once you leave the protective blanket of
Earth’s atmosphere you’ll encounter significant amounts of
other types of solar radiation, including dangerous ultraviolet
and X rays. Your spaceship will require substantial
shielding to protect you from serious radiation burns caused
by these high-energy forms of light.
Through a telescope, you can see that the Sun seethes
with churning gases. At most times you’ll detect at least
a few sunspots blotching its surface (Figure 15.3). If you
focus your telescope solely on a sunspot, you’ll find that it is
blindingly bright. Sunspots appear dark only in contrast to
the even brighter solar surface that surrounds them. A typical
sunspot is large enough to swallow the entire Earth, dramatically
illustrating that the Sun is immense by any
earthly standard. The Sun’s radius is nearly 700,000 kilometers,
and its mass is 2 10 30 kilograms—about 300,000
times more massive than Earth.
Sunspots appear to move from day to day along with
the Sun’s rotation. If you watch very carefully, you may
notice that sunspots near the solar equator circle the Sun
faster than those at higher solar latitudes. This observation
reveals that, unlike a spinning ball, the entire Sun does
not rotate at the same rate. Instead, the solar equator completes
one rotation in about 25 days, and the rotation period
increases with latitude to about 30 days near the solar
poles. Table 15.1 summarizes some of the basic properties
of the Sun.
THINK ABOUT IT
VIS
As a brief review, describe how we measure the mass of the
Sun using Newton’s version of Kepler’s third law. (Hint: Look
back at Chapter 5.)
As you and your spaceship continue to fall toward the
Sun, you notice an increasingly powerful headwind exerting
a bit of drag on your descent. This headwind, called the
solar wind, is created by ions and subatomic particles flowing
outward from the solar surface. The solar wind helps
shape the magnetospheres of planets [Sections 11.3, 12.4]
and blows back the material that forms the tails of comets
[Section 13.4].
A few million kilometers above the solar surface, you
enter the solar corona, the tenuous uppermost layer of the
Sun’s atmosphere (Figure 15.4). Here you find the temperature
to be astonishingly high—about 1 million Kelvin. This
region emits most of the Sun’s X rays. However, the density
here is so low that your spaceship feels relatively little heat
despite the million-degree temperature [Section 4.2].
Nearer the surface, the temperature suddenly drops to
about 10,000 K in the chromosphere, the primary source
of the Sun’s ultraviolet radiation. At last you plunge through
the visible surface of the Sun, called the photosphere,where
the temperature averages just under 6,000 K. Although the
photosphere looks like a well-defined surface from Earth,
it consists of gas far less dense than Earth’s atmosphere.
Throughout the solar atmosphere, you notice that
the Sun has its own version of weather, in which conditions
at a particular altitude differ from one region to another.
Some regions of the chromosphere and corona are particularly
hot and bright, while other regions are cooler and
less dense. In the photosphere, sunspots are cooler than
the surrounding surface, though they are still quite hot
and bright by earthly standards. In addition, your compass
goes crazy as you descend through the solar atmosphere,
indicating that solar weather is shaped by intense magnetic
fields. Occasionally, huge magnetic storms occur, shooting
hot gases far into space.
Into the Sun
Up to this point in your journey, you may have seen Earth
and the stars when you looked back, but as you slip beneath
the photosphere, blazing light engulfs you. You are
chapter 15 • Our Star 499