The Sun
You should know that the Sun is about 1000 times more massive than Jupiter.
It is about 300,000 times the mass of the Earth.
The Sun represents about 99.8% of the mass in the solar system.
The diameter of the Sun is about 1 million miles.
The Sun’s atmosphere has 3 layers. The lowest is the photosphere, the next is the
chromosphere, and the outermost is called the corona.
1) What is the source of the Sun’s energy? The Sun’s energy comes from thermonuclear
fusion. Hydrogen nuclei are fused (combined) to make helium atoms.
2) What is the internal structure of the Sun? The Sun’s core is very dense and at very
high pressure and temperature. About 99 percent of the Sun’s mass is within half its
radius from the center. The temperature of the core of the Sun is around 30 million
degrees Fahrenheit. The density of the Sun’s core is 10 times the density of lead.
3) How can astronomers measure the properties of the Sun’s interior? Astronomers can
look at vibrations on the Sun’s surface to learn about its interior.
4) How can we be sure thermonuclear reactions are occurring in the Sun’s interior?
Nuclear reactions produce particles called neutrinos. We detect these neutrinos coming
from the Sun so we know the reactions are occurring.
5) How do we know what is happening inside the Sun? 1) we make mathematical models
that correctly predict the Sun’s behavior. 2) We look at vibrations. 3) We see neutrinos
from the reactions in the Sun’s core.
6) Does the Sun have a solid surface? No. When we see the Sun, we are seeing the
photosphere. It is a gaseous layer that emits a lot of light—it looks like a solid surface
because it emits light and we can’t see through it.
7) Since the Sun is so bright, how is it possible to see its dim outer atmosphere? Usually
we see the outer layers of the Sun’s atmosphere (the chromosphere and the corona) when
there is a total eclipse.
8) Where does the Solar wind come from? The Solar wind comes from the corona (the
top-most layer of the Sun’s atmosphere).
9) What are sunspots? Sunspots are regions in the Sun’s photosphere which are cooler
than the regions around them. Why do they appear dark? They appear dark because they
do not emit as much light as the regions around them do.
10) Why is it impossible for the burning of substances like coal to be the source of the
Sun’s energy? A piece of coal the size of the Sun would only burn for 10,000 years or
so. This is much smaller than the age of the solar system.
11) What is Hydrogen Fusion? Hydrogen fusion is when the nuclei of two hydrogen
atoms are forced together to form a helium atom. Why is hydrogen fusion fundamentally
unlike the burning of a log in a fireplace? The burning of a log is a chemical reaction that
does not change the atomic nucleus. Atomic fusion is a nuclear reaction which changes
one element into another (one type of atom is changed into another type of atom).
12) Why do thermonuclear reactions occur only in the Sun’s core and not in its outer
regions? In order for nuclear reactions to occur enormous pressures and temperatures
must exist. This is only possible in the Sun’s core (or in the core of other stars).
13) What is a neutrino? A neutrino is a small particle that is a by-product of nuclear
fusion. Why is it useful to study neutrinos coming from the Sun? Because they confirm
that the Sun is powered by thermonuclear fusion, and they can indicate how much fusion
is occurring.
14) Briefly describe the three layers that make up the Sun’s atmosphere.
In what ways do they differ from one another?
a) The photosphere is the lowest layer of the Sun’s atmosphere. It is the most luminous
of the three. It is the lowest temperature of the three. It is the bright yellow disk that we
see in pictures of the Sun.
b) The chromosphere is the layer above the photosphere. It is 1/10,000 the density of the
photosphere. It gives off very little light (pinkish red light) which is usually only visible
during a total eclipse. The temperature is about 50,000F at the top of the chromosphere.
c) The Corona is the outermost layer of the Sun’s atmosphere. The density of the corona
is even less than that of the chromosphere. The corona is the hottest layer of the Sun’s
atmosphere. The temperature is about 4 million degrees at the top of the corona, but the
density is so thin that you wouldn’t feel the heat if you put your hand in it.
15) About how long is the sunspot cycle? 11 years.
16) What is the cause of sunspots? As the Sun undergoes differential rotation, the
magnetic field gets wound around the Sun and sunspots form in regions where the kinks
form in the field.
KeyWords: Corona, hydrogen fusion, luminosity, neutrino, photosphere, radiation, solar
flare, solar wind, sunspot, sunspot cycle, sunspot maximum, thermonuclear fusion.