Worlds of David Darling
> Children's Encyclopedia of Science > The
Sun > Chapter 3
THE SUN: Our Neighborhood Star
a book in the Discovering Our Universe series by David Darling
Of course, we can't
actually see inside the Sun. No real spacecraft has ever gone there, for
it would be melted on approach by the tremendous heat. How, then, are we
able to learn so much about our neighborhood star?
One way is by breaking the "secret code" contained in sunlight. Amazingly
enough, even the tiniest sunbeam carries with it a great deal of information
about the place in the Sun where it came from.
Secrets of the Spectrum
If we pass sunlight through a glass triangle – a prism – we
find that the light is made, not just of one color, but of many. What looks
like white sunlight is really a mixture of all the colors of the rainbow.
The split-up sunlight forms a spectrum of colors stretching from red through
orange, yellow, green, blue, indigo, to violet.
Using a special instrument, called a spectroscope, scientists can get a
much clearer look at the spectrum of sunlight. In this way they can uncover
something even more surprising about it. Not only is the Sun's spectrum
made of many colors, but it is also crossed by hundreds and hundreds of
dark, narrow lines. These are called Fraunhofer lines in honor of one of
the first scientists who noticed them.
The Fraunhofer lines carry the secret code in sunlight. They are made by
substances high up in the Sun that absorb light coming from deeper down.
By studying these lines, scientists have learned what the outside of the
Sun is made of and how hot and dense it is.
The Sun Seen from Earth
Another way we can learn about the Sun is simply by looking at it. The problem
is how to do this safely. If we gazed straight at the Sun or tried to view
it through an ordinary telescope, it would be so bright that it would blind
To view the Sun safely, scientists use special solar telescopes These have
mirrors that track the Sun across the sky, collect its light, and form an
image, or picture, of its brilliant disk on a large, flat viewing table.
The bright surface of the Sun is known as the photosphere. It is a bubbling,
boiling, bursting brew of hot gas. Seen through a solar telescope, the Sun's
surface is always shifting, always alive with movement.
There are, for instance, tiny freckles that cover the entire surface of
the Sun. Scientists call these granules and believe they are caused by hot
gases that well up from deeper in the Sun, cool, and then fall back down
again. The pattern of granules presents a dotlike, ever-changing picture.
| A group of sunspots. Each spot
is darker in the middle than at the edge
Sunspots are much larger and darker spots on the face of the Sun. They may
last a week or more and may be big enough to swallow several Earths. A sunspot
is an area of the Sun's surface that has been made a little cooler than
its surroundings. Because the center of the sunspot is cooler than the rest
of the Sun's surface, it looks dark. But a sunspot is still quite hot –
around 7,000°F (4,000°C) – and, by itself, a large one would
look as bright as the full Moon!
Sunspots are formed when the Sun's magnetic field bursts up through the
photosphere. Since the magnetic field then usually loops back around into
the surface, a second sunspot may be formed. These loops are the reason
sunspots often occur in pairs.
Sometimes there may be no sunspots for several weeks. At other times the
Sun's face seems peppered with them. Their position changes daily as the
Sun slowly turns on its axis. Over a period of years, the average number
of sunspots gradually goes up, hits a peak, and then gradually falls off
again. The sunspot peaks happen about every eleven years, and the whole
pattern of their change is known as the sunspot cycle.
The Sun eclipsed
The bright surface of the Sun that we see isn't where the Sun ends and outer
space begins. The Sun also has a huge atmosphere – a layer of gases
outside the photosphere – that we can't usually see.
Only at a very special time can we get a good view of the Sun's much dimmer
atmosphere. It is during a total eclipse of the Sun.
You may have noticed that the Moon and the Sun seem to be about the size
in the sky. (In fact, the Moon is really much smaller and closer to us.)
Once in a while, the Moon passes exactly between the Sun and the Earth.
Then, from some parts of the Earth, the Sun's bright disk is completely
blotted out by the Moon.
| A total solar eclipse with the
corona in view
During a few minutes of total eclipse, we get a thrilling glimpse of what
is going on above the surface of the Sun. For a short time, just at the
start and at the end of the total eclipse, we see a thin, pinkish, curved
ring around the edge of the Sun. This is the chromosphere - the bottom part
of the Sun's atmosphere. The chromosphere is like a bubbling foam of gases,
a few thousand miles deep, thrown up by the "sea" of the photosphere.
During the middle of the eclipse, we see the beautiful, glowing corona –
the top part of the Sun's atmosphere. Although the corona is made of only
very thin, very hot gases, it is huge. It stretches more than a million
miles out from the Sun in every direction!
Total eclipses also provide us with marvelous views of prominences. These
are great clouds of slightly thicker gas that form in the corona and then
rain down onto the Sun's surface. They look like giant flaming tongues.
Another, rarer type of prominence – the eruptive prominence –
is caused by gas bursting out of the Sun's surface. In fact, it is just
one of the ways in which the Sun can shoot matter into space.
From Sun to Earth
A strange kind of wind – the solar wind – carries about one
million tons of the Sun into space every second! Some of the tiny, fast-moving
particles in the solar wind reach Earth and get trapped in the magnetic
field that stretches all the way around our planet. A small portion of the
solar wind particles manage to leak out at the poles. They rise into the
air and cause a wonderful shimmering glow in the sky. This is how aurorae
The solar wind blows especially hard following a solar flare – an
explosion of the Sun's surface that sends a sudden burst of particles into
space. Solar flares cause magnetic storms around the Earth. For a while,
the aurorae become brighter, and radios and compasses behave in an odd way.
Scientists know of other things, too, that the Sun gives off. Radio waves
and X-rays reach the Earth from the Sun's surface. And huge armies of strange,
ghostly particles, called neutrinos, march out to meet us from the Sun's
By building special instruments on Earth, and by launching others on satellites,
we can study what the "radio Sun," the "X-ray Sun," and even the "neutrino
Sun" look like. In this way, we can gain a better understanding of how our
neighborhood star works.