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AST-103L
Spring 2001: The Sun
Our Sun is a middle-aged, medium sized star, big enough to hold a million Earths. The ancient
Greeks thought that the Sun was a perfect sphere of fire. Today we know that the Sun is a
variable star that produces life giving light and heat as well as harmful radiation. It causes space
weather that can harm astronauts working in space and can interfere with satellites orbiting our
planet and communications.
In this exercise, you will learn the basic features of the Sun’s surface, atmosphere, and
interior, and begin to understand what causes some of these features. You will be introduced to
sunspots and the solar cycle and calculate the rotation of the Sun.
Objectives:
! Identify basic solar features and read about what causes some of them
! Understand what sunspots are, what the solar cycle is, and how the Sun moves
Equipment:
! This write-up, calculator, lab notebook
! Data you downloaded from the SOHO website
Figure 1. Artist’s rendition of solar structure.
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AST-103L
Spring 2001: The Sun
I. FEATURES OF THE SUN’S SURFACE AND ATMOSPHERE
Although the average distance from Earth to the Sun is a whopping 149,600,000 kilometers
(93,000,000 miles), careful observation from Earth reveals a surprisingly large number of
different visible features. Figure 1 is an artist’s schematic of the Sun. The most obvious and best
known feature is the sunspot. Typically moving in groups, these dark, planet-sized features have
been known to humankind for centuries. As sunspots form and disappear over periods of days or
weeks, they also appear to move across the Sun’s surface because the Sun rotates. Caused by
strong magnetic fields, sunspots are shaped much like a horseshoe magnet that rises from below
the Sun’s surface. The rising hot gas is trapped by the sunspots’ intense magnetic field, which
cools the sunspots from 6000°C to about 4200°C. The cool area appears dark compared to the
area around it. Thus, from Earth, we see spots on the Sun (Figure 2). In some photographs, we
can also see light colored areas around groups of sunspots that resemble tufts of cotton candy.
We call these fluffy looking fringes, plages (Figure 3).
Figure 2. Visible Sun. Visible or white-light images of
the Sun show the photosphere and sunspots. Sunspots
are regions of intense magnetic fields. They appear
dark because they are somewhat cooler than the
surrounding gas.
Figure 3. H-alpha Sun. Light from hydrogen gas just
above the photosphere shows the bright plages that
surround the active sunspot regions. Dar kstrong-like
filaments are also seen that are in fact prominences
seen head-on. The bright structures on the limb are
prominences.
Sunspots are the sources of massive releases of energy (called coronal mass ejections) as well
as solar flares (Figure 4), the most violent events in the solar system. In a matter of minutes to
several hours, a solar flare releases about 10,000 times the annual energy consumption of the
United States. Solar flares and coronal mass ejections give off radiation that includes X-rays and
ultraviolet rays, and charged particles called protons and electrons. This sudden surge in
radiation has dramatic effects on Earth. For example, in 1989, the Quebec province in Canada
suffered a blackout because many transformers were destroyed by a large magnetic storm near
Earth. The magnetic storm, caused by solar activity including solar flares, resulted in millions of
dollars in damages.
One of the most spectacular features of the Sun are solar prominences (Figure 5). They
appear to stream, loop, and arch away from the Sun. The most recognizable prominences appear
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Spring 2001: The Sun
as huge arching columns of
gas above the limb (edge) of
the Sun. However, when
prominences are photographed on the surface of the
Sun, they appear as long,
dark, threadlike objects and
are called filaments. Like
sunspots, prominences are
cooler (about 10,000°C) in
relation to the much hotter
background of the Sun’s
outer atmosphere (about
1,500,000°C). While solar
flares may last hours,
prominences can remain for
days or even weeks.
If
you
have
seen
photographs of a solar
eclipse, then you have
probably noticed a bright
halo around the Sun, called
the corona (Figure 6). The
Figure 4. Solar Flare. A solar flare is a highly concentrated explosive release Sun’s corona changes with
of energy within the solar atmosphere that occurs near sunspots. Flares emit sunspot activity. When there
enormous amounts of energy.
are more sunspots, the
corona appears to be held closely to the Sun; when there are fewer sunspots, the corona streams
out into space in a shape that resembles the spike on a warlike, peaked helmet. Sometimes parts
of the corona appear to be missing. Logically, we call this a coronal hole. Scientists believe that
the solar wind, a million mile per hour gale that blows away from the Sun, originates in coronal
holes. Unlike wind on Earth, the solar wind is a stream of ionized (electrically charged) particles
speeding away from the Sun (Figure 7).
Figure 5. Extreme Ultraviolet. This extreme
ultraviolet image of the Sun shows the
chromosphere, which is a thin region of the
Sun just above the photosphere. A giant
prominence can also be seen.
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Spring 2001: The Sun
Figure 6. Solar Eclipse. A solar eclipse shows
the ethereal corona – the crown of light that
surrounds the Sun. Prominences can also be
seen as small white blotches in the
photograph.
Figure 7. Artist’s rendition of the solar wind
bombarding Earth’s magnetic field with
supercharged particles.
Just as the Sun
disappears behind the
Moon during a total solar
eclipse (Figure 6), a flash
of bright red light appears.
This colorful layer of the
Sun,
called
the
chromosphere, becomes
visible for a brief instant.
Although we know little
about the chromosphere,
there
are
curious,
permanent features of the
chromosphere,
called
spicules, that we can study
in more detail. There are
so many of these fine,
bright, hairlike features,
that they are always visible near the Sun’s edge, even though an individual spicule lasts only
minutes. Like sunspots, spicules rise vertically above the Sun’s surface.
Also visible for only minutes, are granulations in the Sun’s photosphere. Granulations are
rising and falling columns of hot gases that look like fluffy marshmallows arranged in a
honeycomb pattern. The tops of these granules form the Sun’s “surface.” Although we refer to
the Sun’s “surface” as the photosphere, you probably know that the Sun has no solid surface,
unlike Earth. It is an uneven sphere of hot, glowing gas.
II. THE PHOTOSPHERE
The photosphere is the visible surface of the Sun that we are most familiar with. Since the Sun is
a ball of gas, this is not a solid surface but is actually a layer about 100 km thick (very, very, thin
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Spring 2001: The Sun
compared to the 700,000 km radius of the Sun). When we
look at the center of the disk of the Sun we look straight
in and see somewhat hotter and brighter regions. When
we look at the limb, or edge, of the solar disk we see light
that has taken a slanting path through this layer and we
only see through the upper, cooler and dimmer regions.
This explains the limb darkening that appears as a
darkening of the solar disk near the limb (Figure 8).
A number of features can be observed in the
photosphere with a simple telescope (along with a good
filter to reduce the intensity of sunlight to safely
observable levels). These features include the dark
sunspots, the bright faculae, and granules. We can also
measure the flow of material in the photosphere using the
Doppler effect. These measurements reveal additional Figure 8. Example of limb darkening.
features such as supergranules as well as large-scale flows
and a pattern of waves and oscillations.
The Sun rotates on its axis once in about 27 days. This rotation was first detected by
observing the motion of sunspots in the photosphere. The Sun’s rotation axis is tilted by about
7.25 degrees from the axis of the Earth’s orbit so we see more of the Sun’s north pole in
September of each year and more of its south pole in March.
Since the Sun is a ball of gas it does not have to rotate rigidly like the solid planets and
moons do. In fact, the Sun’s equatorial regions rotate faster (taking about 24 days) than the polar
regions (which rotate once in more than 30 days). The source of this “differential rotation” is an
area of current research in solar astronomy.
III. FEATURES OF THE SUN’S INTERIOR
We won’t study about the Sun’s interior here. We just want you to be familiar with the three
basic components of the solar interior (see Figure 1).
The core is the central part of the Sun where hydrogen fuses into helium to give off energy.
This is effectively a nuclear power plant. Energy from the Sun’s core then travels outward
through what we call the radiation zone. In the convection zone, the hotter gases rise and fall as
they are heated from the radiation zone below much like a boiling pot of water.
IV. WHAT IS THE “SOLAR CYCLE”?
Every 11 years the Sun undergoes a period of activity
called the “solar maximum,” followed by a period of
quiet called the “solar minimum.” During the solar
maximum there are many sunspots, solar flares, and
coronal mass ejections, all of which can affect
communications and weather here on Earth. One way we
track solar activity is by observing sunspots. Sunspots are
relatively cool, dark areas that appear as dark blemishes
on the face of the Sun and usually travel in pairs. The
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Figure 8. Sunspot Numbers.
AST-103L
Spring 2001: The Sun
innermost and darker center is call the umbra whereas the lighter shadow around the edges is
called en penumbra of a sunspot. Current theory says that sunspots are caused by horseshoe
shaped magnetic fields buried just below the Sun’s surface that prevent the flow of hot gases
from below. Like a magnet, sunspots have both magnetic north and south poles. The spot where
the magnetic field is located cools slightly compared to its surroundings. Because the spot is
cooler, it looks darker. The twisted magnetic fields above sunspots are sites where solar flares
are observed to occur, and we are now beginning to understand the connection between solar
flares and sunspots.
During solar maximum there are many sunspots, and during solar minimum there are few.
Figure 8 shows the number of sunspots observed during the last two solar cycles. We are just
now coming down from the last solar maximum (fall 2000 – early 2001).
V. SOLAR MAGNETIC FIELDS
Magnetism is the key to understanding the Sun. A magnetic field is produced on the Sun by the
flow of electrically charged ions and electrons. Sunspots are places where very intense magnetic
lines of force break through the Sun’s surface. The sunspot cycle results from the recycling of
magnetic fields by the flow of material in the interior. The prominences seen floating above the
surface of the Sun are supported, and threaded through, with magnetic fields. The streamers and
loops seen in the corona are shaped by magnetic fields. Magnetic fields are at the root of
virtually all of the features we see on and above the Sun. Without magnetic fields the Sun would
be a rather boring star.
VI. PREDICTING SPACE WEATHER
A better understanding of the Sun’s magnetic field and its behavior will allow us to make better
predictions of space weather. Observations of magnetic fields associated with solar flares show
that flares are likely to occur when the magnetic field lines linking two sunspots become sheared
or twisted. Observations of the Sun’s magnetic field over the last 20 years illustrate its behavior
over two sunspot cycles (Figure 8). However, predicting long-range behavior, such as the size of
the sunspot cycle, is still based on observing trends and patterns. We hope that in the near future
we will understand the Sun well enough to make these predictions based on current conditions
and past history using a mathematical model of the actual processes.
ANSWER THE FOLLOWING QUESTIONS IN YOUR LAB NOTEBOOK.
1. Why do we regard the photosphere as the “surface” of the sun?
2. Explain limb darkening.
3. What is the difference between space weather and weather here on Earth?
4. What is a sunspot and why do they occur?
5. Using Figure 9, what is the average time between solar maximum and solar minimum?
6. Predict the years for the next two sunspot maxima and minima (cycles 24 and 25).
7. Is the solar cycle a constant or periodic function? How do you know?
8. Why is it important to collect and study data over a long period of time before drawing
conclusions? Give an example other than sunspot data.
9. Why is sunspot variation important to us?
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Spring 2001: The Sun
Figure 9. Sunspots since circa 1750.
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AST-103L
VII.
Spring 2001: The Sun
THE EXERCISE: ROTATION OF THE SUN
We know from personal experience that Earth rotates because we experience day and night.
What about our Sun, that sphere of glowing gas rising just beyond the eastern horizon? Does it
rotate? To find out, we can use one of the Sun’s most prominent features, its sunspots.
Sunspots occur in 11 year cycles where the number of visible spots increases from a
minimum of almost zero to a maximum of over 100 spots. After one 11 year cycle, the poles of
the horseshoe shaped magnetic fields switch and another cycle begins. Consequently, we have an
overall 22 year magnetic cycle as well as an 11 year sunspot cycle. At the beginning of an 11
year cycle, several sunspots appear at about 30° north and south latitudes. As the cycle
progresses, you can track the sunspots’ motion. Generally, individual sunspots or groups of
sunspots do you shift significantly in latitude over time; therefore, we conclude that any change
in longitude is due to the movement of the Sun itself. The Sun must be rotating.
Gather your solar images that you downloaded from the SOHO website. You will be
measuring the positions of sunspots from the images and keeping track of this information over
the time covered by the dataset. Your instructor will show you how to make these measurements
and may provide templates to speed up the process. You may find it helpful to first determine a
“drift line” for a particular sunspot or group of sunspots. You should determine the number of
degrees in longitude that the sunspots travel per day. Use this information to calculate the
average rotational period of the Sun in days. Record your measurements and calculations in a
coherent fashion (tables are nice!!) in your lab notebook. Do not forget the units.
ANSWER THE FOLLOWING QUESTIONS IN YOUR LAB NOTEBOOK.
10. Since the sunspot group appears to move across the Sun’s surface, what kind of solar
motion does this suggest (i.e. orbital, rotation, radial, etc.)?
11. Based on your calculations, what is the average rotational period of the Sun? Do you
find any evidence that different parts of the Sun rotate at different rates? Justify your
answer.
12. What is your calculated tilt of the Sun’s equatorial axis?
13. How do your answers to Questions 11 and 12 compare to the known rotation period
and tilt angle of the Sun?
14. If the sunspot group had changed latitude, would you have been able to calculate the
number of degrees of change per day as easily? Why or why not?
15. At which solar latitudes do most of the spots occur?
16. Were all the same sunspots detected in all the observations? Why or why not?
17. Do you believe the Sun is a static, solid body? Why or why not?
18. Explain how to use Jupiter’s Great Red Spot to determine its period of rotation.
VIII.
REFERENCES
Duckett, K.E. & Moore, M.A. A Laboratory Textbook for Introductory Astronomy, 2nd Edition,
Contemporary Publishing Company, 1986
Hemenway, M.K. & Robbins, R.R. Modern Astronomy: An Activities Approach, Revised
Edition, University of Texas Press, 1991.
Space Science Institute. Solarscapes: Sunspots and Radiation, 1999.
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