SPA Solar Section
Introduction to Solar Observing
by John Chapman-Smith
CHAPTER ONE: THE SUN
The Sun is something that we all take for granted, and we barely notice its movement
during the day. It rises in the East and sets in the West. It is high in the daytime
sky during summer yet quite low during the winter. Our Sun is important to us
here on Earth, as it gives us the light and heat necessary for life to exist on
Earth. But what is this thing we call the Sun?
The Sun is a star, a gigantic ball of glowing gas in the centre of our system
of planets called the Solar System. This body, from Earth, lies at a mean distance
of 93,000,000 miles from us, and it also appears as a large disc in the sky,
which means that because it is lying at such a great distance, it must be very
big indeed! Many theories abound about its creation, but one of the most favoured
is the one that says that the Sun (and the planets) condensed out of a cloud
of gas and dust, around 5000 million years ago. The nebula split and formed
globules of material. These globules, under the influence of gravity, go smaller
and smaller, heating up the interior of the largest globule, which was to be
the Sun, due to compression. As this gas compressed, the temperature climbed
higher and higher. The Proto-Sun was shining, but only feebly at this time.
But a stage came in which the centre of the Sun reached the temperature to start
nuclear reactions, changing a vast amount of hydrogen gas into another element,
Helium, in a very complex way. This reaction, or changing of one substance to
another, is what keeps the Sun shining. The Sun is practically all hydrogen,
with a small amount of helium. The hydrogen will run out one day, but not for
many millions of years.
Despite this, the Sun is using a fantastic amount of hydrogen gas every second,
about 6,000,000 tonnes, but it is estimated that the amount of hydrogen left
would last around another 5000 million years. The Sun is a very stable star,
although it is slightly variable in nature, and we should be very thankful,
for even a small change in the output of energy of the Sun would have grave
consequences for life on Earth.
The Sun is large, with a diameter of 886,000 miles at the equator, and it
would take 1,303,600 Earths to equal its volume. It travels around the Milky
Way Galaxy taking about 225,000,000 years to make one revolution. Other information
is as follows:-
| Density (Water = 1) |
1.409
|
| Mass (Earth = 1) |
333,000
|
| Mass |
2 x 1027 tonnes (99% of the Solar System)
|
| Escape Velocity |
617 Km/Sec
|
| Surface Gravity (Earth=1) |
28
|
| Mean Apparent Magnitude |
-26
|
| Absolute Magnitude |
+4 (If Sun was placed at a distance of 32.6 light years or 10 parsecs
from Earth)
|
| Spectrum Class |
G2V
|
| Surface Temperature |
6000º Centigrade
|
| Core Temperature |
15,000,000º Centigrade
|
| Rotation period (Siderial) |
25.38 days
|
| Rotation period (Synodic) |
27.27 days
|
| Light Distance |
8.3 minutes
|
| Make-up |
78% Hydrogen - 20% Helium - 2% other materials
|
There is much about the Sun that we do not yet know, and observation of this
body helps to find out more. Observations of the Sun started a long time ago,
because the Chinese, who were well advanced, noticed that on some occasions
the Sun had black spots on its face. They did not know what they were, and judged
them to be 'birds crossing the face of the Sun' or gave other explanations.
The Chinese did not have any optical instruments except their eyes, but they
could see the Sun when it was low in the sky and clouds cut down the light.
(Please DO NOT follow this example.)
The real revelation came from the telescope,
and the astronomer Galileo Galilei used it not just on the Sun, but used it
to discover a host of other objects. He observed Sunspots, the name for the
dark areas that crossed the face. His observations proved that they were part
of that body. Sunspots come in many shapes and forms, and consist of a dark
central region called the Umbra, and is surrounded by a lighter region known
as the Penumbra. The spots seem to change from day to day, even from hour to
hour, and vary greatly in size, from small dot like spots called Pores to large
spots covering a vast area of the solar surface (the very large spots are the
ones that can be see with the eye alone, and are called Naked Eye Spots. The
spots appear to be cooler regions, with the umbra having a temperature of about
4,000º, the penumbra being around a thousand degrees higher.
The surface of the Sun, as we see it, is called the Photosphere, meaning 'sphere
of light', is the cooler outer reaches of the Sun, the temperature of which
is about 6000º. It must be said that there is no 'real' surface, as the Sun
is a gaseous body that gets less dense the farther you travel from the centre,
or Core. The sunspots appear darker by contrast, which means that the dark spots
would shine out if the Sun's surface were dark
The interesting thing about the surface of the Sun is Granulation, a mottling
effect due to vast bubbles of gas rising and sinking. These bubbles vary in
size (about the size of the British Isles). There are fantastic magnetic forces
in the Sun and it is these which are the cause of the spots. The Sun rotates
on its axis, but as it is gaseous the equator travels more quickly than the
more northerly or southerly regions, and this can be shown by observing sunspot
activity. Sunspots travel from east to west, taking about 14 days to span the
diameter at the equator, and observation of the movement of the spots allows
us to deduce other information.
In early June and December the spots travel in straight lines across the disc,
indicating that the Earth lies in the plane of the Sun's equator, while from
January to May the spots follow a more curved path to the North, indicating
that the Sun's south pole is tipped towards us, and from July to November the
reverse happens and the spots curve to the South. In fact, at certain times
of the year, the Sun is acutely tilted east or west, due to a combination of
the tilt of the Sun (7.25º) and the tilt of the Earth (23.5º), giving a tilt
of a little over 26º.
Sunspots normally start as pores, then grow, or develop. They may grow to
large complex groups or smaller, singular spots, which after a time get smaller,
or decay, becoming a pore again then disappearing altogether. Only by watching
a particular group over a time can you observe this.
A notable point here is that if you look at the Sun (through a suitable filter)
with the naked eye, you can see that the Sun appears equally bright all over.
However, looking through the telescope shows the solar surface with a bright
centre and less bright to the extreme edges. This is limb Darkening, caused
by the fact you are looking at a sphere, with light travelling from the edges
having to travel a good deal farther through the solar atmosphere, and this
is attenuated somewhat. But this phenomena helps us see another item, Faculae.
Faculae are bright areas on the photosphere, noticable mainly at the edges of
the disc, in the limb darkened regions. The name comes from the Latin for 'little
torches.'
It is noteworthy that the Sun's disc has an angular diameter of about 1/2º.
Because the Earth's orbit is slightly elliptical, this value changes minutely
over the year. The Earth's orbits comes to about 92,000,000 miles from the Sun
in winter (Perihelion) and 94,000,000 miles in the summer (Aphelion). The Moon,
our natural satellite, is also around 1/2º in angular diameter, and on occasions
it can pass in front of the Sun, blocking out the photosphere. This is a Total
Solar Eclipse, although partial ones occur as well. During a total eclipse,
with the photosphere hidden, we are able to see the wonderful outer atmosphere
of the Sun, the Corona. This region has a very high temperature, at around 2,000,000º
(but it is so tenuous that there is very little 'heat'). Another feature that
can be seen is the Solar Prominence, large glowing masses of hydrogen gas coming
from the Chromosphere, a lower region of the solar atmosphere, but above that
of the photosphere. This will be explained more in a later chapter. Prominences
can be small or extremely large affairs, some fairly quiet and some extremely
violent, reaching very high speeds and travelling great distances. These can
be seen using special equipment and this will also be explained later.
During the nineteenth century, A German amateur astronomer, called Heinrich
Schwabe, observed the Sun for around 20 years. When he published these observations,
he suggested that the solar activity varied. The sunspots came and went in periods
of 7 to 17 years, the average being 11 years. This was the discovery of the
Sunspot Cycle. According to sources, Schwabe was trying to find a new planet
within the orbit of Mercury. He did not find it, but the discovery of the solar
cycle was just as remarkable! After hearing about Schwabe, Rudolf Wolf, the
director of Bern Observatory, became interested and after reading Schwabe's
works, and doing further study, made an arbitrary counting system, called the
Universal Sunspot Numbers, which evolved to Relative Sunspot Numbers.
Another amateur astronomer, an English one this time, called Richard Carrington,
studied the work of Schwabe and decided that there was evidence to suggest that
the sunspots at the start of a cycle appear at the high latitudes and then travel
slowly down until they are equatorial as the cycle progresses. Another interesting
fact that was discovered by Carrington was that the sunspots at the centre of
the disc travel faster than those nearer the poles, giving further evidence
that the Sun has a gaseous make-up. A further item on the Sun, only rarely seen,
are Solar Flares. These are violent eruptions on the Sun. They can be seen more
readily by observers with special instruments that are the subject of a later
chapter.
We see the Sun in white light, which is a mixture of colours from red to violet,
and by splitting up the Sun's light using an instrument called a spectroscope,
we can see all of these colours. A German physicist, called Fraunhofer, split
up the Sun's light and noticed black lines within the rainbow band (or Spectrum).
These dark lines corresponded to various chemical elements in the Sun. The Sun's
colour is yellow, and it is a dwarf star, which means it is small compared to
some other stars. The Sun is also our yardstick to the sizes of the stars in
the sky, some of which are much larger and some much smaller.
Observation of the Sun is really about counting the amount of sunspots in
an observation, putting this information on specially prepared forms and then
sending them to the Director. This allows the Director to determined the state
of activity and report finding to the rest of the members of the section, and
the Society.
THE PRIMARY FUNCTION OF THE SOLAR SECTION
The section deals with the observer so he or she can learn how to observe the
Sun safely and accurately. The member has a choice in how he or she observes:-
- Drawing the features of the whole disc, by using special forms that will
be provided. This method is useful when large sunspot activity occurs.
- Counting the number of activity areas, using another form that will also
be provided. This form will be used to assess the amount of activity at any
given time.
All reports should be sent to the Director at the end of each calendar month.
The information is used to construct a graph that will display information to
the casual user on the state of the Sun's activity. Our present graph spans
years of work sent in by section members.
If you are new to observing the Sun and a newcomer to the Section, do not
worry about getting it right first time. It takes time, practice and patience
to get good results.
Chapter Two: What Telescope to Use
SPA Solar Section
