An Introduction to Astronomy
Part IX: The Sun, Our Star
Lambert E. Murray, Ph.D.
Professor of Physics
Our Sun
The Sun is a star – and as such, enables us to study
stars up close (at least within 1 A.U.) Thus,
understanding the Sun is the key to understanding
the stars!
 Our Sun is the principle energy source for our
solar system. It supplies the energy on the Earth
for our weather and for life itself. It also supplies
the high-energy radiation which causes the aurora.

Size of the Sun
The Sun is roughly 100 times the diameter of the
Earth – more than one million Earths could be
dropped into the Sun, with room left over.
 The density of the Sun is actually less than the
Earth, about 1.4 g/cc, but
 The mass of the Sun is about 700 times greater
than the mass of all the rest of the solar system
combined – about 333,000 times the Earth’s mass.
 This mass basically controls the motion of all the
other objects in the solar system.

The Sun’s Atmosphere – That
Part Visible to Us
 The
Sun’s atmosphere is divided into three
separate parts:
– Photosphere – the visible surface of the Sun.
– Chromosphere – a pinkish surface layer of the
Sun’s atmosphere visible only during a total
eclipse.
– Corona – the Sun’s outer atmosphere which
extends great distances away from the Sun’s
surface. This part of the Sun’s atmosphere is
also only visible during a total eclipse.
The Photosphere
 This
is the lowest of the three layers of the
Sun’s atmosphere; it is the layer that
determines the color of the Sun.
 The limb of the Sun is the apparent edge of
the Sun. (This term is applied to the edge of
any object in space.)
 A close-up of this surface shows features
called granules.
Granulation Patterns on the Sun’s Surface
Granulation
The granulation patterns on the surface of the Sun
are dynamic, changing in time like the boiling of
water in a pan.
 They are produced by convections currents in the
Sun’s interior: heated areas rising from below the
surface.

– Doppler measurements confirm that the center of the
granules are rising, while the edges are falling.
– Each granule is about 1000 km across, or about the size
of Texas.
Changing Granulation Patterns on the Sun:
(The images were taken in 2 minutes intervals.)
Supergranulation
 In
addition to the smaller granules on the
Sun’s surface, there appear to be large areas
of the Sun’s surface (about the size of the
Earth) which rise and fall together. These
are called “supergranules”.
Convection in the surface layer of the Sun
The Chromosphere
The Chromosphere
 The
Chromosphere is visible only during a
total eclipse of the Sun.
 A close examination of the chromosphere at
the limb of the Sun reveals “grass-like”
features called spicules.
 These spicules appear to encircle regions of
supergranulation.
The Limb
Spicules Outlining Supergranules
The Corona
The Corona
Visible only during a total eclipse, the corona is
the nearly transparent outer atmosphere of the
Sun.
 It is composed of tenuous gases at extremely high
temperatures (1-2 million Kelvin) – much hotter
than the surface of the Sun.
 The corona may extend millions of miles out into
space.

Choronographs
For years, the only time scientists could study the
chromosphere or the corona was during a total
eclipse.
 In recent years, the coronagraph has been
developed. This instrument blocks out the disk of
the Sun to allow the outer edge (the
chromosphere) and the corona to be studied
 The next several images were taken with a
choronograph.

The Dynamic Chorona
 From
these previous pictures, it should be
clear that the corona is dynamic.
 Coronal activity appears to be linked with
the appearance of Sunspots.
 The next image shows variations in coronal
activity as correlated with sunspot activity.
The Solar Corona During
Solar Maxima
Solar Minima
Model of Sun’s Outer Atmosphere
What causes the large temperature increase in the corona is
not completely understood at present. Some type of
magnetic disturbance is the most likely explanation.
The Corona and the Solar Wind
The Corona appears to be a continuous stream of
particles being release from the Sun’s surface.
 This “Solar Wind” is made up mostly of highenergy electrons and protons.
 Millions of tons of matter each second is being
spewed into interplanetary space.
 This Solar Wind is also very dynamic and
variable.

Sunspots and Solar Activity
 During
certain times the solar disk is
completely devoid of apparent activity –
this is the time of the quiet sun.
 At other times, there appear to be magnetic
“storms” that move across the Sun’s
surface.
 The severity and number of these storms
varies greatly from time to time.
The Solar Disk
Active
Quiet
The Solar Cycle
Sunspot activity has been monitored for several
centuries.
 The number of sunspots on the Sun’s surface
appears to go through a cycle that has a period of
approximately 11 years.
 The Sunspot cycle is plotted in the next slide over
a period of about 100 years and shows not only the
number of sunspots, but their locations on the
solar disk in the form of “butterfly” diagrams.

Long-Period Solar Cycles
• There appears to be another solar cycle superimposed
upon the 11-year cycle, as seen on the next slide –
notice the minimum every 100 years.
• The period from 1645 - 1715 when there were almost
no visible sunspots and a dearth of any solar activity is
known as the “Maunder minimum”.
• There is some evidence that there were very abnormal
weather patterns during this time period – indicating a
possible link between solar activity and the weather.
The Maunder Minimum
SunSpot
Record
Sunspots: A Closer Look
Umbra
Penumbra
Sunspots look like dark blemishes on the solar disk. These areas
are not really dark, however, they are just not as bright as the
surrounding areas.
Groups of Sunspots
often appear in groups – quite
often in pairs.
 These sunspot pairs appear to move on the
Sun’s surface as the Sun rotates – gradually
moving from higher latitudes toward the
Solar equator.
 Sunspots
Source of Sunspots
 In
1908 George Hale discovered that
sunspots were associated with intense
magnetic fields – thousands of times larger
than the average solar magnetic field.
 These magnetic fields can be measured
using the Zeeman effect – where spectral
lines are split proportional to the strength of
the magnetic field.
Zeeman Splitting at a Sunspot
Magnetic Polarization Studies
The Zeeman effect also causes the light from these
strong magnetic field regions to be polarized.
 The polarization direction is associated with the
direction of the magnetic fields.
 A magnetogram, a photograph based upon this
polarization effect, is shown on the next page:

– On this image one polarity is yellow, while the other is
purple.
– The sunspots appear to occur in matching pairs of
opposite polarity
– The polarity is reversed in opposite hemispheres and
remains the same over an 11-year cycle, then reverses!
A Magnetogram of the Sun
The 22-year Solar Cycle
 Since
the directions of the magnetic fields
in the northern and southern hemisphere of
the Sun reverse every 11-years, it appears
that the 11-year sunspot cycle is actually a
22-year cycle.
 The cycle of sunspot activity appears to be
associated with a twisting of the Sun’s
internal magnetic field lines.
Plages and Filaments
 Associated
with the Sunspots are other
features on the Solar disk.
 The next slide is an image of the Sun taken
with an H-alpha filter (looking at the
hydrogen emission line). You will see
bright areas called plages, which are closely
associated with sunspot activity, and dark
snake-like features called filaments.
Filaments
and Plages
Filaments and Prominences

The dark filament observed in the last slide is
actually a stream of ionized gas trapped in the
Sun’s magnetic field. These gases have been
cooled, and are thus not as bright as the surface
gases. When observed from above the Sun’s
surface they appear darker than the rest of the
surface. However, when observed from the side
(above the edge of the Sun) they appear quite
bright, and are called prominences.
Filament
Prominence
Solar Flares
 Flares
are the most violent events on the
surface of the Sun.
 They are usually associated with sunspot
groups.
 The material of a flare is heated to
extremely high temperatures.
 Large amounts of high-energy particles and
radiation are emitted into space.
Active SUN 2/6/2000
large flare
Time Sequence of a Flare
Time sequence of eruptive prominence (~1½ hr intervals)
Large Flares can be Deadly
A Flare is a violent eruption from the surface
which is usually over in about 20 minutes.
 The x-rays and ultraviolet rays emitted from the
flare arrive at the earth in about 8 minutes.
 The highest energy particles streaming out from
the Sun can reach the Earth within about 20 to 30
minutes but will reach a peak only after several
hours or perhaps days.
 An astronaut exposed in space to the high-energy
particles from a large solar flare can literally be
cooked.

Protection from Solar Events
 Fortunately,
the Earth is shielded from these
high-energy events by the Earth’s magnetic
field, which diverts the charged particles
toward the poles, and by the atmosphere
which absorbs much of the excess energy.
The High-Energy Particles
Follow the Sun’s Magnetic Field
Other Effects of Solar Flares
 These
high-energy bursts can disrupt radio
communication on Earth
 They increase auroral activity
 They may create power surges in electrical
power grids, burning out circuits.
The Sun’s Interior
Solar Interior
 Current
solar models describe three regions
inside the Sun
– Core - where thermonuclear reactions power
the sun
– Radiative zone - where photons carry energy
away from the core
– Convective zone - where convection of gases
carries energy away from the core
Gravitational Attraction vs.
Radiation Pressure
The material making up the Sun is being pulled
toward the center by gravity.
 Radiation pressure (the outward force of the
photons and other elementary particles) is pushing
the gases outward.
 These two forces are in equilibrium inside the
Sun.

– If the radiation pressure were to decrease, the Sun
would collapse.
– If the radiation pressure were to increase, the Sun
would expand.
Energy Source for Stars
During the last 200 years, several different energy
sources for the Sun have been proposed.
 However, all fail to provide enough energy for the
Sun to have provided light to our solar system for
the past 3-5 billion years except one: nuclear
fusion.
 Gravitational pressure of the Sun’s great mass
causes the core to reach temperatures of 15 million
Kelvins.
 Under these conditions Hydrogen (H) can be fused
together to make a heavier element Helium (He)
liberating neutrinos and energy.

Nuclear Fusion






According to Einstein’s theory of relativity,
E = mc2
This equation indicates the possibility of converting mass
into energy.
When hydrogen is converted into helium, only about 0.7%
of the mass is converted into energy.
However, because c is so large, every gram of matter
converted produces and amount of energy equivalent to
that produced by 300,000 tons of coal.
The Sun must convert 600 million metric tons of hydrogen
into helium every second to maintain its present
luminosity.
However, there is enough hydrogen still in the Sun to
provide energy for at least another 5 billion years!
End of Part IX