An Introduction to Astronomy Part IX: The Sun, Our Star

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An Introduction to AstronomyPart IX The Sun,
Our Star

• Lambert E. Murray, Ph.D.
• Professor of Physics

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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.

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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 Earths mass.
• This mass basically controls the motion of all
  the other objects in the solar system.

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The Suns Atmosphere That Part Visible to Us

• The Suns atmosphere is divided into three
  separate parts
• Photosphere the visible surface of the Sun.
• Chromosphere a pinkish surface layer of the
  Suns atmosphere visible only during a total
  eclipse.
• Corona the Suns outer atmosphere which extends
  great distances away from the Suns surface.
  This part of the Suns atmosphere is also only
  visible during a total eclipse.

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The Photosphere

• This is the lowest of the three layers of the
  Suns 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.

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Granulation Patterns on the Suns Surface
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Granulation Patterns on the Suns Surface
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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
  Suns 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.

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Changing Granulation Patterns on the Sun (The
images were taken in 2 minutes intervals.)
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Supergranulation

• In addition to the smaller granules on the Suns
  surface, there appear to be large areas of the
  Suns surface (about the size of the Earth) which
  rise and fall together. These are called
  supergranules.

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Convection in the surface layer of the Sun
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The Chromosphere
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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.

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The Limb
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Spicules at the Suns Limb
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Spicules Outlining Supergranules
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The Corona
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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.

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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.

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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.

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The Solar Corona DuringSolar Maxima
Solar Minima
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Model of Suns Outer Atmosphere
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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.
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The Corona and the Solar Wind

• The Corona appears to be a continuous stream of
  particles being release from the Suns surface.
• This Solar Wind is made up mostly of
  high-energy 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.

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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 Suns surface.
• The severity and number of these storms varies
  greatly from time to time.

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The Solar DiskActive Quiet
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The Solar Cycle

• Sunspot activity has been monitored for several
  centuries.
• The number of sunspots on the Suns 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.

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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.

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The Maunder Minimum
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SunSpotRecord
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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.
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Groups of Sunspots

• Sunspots often appear in groups quite often in
  pairs.
• These sunspot pairs appear to move on the Suns
  surface as the Sun rotates gradually moving
  from higher latitudes toward the Solar equator.

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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.

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Zeeman Splitting at a Sunspot
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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!

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A Magnetogram of the Sun
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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 Suns internal
  magnetic field lines.

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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.

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Filaments and Plages
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Filaments and Prominences

• The dark filament observed in the last slide is
  actually a stream of ionized gas trapped in the
  Suns magnetic field. These gases have been
  cooled, and are thus not as bright as the surface
  gases. When observed from above the Suns
  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.

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Filament
Prominence
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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.

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Active SUN 2/6/2000
large flare
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Time Sequence of a Flare
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Time sequence of eruptive prominence (1½ hr
intervals)
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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.

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Protection from Solar Events

• Fortunately, the Earth is shielded from these
  high-energy events by the Earths magnetic field,
  which diverts the charged particles toward the
  poles, and by the atmosphere which absorbs much
  of the excess energy.

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The High-Energy Particles Follow the Suns
Magnetic Field
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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.

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The Suns Interior
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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

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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.

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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 Suns 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.

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Nuclear Fusion

• According to Einsteins 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!

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End of Part IX