The Sun - Our Star

1
The Sun - Our Star

• Suns diameter 100 times the Earths
• Suns mass is 700 times the mass of all the
  planets put together
• The energy source of the Sun is the conversion of
  hydrogen atoms into helium atoms through nuclear
  fusion in the core of the Sun. Without this
  energy source the Sun would collapse.
• Composition of the Sun is 71 Hydrogen, 27
  Helium, 2 heavier elements

2
How do we know what we know?

• Our understanding of the Sun is a combination of
  measurements of observed quantities (diameter,
  surface temperature, spectra) and computer models
  based on physical laws (internal temperature and
  density).
• Models are correct so far as they can predict the
  observed properties of the Sun.

3
Solar Structure

• The Sun is completely gaseous.
• The atoms in this gas strongly absorb light.
• Because of this absorption energy travels very
  slowly through the Sun. It takes about 100,000
  years for energy to travel from the core to the
  solar surface.
• Above a certain height however the gas is too
  thin to absorb light effectively. The point at
  which the gas becomes transparent is called the
  photosphere.
• The photosphere is considered the surface of
  the Sun. Surface temperature about 6000 Kelvin.

4
Solar Structure

• At the core of the Sun, where nuclear fusion
  occurs, temperatures are about 15 million degrees
  Kelvin.
• As this energy propagates outwards it initially
  is carried by photons of light. This region of
  the Sun is called the radiative zone.
• Near the photosphere this process of energy
  transfer becomes less efficient and energy begins
  moving outward through convective motions of the
  gas. As gas heats it rises to the surface. Once
  it reaches the surface it releases heat and
  sinks. This region is called the convective zone.
  

5
Hydrostatic Equilibrium

• Requires that the pressure generated by the
  fusion reactions in the core of the Sun must be
  in exact balance with the weight of material
  falling inwards due to the Suns gravity.
• Without this balance the Sun would either
  collapse (gravity wins) or explode (pressure
  wins).

6
The Suns Energy Source

• The Sun is not burning. If it were burning fuel
  like coal it would have exhausted its fuel long
  ago.
• The slow collapse of the Sun was once thought to
  be the energy source but that wouldnt have
  lasted more than a few million years.
• It wasnt until the 20th century that physicists
  understood the process behind what powers the Sun.

7
Einsteins Mass-Energy Relation

• In 1905 Albert Einstein recognized that mass and
  energy were related through the formula
• Emc2
• (m mass, Eenergy, cspeed of light)
• What this means is that a small amount of mass
  could be converted into an enormous amount of
  energy.
• The means by which the Sun generates this energy
  is through nuclear fusion.

Albert Einstein (1879-1955)
8
Nuclear Fusion

• Nuclear fusion involves combining two atomic
  nuclei together to form one, larger nucleus.
• This can not occur under normal conditions
  because the electrical repulsion between protons
  prevents the nuclei from fusing.
• At extremely high temperatures however this
  electrical repulsion is overcome and the nuclei
  fuse.

9
Nuclear Fusion

• The specific steps of nuclear fusion follow a
  process called the proton-proton chain
• Through this process 2 neutrinos, 2 positrons, 2
  1H and a 4He is created by the fusion of 6 1H.
• The mass of all the particles created is less
  than the sum of the masses of colliding
  particles. This difference in mass was converted
  into energy through Emc2.

10
Studying the Solar Interior

• The neutrinos generated in the fusion process
  escape rapidly through the Sun.
• These neutrinos are very difficult to detect but
  they can be detected.
• This type of neutrino telescope can check
  whether our understanding of the solar interior
  is correct.
• It seems that the number of neutrinos measured
  match computer models but only if neutrinos have
  mass.

A neutrino detector in Japan. This is a giant
tank of water buried deep underground. Tiny
bursts of light due to neutrinos from the Sun are
detected by light sensitive detectors on the
walls of the tank.
11
Solar Magnetic Activity

• Like several of the planets the Sun generates a
  magnetic field.
• Unlike the planets the strength of the field
  strongly varies with time and with location on
  the Sun
• The most easily detectable evidence for solar
  magnetic activity are the Sunspots

12
Sunspots

• Sunspots are dark splotches on the photosphere of
  the Sun.
• Sunspots are dark because they are cooler than
  their surroundings.
• The reason they are cooler is because intense
  magnetic fields prevent hot, ionized gases from
  entering so the region cools forming a spot

This sunspot is twice the size of the Earth!
13
Prominences Flares

• Prominences are associated with pairs of sunspots
  are are arcs of magnetic field with glowing gas
  trapped inside.
• Flares are explosive events where strong bursts
  of X-rays are emitted. These are due to rapid
  changes in the magnetic field.

14
The Corona and Solar Wind

• The outer atmosphere of the Sun is called the
  corona. The temperature of the corona can be
  several million degrees.
• Magnetic fields are thought to be responsible for
  these very high temperatures.
• The corona is so hot it expands outward along
  with the Suns magnetic field and streams through
  the Solar System. This is known as the Solar
  Wind.
• Depending on the Suns magnetic activity level,
  the Solar Wind may change in velocity and density.

The corona is only visible from Earth during a
Solar Eclipse.
15
The Solar Cycle

• Sunspots, prominences, flares and the strength of
  the solar wind all vary over an approximately
  11-year cycle
• The magnetic field of the Sun will strengthen and
  weaken over 11 years.
• The polarity of the Suns field will also flip.
  The North pole becomes the South pole and vice
  versa. It takes 22 years to return to its
  original polarity.

A plot of the number of sunspots with time from
1860 - 2000