The Sun

1
The Sun

• Closest star to us!
• Radius 6.95 105 km 108 Earth radii
• Mass 21030 kg 333,000 Earth masses
• How do we get this mass?
• Made of 98 hydrogen and helium, 2 heavier
  elements
• How might we find this out?

2
The Sun

• Sphere of gas
• NO solid surface
• T5,800oK
• (11,000oF)
• at surface

3
How Does The Sun Shine?

• Is it burning like a piece of wood?
• No once people knew how far the Sun was, and
  how much energy it had to produce, it was clear
  this was not possible

4
How Does The Sun Shine?

• Is it due to gravitational contraction?
• Recall a gas cloud contracting under its own
  gravity gets hotter.

5
How Does The Sun Shine?

• Is it due to gravitational contraction?
• No. At most this would work for only 25
  million years.
• Earth is a few billion years old Sun is
  older. So this cant be it either.

6
How Does the Sun Shine?

• Conservation of mass-energy
• Emc2 (Einstein, 1905)
• Small mass can be turned into a lot of energy
• Nuclear fusion requires very high density and
  temperature
• Just as found in center of Sun

7
Gravitational Equilibrium

• Keeps Sun from
• expanding or
• shrinking
• Outward pressure
• of hot gas vs.
• Inward pull of
• gravity

8
Gravitational Equilibrium

• Just as the upward pressure of arms and legs
  balances downward pull of gravity.
• Outward pressure greatest at bottom of pile,
  which is carrying the most weight

9
But How Did the Sun Turn On?

• Gravitational contraction - it is how the Sun
  formed and became dense and hot enough to start
  nuclear fusion
• Then fusion produced energy and outward pressure
  to stop the contraction and a balance was
  achieved
• We see places where stars are forming and observe
  stages in this process

10
Structure of the Sun (and many other stars!)

• Sun is a very hot gas, or plasma
• So hot that all the atoms are ionized
• Sun rotates, but NOT as a solid ball
• Equator rotates faster than poles
• Suns power output 3.81026 watts

11
Digression Energy, Power, Luminosity

• Energy measured in joules
• Power is the rate at which energy is released
• What would be the units for this?

12
Digression Energy, Power, Luminosity

• Energy measured in joules
• Power is the rate at which energy is released
• Joules/second, also called watts
• This is the rating on a light bulb! Tells you the
  power consumption (or output)

13
Power of the Sun

• Suns power output 3.81026 watts
• If we could capture ALL of this for just 1
  second, it would be enough power for all humans
  for 500,000 years!!!

14
The Sun Shines

• We see only the surface of the Sun
• The surface does NOT have fusion going on, only
  at the core!
• The energy from the core goes into the gas, heats
  it up.
• What kind of spectrum will this produce?

15
Structure of the Sun

• Multiple regions
• Photosphere
• visible surface
• Convection zone
• Turbulence
• moves energy
• Radiation zone
• Photons move
• energy

16
Structure of the Sun

• The core
• Energy production
• T15,000,000K
• 100x as dense as
• water (but still
• a gas!)
• Pressure 2x1011
• higher than at
• Earths surface!