The Aging Sun:
At birth:
Core:
Envelope:
>> Hydrogen Fusion ENDS <<
Hydrogen
75%
75%
Helium
25%
25%
Today
Today (Age = 4.64 billion years):
years):
Hydrogen Helium
Core:
35%
65%
Envelope:
75%
25%
Future: 5 billion years from now
Hydrogen Helium
Core:
~ 0%
~ 100%
Envelope:
75%
25%
End of Main Sequence Life
What happens:
• Core no longer has Hydrogen
• Core contracts and gets hotter!!
Why?
• Core is now ~100% Helium
• Core: Gravity > Gas Pressure
• Grav. Pot. Energy converted to heat
RED GIANT STAR:
STAR:
What happens:
• H fusion starts in a shell around core
• Outer layers expands and cools
Why?
• Envelope still 75% Hydrogen
• Larger pressure in outer envelope
>> “Helium Flash” <<
What happens:
• He fusion begins in the core
Why?
• Core reaches T needed for He fusion
HORIZONTAL BRANCH (RED GIANT) STAR:
STAR:
What happens:
• Becomes stable with two energy sources
• Envelope expands more and cools more
Why?
• Re-establishes Hydro & Thermal Equilibrium
http://www.spacetelescope.org/videos/html/hst15_red_giant_sun.html
SUPERGIANT (AGB) STAR:
STAR:
What happens:
• He fusion starts in a second shell around core
• Outer layers expands and cools
Why?
• H fusing shell creates more He
• Larger pressure in outer envelope
>> Helium
Helium Fusion ENDS <<
What happens:
• Core no longer has Helium
• Core contracts and gets hotter!!
Why?
• Core is now ~100% Carbon (some O)
• Core: Gravity > Gas Pressure
• Grav. Pot. Energy converted to heat
PLANETARY
PLANETARY NEBULA and WHITE DWARF:
What happens:
• Most of envelope ejected into space
• Leftover dead core exposed, can be seen
Why?
• Unstable shell sources causes pulsations
• Ejected envelope seen as Planetary Nebula
• Visible core seen as White Dwarf
• As cools, becomes “Black Dwarf”
>> DEATH OF SUN <<
What happens:
• Core shrinks to final size
• Fusion in shells become unstable, then end
Why?
• Core is no longer an “ideal” gas
• No more fuel is available
Youngest Planetary Nebula – 20 yrs old!!
Sirius (9,400 K) and Sirius B (27,000 K)
Globular Cluster M4