Stellar Evolution The lives of low-mass stars And the lives of massive stars
The Structure of the Sun Let s review: The Sun is held together by what force? The inward force is balanced by what other force?
Thinking about Gas Pressure The force exerted by gas pressure is really produced by collisions. Faster moving atoms & molecules produce stronger collisions: hotter gas = more pressure
Thinking about Gas Pressure Denser gas contains more particles in equal volume. So there are more collisions per second. More collisions also provide more force: denser gas = more pressure low density high density
Leaving the Main Sequence The fusion of hydrogen to helium in the Sun s core causes helium to build up. After 10 billion years, all of the hydrogen in the center of the Sun is gone. Something has to change!
Up the Red Giant Branch As hydrogen converts to helium, the pressure in the core drops core must shrink and heat up. Higher fusion rate higher luminosity Outer layers are pushed outward, grow cooler. The Sun becomes a red giant star.
Horizontal Branch = helium-fusion Sun-like stars settle into a helium-fusing stage. Helium core fuses to carbon, Hydrogen shell fuses to helium. Why is star brighter as He-burning star?
The Horizontal Branch The star has come a long way from its days on the main sequence: The star s helium core is most of the mass of the star and exceeds 100 million K. The outer core contains hydrogen and is hot enough for fusion: hydrogen shell. The outer layers of the star have swollen to 10 times their original size.
The Horizontal Branch Why is 100 million Kelvin needed for helium fusion? (Think back: why does proton-proton fusion need a high temperature?) Helium fuses to carbon by the triple-alpha process. (Helium nuclei are also known as alpha particles.) Helium nuclei Beryllium nucleus (fragile) Carbon nucleus (stable)
Which has more mass? 3 helium nuclei or 1 carbon nucleus A. the 3 helium nuclei B. the carbon nucleus C. the masses must be equal D. there is no way to be certain
The Death of Sun-like Stars
The End of a Sun-like Star While a Sun-like star lasts 10 billion years on the main sequence, the final stages are much shorter. Higher luminosity means higher fusion rate, Higher fusion rate means fuel is used up faster, Helium produced from H-burning, so less He. End stages (red giant, horizontal branch) last only 10-20% of the time the star was main sequence.
A Dying Sun-Like Star Finally, the luminosity of the star is so great that it simply blows off its outer layers. These glowing stellar remnants are known as planetary nebulas, because these objects were first mistaken for newly-forming planets. The Ring Nebula in the summer sky. Note the dead core, known as a white dwarf, sitting at the center of the nebula.
Planetary Nebulas Planetaries come in many different shapes and sizes: some simple Note: the plural of nebula is nebulae, but feel free to use nebulas. We re speaking English, not Latin, after all.
Planetary Nebulae Some are very complex why? Its hard to say. Images courtesy the Hubble Space Telescope
Stars of Other Mass Stars that begin with less than 8 solar masses all end their lives with planetary nebulas and leave behind a white dwarf. Stars on the higher end have core temperatures high enough to fuse carbon and oxygen into neon, sodium, and magnesium. Even these stars cannot proceed any further.
hot #4 #5 #3 #2 #1 Lowmass star temperature cool faint luminosity bright
Consider the Sun and Sirius (a 2-solar-mass star). Which of these stars will leave behind a white dwarf when they die? A. only the Sun B. only Sirius C. both the Sun and Sirius D. neither star
When the Sun evolves into a red giant, what will Earth be like? A. very cold the Sun has become very faint B. the same as today the Sun hasn t changed C. very hot the Sun has become very bright
Consider the white dwarf that the Sun leaves behind when it dies. What will this white dwarf be made of? A. hydrogen & helium B. carbon & oxygen C. iron & nickel D. uranium & plutonium