Low-mass 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? The inward force is balanced by?
Thinking about Gas Pressure The force exerted by gas pressure is really produced by collisions. Faster moving particles = stronger collisions: hotter gas = more pressure
Thinking about Gas Pressure Fusion combines 4 protons into a helium nucleus, è Fewer particles in the same volume. Fewer particles = fewer collisions less density = less pressure high density low 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!
If all of the gas pressure in the Sun disappeared, what would happen to the Sun? A. Nothing. Gas pressure isn t important in the Sun. B. The Sun would expand very fast and blow apart. C. The Sun would collapse to form a black hole. D. The Sun would expand and contract for a while.
Becoming the Red Giant As hydrogen converts to helium, the pressure in the core drops core must shrink and heat up. Higher fusion rate higher luminosity Outer layers pushed outward, grow cooler. The Sun becomes a red giant star.
Why does the core heat up as it contracts? A. Gravitational potential energy is released. B. Gravitational potential energy is absorbed. C. Kinetic energy is released. D. Mass-energy is absorbed.
Main Sequence and Beyond The main sequence life of 1 solar-mass star is 10 billion years. Later stages are much shorter: Higher core temperature è higher fusion rate è higher luminosity shorter time to use up fuel End stages (after main sequence) last only 10-20% of the star s main sequence lifetime.
As core depletes hydrogen, fusion proceeds faster The End of a Red Giant luminosity rises core temp rises fusion even faster Runaway process until core reaches 100 million Kelvin: Helium flash!
Helium Flash and Helium Fusion Helium fusion begins at 100 million Kelvin. Why does helium fuse only at higher temperatures? Three helium nuclei fuse to form a carbon nucleus: He #1 He #3 He #2 beryllium nucleus (4 p + 4 n) carbon nucleus (6 p + 6 n)
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"
Double Fusion 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 10-100x main sequence size.
A Repeat of the Main Sequence The star fuses helium into carbon in the core and hydrogen into helium in a shell around core. Too cool for carbon to fuse carbon builds up. Fewer particles in core pressure drops, core shrinks. Core temperature rises again faster fusion luminosity rises.
The Lives of Sun-like Stars
The Lives of Sun-like Stars The history of the Sun s luminosity
A Dying Sun-Like Star Finally, the luminosity of the star is so great that the star s gravity can t hold on to the outer layers. The outer layers are excited by the star s hot core. These glowing shells are called planetary nebulas. 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 (lower mass stars, G & F) Plural of nebula is nebulae, but feel free to use nebulas.
Planetary Nebulas Some are very complex hotter, higher-mass stars (A & F types). Images from Hubble Space Telescope
Stars of Other Mass The line between low-mass & high-mass is around 8 solar masses. (Evolution depends on original mass, rotation, composition, etc ) Stars that begin with less than 8 solar masses end their lives as planetary nebulas and leave behind a white dwarf. Stars with more than 8 solar masses have higher core temperatures and fuse carbon & oxygen. Their evolution takes a different path.
luminosity bright #4 #2 #3 Lowmass star #1 faint hot #5 temperature cool
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 will be very faint and red" " B. the same as today, but the Sun will be big and red in the sky" " C. very hot the Sun will be very bright"
Consider the white dwarf that the Sun will leave behind when it dies." " What will this white dwarf be made of?" A. hydrogen & helium" B. carbon & oxygen" C. iron & nickel" D. uranium & plutonium"