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Stellar Evolution

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Stellar Evolution Stellar Evolution The Lives and Deaths of Stars Protostars. These objects radiate energy away in the form of light. That energy comes from gravity – released by contraction. On the Main Sequence, the contraction stops because gravity and internal pressure exactly balance each other. Nuclear reactions occur at exactly the right rate to blbalance grav ity. AstarsA stars’ mass determines its luminosity Nuclear reactions slowly convert H to He in the core. Newlyyypy formed stars are typically: ~91% Hydrogen (H) ~9% Helium (He) In the Sun’s core, the conversion of H to He w ill take ~10 billion years (its Main Sequence lifetime). Composition of a Sun-Sun-likelike star What happens when the core Hydrogen is used up? Nuclear reactions stop. Core pressure decreases. Core contracts and gets hotter - htiheating overl liaying l ayers. 4H → 1He burning moves to a hot “shell.” Post-Main Sequence evolution •4H → 1He reactions occur faster than before. •The star gets brighter (more luminous)! •The hot shell causes the outer layers to expand and cool! •The star moves o ff t he Ma in Sequence, …up the “Red Giant” branch. AiAscension up thditthe red giant branch takes ~100 million years What happens in the core as it continues to contract and getht ho tter? Remember why Hydrogen burning requires 107 K? (Protons repel each other.) Helium nuclei ((p2 protons) rep el each other even more… Helium begins to fuse into Carbon at >108 K. This reaction is called “triple alpha ”= 3He → C. The Helium Flash: Post-Main Sequence evolution 8 After the core reaches 10 K, Helium “ignites” to make Carbon. The onset of this burning causes the temperature to rise sharply in a runaway explosion - Helium Flash (stage 9) Eventually the core expands, density drops and equilibrium is re-established Core structure is now readjusted during Helium core burning and total luminosity is actually decreased. During core Helium burning, the star is on the Horizontal Branch. Relative sizes of main-sequence, red giant, and horizontal branch stars . Stage 9 Stage 7 Stage 10 Stage 10 - Helium-to-Carbon burning Carbon occurs stably in core, with Hydrogen-to- Helium burning in shell… Helium …until the core Helium runs out …. in just 20-50 million years… Carbon Helium Carbon Helium The increased shell burning causes the outer layers to expand and cool (again). The star moves up the asymptotic giant branch (in only ~10,000 years!). becoming a red suppgergiant (stage 11) •During this phase the Carbon core contracts and heats up. •Helium is burning to Carbon in a shell around the Carbon core while H-to-He burning occurs in an outer shell. What do you suppose happens next in the core? For a Sun-like star: Solar mass stars cannot squeeze and nothing…. Why? heat the core enough to ignite Carbon. So what does happen? The Carbon core continues to contract and heat. Shell He burning grows more intense. He flashes occur in the shell. Surface layers pulsate and are finally ejected (slowly, at ~10s of km/s). The hot, tiny core (White Dwarf) is revealed. And a Planetary Nebula appears! (expanding emission line nebula heated by intense radiation from the White Dwarf hot white dwarf) Planetary Nebulae have nothing to do with planets. They em it li ne radi ati on (hot gas) but are much smaller than the emission nebulae (HII regions) we discussed in Chapter 11. They are important sources of heavy elements ((,C, N and O), which will go into the next generation of stars. White dwarfs have about ½ the Sun’s mass (the rest was expelled). They are about the size of the Earth! (~0.01 solar radii) Dens itity: ~6600 lbs /cm3 !! Veryy, low luminosities, (L = 4πR2 σT4) What eventually happens to a white dwarf? It gets cooldfiler and fainter (at the same radius). This is the End: •White Dwarf fades away. •Planetary Nebula dissipates into interstellar space. •End of story for stars like the Sun. Or is it? •A main-sequence or giant star in Some white dwarfs end up in binary system with a WD. explosively active situations. •Material gets pulled off the bigger star Nova bby gravitat iona l attraction – forms an accretion disk around WD. •This material builds up on the WD surface, getting hotter and denser. •Reaches 10 million K and Hydrogen burning ignites on the white dwarf surface! •Burning is fast and furious - Nova •After material burns off, accretion resumes and the process repeats. Summary: What happens to higher mass stars? Gravity squeezes and heats the core enough to ignite Carbon. Then Oxygen, then Neon…as each fuel gets exhausted in the core, its b urni ng moves t o a shell. Concentric fusion shells form an “onion skin” structure. Formation of an Iron core is the last stage… Why is Iron formation the end of the line? Creating elements heavier than Iron requires energy! With no more sources of energy, and Fe fusion taking energy from the gas, pressure support in the star’s core is lost… The core quickly collapses und er i ts own weight…. Protons and electrons are crushed together in the collapsing core, making ne utrons . Eventually, the neutrons are so close together they “touch” neutron degeneracy pressure (which is very stiff). The densities reach 100 billion kg/cm3 (at those densities the whole Earth would fit in our football stadium!!) The collapsing neutron An explosive shock wave core then bounces! propagates outward, expelling all out er l ayers. Supernova Two types of Supernovae: Type II – we just discussed – massive star explosion Type I – The long term effects of the Nova situation •WD eventually “builds up” material that was not completely ejected during nova explosions •Added mass causes gravity to squeeze the WD allowing Carbon to finally start burning. •Carbon burns all over the WD at once (not just in core) and the star explodes in a carbon-detonation supernova (Type I) Comparison of Type I and Type II Supernovae This supernova was recorded by Chinese astronomers in 1054 AD. Supernova ejecta like this spread heavy elements throughout the Galaxy. Crab Nebula Observing Stellar Evolution in Star Clusters.
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