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Evolution of the Sun, Stars, and Habitable Zones

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Evolution of the Sun, Stars, and Habitable Zones Evolution of the Sun, Stars, and Habitable Zones AST 248 fs The fraction of suitable stars N = N* fs fp nh fl fi fc L/T Hertzsprung-Russell Diagram Parts of the H-R Diagram •Supergiants •Giants •Main Sequence (dwarfs) •White Dwarfs Making Sense of the H-R Diagram •The Main Sequence is a sequence in mass •Stars on the main sequence undergo stable H fusion •All other stars are evolved •Evolved stars have used up all their core H •Main Sequence ® Giants ® Supergiants •Subsequent evolution depends on mass Hertzsprung-Russell Diagram Evolutionary Timescales Pre-main sequence: Set by gravitational contraction •The gravitational potential energy E is ~GM2/R •The luminosity is L •The timescale is ~E/L We know L, M, R from observations For the Sun, L ~ 30 million years Evolutionary Timescales Main sequence: •Energy source: nuclear reactions, at ~10-5 erg/reaction •Luminosity: 4x1033 erg/s This requires 4x1038 reactions/second Each reaction converts 4 H ® He 56 The solar core contains 0.1 M¤, or ~10 H atoms 1056 atoms / 4x1038 reactions/second -> 3x1017 sec, or 1010 years. This is the nuclear timescale. Stellar Lifetimes τ ~ M/L On the main sequence, L~M3 Therefore, τ~M-2 10 τ¤ = 10 years τ~1010/M2 years Lower mass stars live longer than the Sun Post-Main Sequence Timescales Timescale τ ~ E/L L >> Lms τ << τms Habitable Zones Refer back to our discussion of the Greenhouse Effect. 2 0.25 Tp ~ (L*/D ) The habitable zone is the region where the temperature is between 0 and 100 C (273 and 373 K), where liquid water can exist. All stars have habitable zones. Tp depends on both • the stellar luminosity L* • the distance D. D does not change, but L* does, as the star evolves. The Habitable Zone in the Solar System Location depends on assumptions about atmospheric composition and albedo • Inner edge: • 0.84 – 0.95 AU • Outer edge: • 1.37 – 1.67 AU ____________ 0.9 – 1.5 AU Kasting, J.F., Whitmire, D.P., & Reynolds, R.T. Science, 101, 108 (1993) Stars Brighten with Time • Sun is in hydrostatic balance – Pressure is proportional to nT – n is density of particles – Pressure depends on total mass • Nuclear fusion reduces the number of particles in the core – 4 protons + 4 electrons -> 1 He nucleus + 2 electrons – As n decreases, T increases – Reaction rates increase, and Luminosity goes up Bright Old Sun Problem The Sun is brightening with time. In about 109 years, the mean temperature on Earth will exceed 100 C. Why is this a problem? The Continuously Habitable Zone Other Stars • All stars have habitable zones • Width ~ √(L) – More massive stars have wiDer HZs – Less massive stars have narrower HZs • Implications for probability of planets in HZ Appropriate Stars Massive stars (M >≈ 1.5 M¤): • Lifetimes are short • Habitable zones are wiDe • Many habitable planets? Low Mass stars (M <≈ 0.5 M¤): • Lifetimes are long • Habitable zones are narrow • Few habitable planets? Other Considerations • Tidal Locking • Stellar Flaring Both work against M stars Appropriate Stars For nuclear timescale to exceed 109 years, then O,B,A stars must be excluded. For a wide-enough habitable zone to include at least one planet (for a system like the Solar System), exclude the M stars. Late-F, G, and K main sequence stars comprise about 20% of the stars in the Galaxy So Far… 11 N*: 4x10 stars fs: 0.2 10 N: 8x10 fp nh fl fi fc L/T.
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