Reading Chapter 8: Sec. 8.1 (Spectra p. 202-211) Sec. 8.1 Optional: 212-219 Sec. 8.2 H-R Diagram Chapter 9: Not covered Chapter 10: Sec. 10.1 10.2 optional Chapter 10: Sec. 10.3 required Croswell: Chapters 6-9 H-R diagram Stars fall into four categories of stars: 1.main-sequence stars 2. giants 3. supergiants 4. white dwarfs. HRDiagram.anim.mp4 UY Scuti is the largest Star Currently Known Antares, a SUPERGIANT in Scorpius RUY Scuti = 1700 RSUN UY Scuti is the largest Star Currently Known Spectroscopic Parallax ! Spectroscopy allows us to: ! 1. Determine spectral type, hence temperature ! 2. Determine luminosity class (I - VI) (see Sec. 9.5) ! If a star can be placed on the HR diagram, then its absolute magnitude (MV) can be determined. ! By measuring its apparent mag, distance can be determined: ! This method of determining distances is called spectroscopic “parallax” ! Less precise than usual parallax... Stellar Evolution ! As a star ages, it undergoes changes in its Luminosity & Temperature. ! It “moves on the HR Diagram” ! Stars on the Main Sequence (MS) are all fusing Hydrogen. ! When their Hydrogen supply runs out, they leave the MS. ! More massive stars leave first. What powers the Sun? (C&O 10.3) Could it be Chemical Reactions? If so, then how much energy, E, is available? How long could the Sun be powered by chemical reactions? The Sun produces 4 x 1026 Joules/s t = E /LSun Chemical reactions typically release: ~4 eV of energy = 4 * 1.6 x 10-19 J ~ 10-18 J, per atom If every atom in the Sun burns (like coal), then the energy available is: E = n * 10-18 J . (n = # atoms in the sun) Powering the Sun (C&O 10.3) Activity: Estimate, n, the number of atoms in the Sun 30 -27 57 n ~ M SUN /mH ~ 10 kg / (10 kg/atom)~ 10 atoms Chemical Energy avail.: E = n * (10-18 J ) = 1039 J 39 26 12 tlife = E /L = 10 J / 4 * 10 = 2.5 * 10 sec = 2.5 1012 sec * (1 year / π * 107 ) = 104 years! Kelvin-Helmholtz Timescale Another source of energy in the Sun could be gravity: If an extended object collapses to lower U, it will gain kinetic & thermal energy. The gravitational potential energy of 2 particles is: If we allow a distant sphere of mass M to fall in to radius: R, then the grav. potential energy will decrease by: ΔEg = Kelvin-Helmholtz Timescale This allows us to estimate the lifetime of the Sun, assuming gravitational collapse is its only source of energy: The solar system is much older than this. So another source is required.... ...nuclear reactions. What’s Inside Atoms? Which has more space: An atom or the Solar System? \ a nucleus The Players: Note: We can use units of Energy (MeV) to measure mass (x c2)!.