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Lecture 9

Introduction to Astronomy-Lecture 9 Notes!.docx

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Department
Physics
Course
PCS 181
Professor
Margaret Buckby
Semester
Winter

Description
Tangential Velocity:  Tangential velocity is the component of the Star’s velocity perpendicular to our line of sight. 3  V = 4t74 x 10 U d  Where V is intm/s.  d is in pc.  U is in arcsec per year (“/year). Space Velocity:  Space Velocity (V ) is tse actual velocity of the star in space.  Earth--------------V ----s-d--------V t Star V r 2 2 1/2  V = (s + v r t Distance/Velocity Problem: SIMILAR PROBLEMS ON TEST Question: A nearby White Dwarf has a parallax angle of 0.232 arcsec ( =0.232”). Its proper motion is 2.95 arcsec/year (=2.95”/year) and its radial velocity is 54 km/s. What is the objects actual speed in space (in km/s)? Answer:  V = (s + v r , wetneed V t 3  V = 4t74 x 10 Ud, we need d  d = 1 = 1 = 4.31 pc 0” 0.232”  V = 4.74 x 10 Ud 3 t  V = 4t74 x 10 (2.95) (4.31) = 60.3 x 10 m/s = 60.3 km/s. 2 2 1/2  V = (s + V )r t  V = (s4 + 60.3 ) 2 1/2  V = 8s.9 km/s ~ 81 km/s. Death of Stars:  Stars die in several different ways.  The manner of death is determined by the star’s mass. Death of Lightweight Stars: REMEMBER FOR THE TEST WE NEED 10 MILLION K IN THE CORE  These are < ~ 4 M o.  These are normal stars…95% of stars are in this category.  1) Core runs low on Hydrogen fuel.  Star’s leaves Main Sequence (=Hydrogen fusion) phase. 6 12  2) Core contracts and at 100 x 10 k, Star fuses Helium (3 (Alpha) = C) in core.  3) Star runs low on Helium fuel in core which now consists of …  4) Mainly Carbon and some Oxygen.  5) Gravity causes outer layers to collapse onto carbon core, squashing it and raising the core temperature.  6) Core temperature never becomes high enough to initiate carob-based nuclear fusion reactions.  7) Computer simulations = energy is released by gently blowing off the star’s outer layers at ~25 km/s. Planetary Nebula: REMEMBER FOR TEST  No more nuclear reactions in the core of star.  Stars dies with a compressed Carbon (Some Oxygen) Core.  Planetary Nebula = dying/dead star remnant surround by a shell of ejected gases.  The central remnant core= White Dwarf…small, very hot, plots on the lower left of the HR Diagram. Typical White Dwarf:  Mass ~0.8 M = 1o o.  Mass limit is 1.4 M ; the Chandrasekhar Limit. o  Size~same size as Earth.  Density ~ 10 kg/cm . 3  Magnitude = M ~ + 11 (very dim).  Temperature ~15000 k on surface.  No longer has a central energy source.  We can detect ~500 White Dwarfs = our MW (Milky Way) galaxy must contain billions of them. Death of Middleweight Stars:  These are ~8 M = ~o20M o.  < ~ 5% of stars are > ~4M o.  1) Star undergoes several Red Giant phases, producing ever-heavier elements in the core. Evolutionary Stages of a massive Star (~25 M ): o
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