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Transcript
The Life Cycles of Stars Dr. Jim Lochner, NASA/GSFC Twinkle, Twinkle, Little Star ... How I Wonder What You Are ... Stars have • Different colors Which indicate different temperatures The hotter a star is, the faster it burns its life away. Stellar Nursery - Nebulae Space is filled with the stuff to make stars. Stars start from clouds Clouds provide the gas and dust from which stars form. But not this kind of dust Rather: Irregular Grains Of Carbon or Silicon Collapse to Protostar Stars begin with slow accumulation of gas and dust (accretion). • Gravitational attraction of Clumps attracts more material. Gm1m2 F 2 r • Contraction causes Temperature and Pressure to slowly increase. Concept Quiz Collapse to Protostar If an interstellar cloud contracts to become a star, it is due to which force? A. electromagnetic B. nuclear C. gravitational D. centrifigal Nuclear Fusion ! At 15 million degrees Celsius in the center of the star, fusion ignites ! 4 (1H) --> 4He + 2 e+ + 2 neutrinos + energy Where does the energy come from ? Mass of four 1H > Mass of one 4He E = mc2 How much Energy 4 (1H) --> 4He + 2 e+ + 2 neutrinos + energy Energy released = 25 MeV = 4 x 10 -12 Joules = 1 x 10 -15 Calories But the sun does this 1038 times a second ! Sun has 1056 H atoms to burn ! Concept Quiz How Much Energy Why does the fusion of Hydrogen release energy? A. Fusion breaks the electromagnetic bonds between hydrogen atoms, releasing energetic photons. B. The mass of a helium nucleus is smaller than the mass of four protons. C. The mass of a helium nucleus is larger than the mass of four protons. D. None of the above is true. A Balancing Act Energy released from nuclear fusion counteracts inward force of gravity. Throughout its life, these two forces determine the stages of a star’s life. New Stars are not quiet ! Expulsion of gas from a young binary star system All Types of Stars Recall Stars have Different colors which indicate different temperatures All Types of Stars Annie J Cannon (1863-1941) Out Fiery Gases Many Red Oh! Beyond Be Oh! a Fine BeAndromeda, a Girl Fine- Girl Kiss- Me KissRight Me !NowKindle Sweetheart ! New Stars Reprise: the Life Cycle Sun-like Stars Massive Stars The Beginning of the End: Red Giants After Hydrogen is exhausted in core ... Energy released from nuclear fusion counter-acts inward force of gravity. • Core collapses, Kinetic energy of collapse converted into heat. This heat expands the outer layers. • Meanwhile, as core collapses, Increasing Temperature and Pressure ... More Fusion ! At 100 million degrees Celsius, Helium fuses: 3 (4He) --> 12C + energy (Be produced at an intermediate step) (Only 7.3 MeV produced) Energy sustains the expanded outer layers of the Red Giant Concept Quiz More Fusion • • • • • After a Sun-like stars core hydrogen is depleted by nuclear fusion the core will consist primarily of A. carbon B. deuterium C. helium D. oxygen A Red Giant You Know The end for solar type stars After Helium exhausted, outer layers of star expelled Planetary Nebulae White dwarfs At center of Planetary Nebula lies a White Dwarf. • Size of the Earth with Mass of the Sun “A ton per teaspoon” • Inward force of gravity balanced by repulsive force of electrons. Discussion • Get with 3’clock partner and discuss life cycle of a sun-like star. • On paper provided create life cycle using colored pencils/crayons/markers. • Label each stage and write brief summary about what is happening inside star. Review – Warm Up Read and think of the answer to the following question without looking in your notebook. The event that marks the end of a star’s evolutionary life before becoming a white dwarf is A. a supernova B. a planetary nebulae C. the exhaustion of hydrogen in the core D. a red giant Discuss with table group which answer is correct and be ready to share with class. Reprise: the Life Cycle Sun-like Stars Massive Stars Life Cycle of a Sun-like Star Take poster paper from front of room…1 per student. Create life cycle of sun-like star based on your notes from yesterday. Use arrows to show movement from one step to the next. At each step write a short explanation of what is happening during that stage. Fill the entire poster with you cycle. You may want to sketch it out in your notebook before you start on the poster paper. Due at end of period at back table. Make sure your name is on it. Fate of high mass stars After Helium exhausted, core collapses again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen. 12C + 12C --> 24Mg OR 12C + 4H --> 16O Through a combination of processes, successively heavier elements are formed and burned. Periodic Table Light Elements Heavy Elements 28Si +4 12 416 1 12 16 12 16 12 4He 20 24 32 16 He He 7( 3( 4 CO4(4+ He) + He) +H) C-N-O C O C O 56 Ni C Cycle Ne Mg S O ++ +energy energy +++energy energy energy energy 56Fe The End of the Line for Massive Stars Massive stars burn a succession of elements. Iron is the most stable element and cannot be fused further. Instead of releasing energy, it uses energy. Concept Quiz The End of the Line • Fusion in the core of a stable massive star cannot proceed beyond iron because • A. it would require temperatures that even stars cannot generate. • B. the fusion of iron nuclei is impossible under any circumstances. • C. the gravity limit has been reached so a black hole must result. • D. iron nuclei are the most tightly bound of all nuclei so iron fusion does not release energy. Supernova ! Supernova Remnants: SN1987A a b c d a) Optical - Feb 2000 • Illuminating material ejected from the star thousands of years before the SN b) Radio - Sep 1999 c) X-ray - Oct 1999 d) X-ray - Jan 2000 • The shock wave from the SN heating the gas Supernova Remnants: Cas A Optical X-ray Elements from Supernovae All X-ray Energies Calcium Silicon Iron What’s Left After the Supernova Neutron Star (If mass of core < 5 x Solar) • Under collapse, protons and electrons combine to form neutrons. • 10 Km across Black Hole (If mass of core > 5 x Solar) • Not even compacted neutrons can support weight of very massive stars. Concept Quiz Supernova • A star will end its life in a supernova A. B. C. D. if it is a massive star when it has fused all of its helium after the white dwarf stage0 and create a planetary nebulae A whole new life: X-ray binaries In close binary systems, material flows from normal star to Neutron Star or Black Hole. X-rays emitted from disk of gas around Neutron Star/Black Hole. Black Holes - Up Close and Personal Accretion Disk Singularity (deep in center) Event Horizon Jet (not always present) SN interaction with ISM Supernovae compress gas and dust which lie between the stars. This gas is also enriched by the expelled material. This compression starts the collapse of gas and dust to form new stars. Concept Quiz Life Cycle of Stars • The birth and death of stars is called a ‘life cycle’ because • A. when a star dies it changes into another star • B. the end of stars creates the material that new stars are made of • C. stars are born and die all the time • D. of no reason, it just sounds good. Which Brings us Back to ... CREATE • On paper provided create life cycle of Massive Star using chalk provided. • Label each stage and draw arrows to show direction. • Be sure to include both possible endings. • Make your chalk drawings dark(thick) so the steps will show after it is sprayed with hairspray Materials for Life Cycles of Stars This presentation, and other materials on the Life Cycles of Stars, are available on the Imagine the Universe! web site at: http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/stars.html