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Stars and Nebulae

Stars and Nebulae. The study of the stars & nebulae provides information about the origin and history of the Earth and the solar system. Astronomers discover how stars like our Sun are born, grow old and die. Review. Pulsars are ...like neutron stars because....

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Stars and Nebulae

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  1. Stars and Nebulae

  2. The study of the stars & nebulae provides information about the origin and history of the Earth and the solar system. Astronomers discover how stars like our Sun are born, grow old and die.

  3. Review • Pulsars are ...like neutron stars because.... ...not like red giants because ... • Draw a graphic organizer to show the life of stars

  4. lesson outcomes... • know, understand and explain: • steps in the formation of a star like our Sun • life-cycles of different size stars • measuring distances to nearby stars • classifying stars by their features

  5. The Life of Stars

  6. The Interstellar Space • space among stars • filled with a thin gas laced with microscopic dust particles • combination of gas and dust is called the interstellar medium • this is concentrated in the disk of the galaxy • evidence for this is that distant stars sometimes are dimmed or redenned • clouds within the interstellar medium are called nebulae

  7. Nebulae • interstellar cloud  a nebula (plural nebulae) or nebulosity • a dark nebula: • a cloud that is visible as a dark blot against distant stars • e.g. Horseheadnebula • reflectionnebulae • starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow • emission nebulae • glowing, ionized clouds of gas • uv light absorbed from nearby hot stars • e.g. Orion nebula

  8. Nebulae A Dark Nebula: Horsehead Nebula

  9. Nebulae Emission Nebula: Crab Nebula

  10. Nebulae Reflection Nebula: Orion Nebula

  11. Stellar Evolution

  12. Stellar Evolution • Stellar evolution means how stars change during their life-span i.e. how they are born, live and die. • stars have finite live-spans • the amount of H in the sun’s core is vast but not infinite • so the sun cannot always have been shining nor can it continue to shine forever. • Protostars form in cold dark nebulae • gravity causes a clump of material to condense into a protostar

  13. Stellar Evolution • Protostargrows by the gravitational accretion of gases • the ball of gas contracts, heat & begin glowing • at this point, relatively low temperature and high luminosity • Protostars evolve into main sequence stars when core temperatures become hot enough to ignite steady hydrogen burning to helium • the more massive the protostar, the more rapidly it evolves • Newborn stars may form an open or galactic cluster, with the collection of stars held together by gravity.

  14. Stellar Evolution • total time that a star will spend burning hydrogen in its core is called the main sequence lifetime • during a star’s main sequence life-time, the star expands somewhat and undergoes a modest increase in luminosity • a more massive star  shorter main sequence lifetime • the duration of a star’s main sequence lifetime depends on • the amount of hydrogen in the star’s core • rate at which the hydrogen is consumed

  15. Stellar Evolution • When core hydrogen-burning ceases, a main sequence star becomes a red giant • main sequence star  hydrogen is converted into helium in the core • when core hydrogen is exhausted, star expands to become a red giant • helium burning starts • converts He to C & O (triple alpha process ) • how quickly burning starts depends on size • in massive red giant, helium burning begins gradually • less massive red giant, begins suddenly • Depending on the size of the original star, the red giant ends its life as a neutron staror a black hole

  16. Stellar Evolution

  17. Observing Stars

  18. Measuring Distances to Stars • parallax: apparent displacement of an object because of a change in the observer’s point of view • closer the object, the greater the parallax shift • allows brain to determine distances to objects (depth perception)

  19. Measuring Distances to Stars • stellar parallax • direction from Earth to star changes with Earth orbit • star appears to move back and forth against the background of more distant stars. • use two points as far apart as possible – opposite sides of Earth orbit • measure distance measured in parsecs • e.g. a star with a parallax angle of 1° of arc d = 1 pc • can only be measured for stars within a few hundred parsecs • if done from Earth orbit more accurate because eliminates blurring effects of the atmosphere

  20. Measuring Masses of Stars • can determine stellar mass from binary stars • two stars held together in orbit around each other by their mutual gravitational attraction • the two stars move in an elliptical orbit about the centre of mass of the system • like unequal partners on a see-saw, different masses of stars will have different orbital dimensions and periods • masses computed from measurements of the orbital period and orbital dimensions of the system

  21. Measuring Brightness of Stars • If a star’s distance is known, its luminosity can be determined from its brightness • we see the apparent brightness of stars: • light moving away from its source diverges in space • a distant bright star can appear dimmer than a proximal dim star • astronomers use the magnitude scale to denote brightness • the greater the apparent magnitude the dimmer the star. • luminosity is amount of light energy emitted by a star each second • measures a star’s true energy output • called absolute magnitude (magnitude at 10 parsecs) • depends on light given off now how much light spreads out

  22. Observing Stellar Features • stellar spectrosocopy • determining the structure, chemical composition & temperature of stars from analyzing the light they emit • all stars contain hydrogen and helium in their spectra • in addition, there are other metal spectra present • astronomers use the term metal for everything that is not hydrogen or helium. • To cope with a diversity of spectra, astronomers group similar-appearing stellar spectra together in spectral classes and even finer spectral types. • OBAFGKM (oh be a fine girl/guy kiss me)

  23. Organizing Stars

  24. Classifying Star Populations • luminosity: • stars of relatively low luminosity are more common than more luminous stars • the Sun has intermediate luminosity • magnitude scale • used to denote brightness • spectral type • based on the major patterns of spectral lines in their spectra • directly related to surface temperature & colour • O stars are the hottest • M stars are the coolest

  25. Classifying Star Populations • Astronomers have collected a wealth of data about stars • radius : stars come in a variety of different sizes • luminosity: stars emit different amounts of light • surface temperature: influences star colour • To analyze this data, astronomers construct a graph to show how these quantities may be related • Hertzsprung-Russell Diagram • plots • absolute magnitudes of stars against their spectral types • or luminosities against surface temperature

  26. Hertzsprung-Russell

  27. Questions... • What steps are involved in forming a star like the Sun ? • What kind of nuclear reactions occur in a star like the Sun as it ages? • Why are red giants not main sequence stars ? • Do all stars evolve into red giants at the same rate ? • Why are some stars red and others blue ? • What are giant stars, supergiant stars and white dwarf stars?

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