Stars and Nebulae

1. Stars and Nebulae

4. 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.

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

6. 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

7. The Life of Stars

8. 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

9. 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

10. Nebulae A Dark Nebula: Horsehead Nebula

11. Nebulae Emission Nebula: Crab Nebula

12. Nebulae Reflection Nebula: Orion Nebula

13. Stellar Evolution

14. 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

15. 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.

16. 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

17. 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

18. Stellar Evolution

19. Observing Stars

20. 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)

21. 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

22. 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

23. 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

24. 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)

25. Organizing Stars

26. 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

27. 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

28. Hertzsprung-Russell

29. 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?