Chapter 20 Section 1: The Life & Death of Stars pp. 693-701

1. Chapter 20 Section 1: The Life & Death of Stars pp. 693-701

2. What are Stars? • Stars are: huge spheres of very hot gas that emit light and other radiation • Formed from clouds of dust & gas (nebulas) and go through different stages as they age. • Located different distances from Earth • Light-year: distance light travels in 1 year (9.5 x 1012 m) • Light from Sun takes 8 minutes to reach Earth • Light from star nearest Sun takes 4 years to each Earth

3. Relative Sizes of Stars/Planets

4. How do stars get their energy? • Nuclear fusion: • Occurs in core of star • Combines the nuclei of hydrogen atoms into helium atoms • H+ + H+ deuterons (1 proton & 1 neutron) • Deuteron + 1 proton He isotope (helium-3) • helium-3 + helium-3helium-4 • Each time two particles fuse energy is released!

5. Star Anatomy • Energy moves slowly through layers • Radiation & Convection: • Convection similar to convection currents in layers of Earth;hot gas moves away from star’s core • Radiation: energy is transferred to individual atoms atoms absorb energy and keep on transferring to other atoms randomly • Energy from nuclear fusion in core takes thousands of years to reach surface@ surface it’s released into space as radiation & light • Interior= plasma or ionized gases (only nuclei & electrons) at temps of 10,000,000 K • Light produced each second = 4 trillion trillion 100-W light bulbs • ~5 billions years to go for Sun—will have used up all H+ core

6. Apparent vs. Absolute Brightness • Brightness of a star depends on: • temperature • size • Distance from Earth • Apparent brightness: the brightness of a star as seen from Earth • Absolute brightness: how bright a star actually is (if all stars were at same distance) • Apparent vs. Absolute Light bulb Activity

7. What does the color of a star mean? • Star’s peak wavelength corresponds to a color (the wavelength of light with the most energy) • Temperature determines wavelength which determines color! • Cooler objects, less intensity, shorter wavelength=RED • Sun’s peak wavelength=yellow • Hot objects, more intensity, longer wavelength=BLUE

8. What elements make up stars? • Almost all stars are • 73% Hydrogen • 25% Helium • 2% other • Emission spectra • Burning gas releases energy @ different wavelengths • Corresponds to a color • Match pattern of lines up with known elements to determine elemental composition of stars

9. Key for Lifecycle Boxes & Arrows • Key = Low mass stars • Have longer life spans than high mass stars • Burn their fuel more slowly • Outline all top boxes RED, connect w/RED arrows • Key = High mass stars • Have shorter life spans than low mass stars • Burn their fuel more quickly • Outline all bottom boxes BLUE, connect w/BLUE arrows

11. Stellar Nebula • Birthplace of stars cloud of dust and gas • Cloud condensed by passing stars or supernova explosion • As stars age, H+fused to He, C, Fe & other heavier elements to increase star’s density • Nebula collapses (force of gravity) and spins protostar • Nuclear fusion occurs & star is born! Horsehead Nebula Eagle Nebula

13. Sun • Fusion within Sun generates energy that produces outward pressure • Force of gravity pushing in on Sun • These opposing forces balance each other • Balanced forces keep Sun at stable size • Sun will die when: • Fusion reactions decrease • Still have 5 billion years

14. Main Sequence/Average or Massive Star • Infancy • Star continues process of nuclear fusion • Longest stage of life cycle (~100,000 years) • Star grows & becomes stable • 90% of stars in our galaxy, including the Sun, are in the main sequence stage and stay there for most of their ‘lives’ (~100,000 years) • Massive stars evolve much faster than average stars (larger so using up more ‘resources’)

15. Red Giant/Red Supergiant • Large, bright star with cool surface • Formed during later stages of star’s life • Hydrogen fuel begins to run out less fusion • Star expands in size • Temps cool as expansion occurs=red color • Sun will become red giant before it dies

17. Death of a Low Mass Star • Planetary Nebula • Gas is used up • Star condenses & outer atmosphere blows off • White Dwarf [as dense as the Sun but as big as the Earth] • Last of remaining fuel burns • Core of heavy elements shrinks in size • No longer fuse elements slowly cools • Most stars in our galaxy will end as white dwarfs • Black Dwarf [is a cooled white dwarf no longer emits light] • No remaining fuel, stops burning • Just a core of leftover matter

18. Planetary Nebula (1) Black Dwarf (3) White Dwarf (2)

19. Death of a High Mass Star • Supernova • Gas used up, triggers violent collapse of RSG • Collapse triggers a huge explosion • shockwaves can condense distant nebulae=new stars! • Final fate of star determined by mass of star: • 1.4 - 3 solar masses formsNeutron star • Label arrow on top branch • Greater than 3X solar mass collapses to formBlack hole • Label arrow on bottom branch

20. The end of a High Mass Star… • If 1.4 - 3 solar masses: Neutron star (shell) • 20 km in diameter (small) • Equal to a teaspoon on Earth weighing 1 million tons (very dense) • Composed of mainly neutrons • Dense core can’t support self, collapses

21. The end of a High Mass Star… • If leftover core is +3 solar masses: Black hole • Collapse so great all matter (even light!) is pulled in • Nothing can escape gravitational pull (no light so can’t be seen) • Can be detected by radiation from the light

22. H-R Diagram shows how stars evolve! H-R Diagram • A way of classing stars based on: • Temperature: • Hottest (blue)Coolest (red) • Luninosity (brightness) • In comparison to our Sun (1) • Hertzsprung-Russell diagram • X-axis=temperature • Y-axis=luminosity/brightness • Main sequence is when a star is stable (fusing H+ into He)=where 90% stars occupy • As stars age & change position on H-R diagram changes • Our Sun will stay on MS for 10 billion years until changes into red giant (100 million years)white dwarf • Star Magnet Activity!

23. HOT/BRIGHT COOL/BRIGHT COOL/DIM HOT/DIM