The Life Cycle of a Star

1. The Life Cycle of a Star

2. Star: Self- Luminous Celestial Object

3. Constellations: Patterns of Stars

4. Our Sun • Nuclear fusion is the fuel • Heat and pressure are so intense that matter exists in its fourth physical state • Plasma: gas-like clouds that consist of charged particles (nuclei and electrons) that respond strongly and collectively to electromagnetic fields

5. The Sun’s Layers • Corona: • Thin and very bright outer atmosphere • ~ 1.0 x 106 - 3.0 x 106oC • Chromosphere • Inner layer of the atmosphere • ~20,000 oC • Hydrogen emits light with a distinctive reddish colour • Prominence: dense clouds of material suspended above the surface by magnetic fields • Protosphere • The visible surface • Granules • 1,000 km wide • Last about 20 min • ~ 6,000 oC

6. The Sun’s Core • Mostly hydrogen and helium in a plasmid state • ~ 1.6 x 107 oC • Surrounded by radiative zone (~ 8 x 106oC) • Convection zone • Surrounds radiative zone • ~ 15 x 106 oC • Rising and falling currents of plasma that carry energy to the sun’s surface

8. Sunspots and Solar Winds • Sunspots • Dark spots in the protosphere • Some as large as 50,000 miles • Contract and expand as they move across the surface of the sun • Cooler than its surroundings due to a strong magnetic field there that inhibits the transport of heat • Solar winds: • Constant stream of electrically charged particles deflected by the Earth’s magnetic field

10. Solar Flares Outbursts of light that rise up suddenly in areas of sunspot activity

11. The Life Cycle of A Star Hertzsprung-Russell Diagram

12. Hertzsprung-Russell Diagram • Dot = star • Position of dot = luminosity and temperature • Luminosity (aka absolute magnitude) = amount of energy a star radiates in one second; how bright or how dim the star appears. • Hotter things are brighter. • Bigger things seem brighter. • Brightness depends on size and temperature

13. The Right Ingredients for Birth • Place of birth: • Molecular clouds (place where molecular hydrogen can form) • Matter and energy can be replenished through stellar winds, planetary nebulae, and supernova explosions • The interstellar medium: • Interstellar dust: dark patches that block light from stars behind them (silicate, graphite, and polycyclic aromatic hydrocarbons) • Interstellar gas (mainly hydrogen, but also water, carbon monoxide, ammonia, and formaldehyde)

14. The Birth of a Star • Stars are born in the coldest places in the Galaxy (~ 20K or -253oC) • Place of birth consists of molecular hydrogen • Cold Temperature means slow speeds • Gravity brings interstellar material together by overwhelming pressure

15. The Protostar • The core of a giant molecular cloud starts to collapse • Increased gravitational force causes the molecules to pick up speed as they fall inward • Higher pressure caused by increase in temperature prevents further collapse • The hot core is now called a protostar

16. The center of the protostar continues to shrink and become hotter • The inner portion of the collapsing matter becomes so hot and dense that nuclear fusion starts (Hydrogen into helium)

17. The Main Sequence Life of Stars • 90% of stars in the sky are on the main sequence • Nuclear fusion causes the number of particles to decrease • Core shrinks slightly • Temperature in the core increases due to gravity • Fusion rate increases • Core releases more energy (star becomes more luminous) • Outer portions of the star expands • The surface cools

18. Star Death • The primary difference between the evolution of stars of various masses is in the amount of time they spend as protostars and main sequence stars • The mass of the star will determine how its life will end

19. Very Low Mass Stars • Hydrogen from throughout the star is cycled through the core • The star runs low on hydrogen • The rate of fusion decreases • Core contracts and temperature increases • Gravitational energy is converted into thermal energy • Thermal energy is distributed by convection • Entire star contracts and heats up forming a white dwarf

20. The Red Giant Stage • The core begins to run low on hydrogen fuel • Core shrinks dramatically • The electrons are packed as densely as possible • Matter of the core is said to be degenerate • Contraction converts gravitational energy into thermal energy and radiation • Increase in radiation causes the shell of material around the core to heat up enough that hydrogen fusion begins there • Thermal energy in the core and fusion in the surroundings cause the outerpart to expand and cool • Once the exhaustion of fuel takes place, red giants become white dwarfs

21. Supergiants • Massive stars have short main sequence lives • After its main sequence life stage, they expand and become red giants • Gradual change in the core from hydrogen fusing to helium fusing • Core temperature is greater therefore, it becomes brighter • As core pressure and temperature increase, heavier elements like Ne, Si, Fe are produced in the core

22. Death of a Star Like the Sun • When hydrogen is used up, core shrinks due to gravity • Core temperature increases to the point in which He fuses into C and O • H and He fusion continues in the layers surrounding core • Gases at the surface begin to blow away • A white dwarf is left behind surrounded by a halo of gases called planetary nebula, which eventually will fade

23. Death of a Massive Star - Hydrogen is fused more quickly and continue until Fe nuclei are formed • Formation of Fe nuclei does not release energy • Fe core quickly and suddenly collapses • Supernova • Cu, U, Ag, Pb are formed • Core is left behind - dense mass of neutrons (neutron star) • Neutron star spins and gives off radio waves (pulsars) • If star was 15x the mass of sun -> black hole