5.2STELLAR EVOLUTION<br />
Stellar Models<br />
Star Birth<br />Recall stars are born in a rotating cloud of interstellar gas and dust in space known as a nebula.<br />Th...
Upper End of Main Sequence<br />Astronomers don’t find stars more massive than 100 M(solar masses).<br />Massive clouds f...
Lower End of Main Sequence<br />In stars below 0.08 M, stellar progenitors do not get hot enough in their cores to ignite...
Brown Dwarfs<br />Hard  to detect because they are very faint and cool (2500 K  dull, muddy-red/brown color); mainly emit...
Evolution on the Main Sequence<br />A normal main sequence star supports its weight by fusing H into He.<br />Chemical com...
Evolution on the Main Sequence<br />How long a star spends on the main sequence is dependent upon its mass.<br />Massive s...
Trouble Looms for Earth<br />When the Sun began its life approximately 5 BYA, it had only about 70% of its present luminos...
Life Expectancy of Stars<br />The amount of fuel a star has is proportional to its mass; the rate at which it burns its fu...
Life Expectancy of Stars<br />For example, how long can a 4 solar-mass star live, relative to the Sun?<br />1/32 solar lif...
Post-Main Sequence Evolution<br />When the H in a star’s core is completely converted into He, “Hydrogen Burning” (fusion ...
Degenerate Matter<br />He core continues to contract  gravity squeezes it tighter and it becomes very small.<br />When a ...
Red Giant Evolution<br />As temperatures continue to increase in the He core, approaching 100,000,000 K, He nuclei begin t...
Fusion Into Heavier Elements<br />Fusion into elements heavier than C and O require very high temperatures; occurs only in...
Death of Low-Mass Stars<br />Stars < 0.4 M are known  as red dwarfs, and can survive a long time.<br />Small mass  littl...
Death of Sun-like Stars<br />Medium-mass stars like the Sun, roughly between 0.4 – 4 M, fuse H and later He, but cannot g...
Death of Sun-like Stars<br />The “final breath” of medium-mass stars like our Sun, may come when the outward pressure of a...
Dumbbell Nebula<br />
Ring Nebula<br />
Eskimo Nebula<br />
Cat’s Eye Nebula<br />
Helix Nebula<br />
Hourglass Nebula<br />
Ant Nebula<br />
White Dwarfs<br />Among the most common stars.<br />Sirius B is an example of a white dwarf star.<br />Interior mainly C a...
White Dwarfs<br />Very dense  1 tsp ~ 16 tons<br />Chunk of white dwarf material the size of a beach ball would outweigh ...
Chandrasekhar Limit<br />Math equations predict if you add mass to a white dwarf, its radius would shrink due to an increa...
Evolution of Binary Stars<br />Binary stars can sometimes interact by transferring mass from one star to another.<br />The...
Nova Explosions<br />An explosion on the surface of a white dwarf in a binary system is called a nova.<br />Occurs when ma...
Death of Massive Stars<br />High-mass stars > 8 M live short, violent lives and have much too much mass to die as white d...
Crab Nebula<br />
Types of Supernovae<br />Supernovae are rare and only a few have been seen in our galaxy along with some other galaxies.<b...
Neutron Stars<br />When a star > 8 M explodes (supernova), its central core usually collapses into a compact object more ...
Neutron Stars<br />
Neutron Stars<br />A piece of neutron star material the size of a sugar cube would have a mass ~ 100 million tons!<br />Th...
Pulsars<br />Pulsars are thought to be spinning neutron stars.<br />Emit winds and jets of high-energy particles; slows ro...
Lighthouse Model of Pulsars<br />A pulsar’s magnetic field has a dipole, similar to that of Earth’s.<br />Radiation emitte...
Black Holes<br />Similar to how collapsing cores of white dwarfs have a mass limit (Chandrasekhar Limit), there is a mass ...
Black Holes<br />Since light cannot even escape, the inside of a black holes is totally beyond the view of an outside obse...
Schwarzschild Radius<br />
General Relativity Effects<br />At a distance, the gravitational fields of a black hole and a star of the same mass are vi...
General Relativity Effects<br />An astronaut descending down towards the event horizonof a black holewill be stretched ver...
Black Hole Candidates<br />
AST 5.2 PPT
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AST 5.2 PPT

  1. 1. 5.2STELLAR EVOLUTION<br />
  2. 2.
  3. 3. Stellar Models<br />
  4. 4. Star Birth<br />Recall stars are born in a rotating cloud of interstellar gas and dust in space known as a nebula.<br />The nebula is rotating due to angular momentum and gravity enables the in-falling of material to the center of the nebula.<br />The contraction of a dense core as matter falls in results in heat production and the object gradually behaves more like a star.<br />This collapsing cloud of gas and dust, destined to become a star is known as a protostar, and marks the first stage of star development.<br />Only detectable at IR wavelengths.<br />
  5. 5.
  6. 6. Upper End of Main Sequence<br />Astronomers don’t find stars more massive than 100 M(solar masses).<br />Massive clouds fragment into smaller pieces during star formation.<br />Very massive stars lose mass in strong stellar winds.<br />Eta Carinae is a binary star system of 60 Mο and 70 Mο<br />
  7. 7. Lower End of Main Sequence<br />In stars below 0.08 M, stellar progenitors do not get hot enough in their cores to ignite Hydrogen fusion.<br />These objects are known as brown dwarfs and slowly contract, converting their gravitational energy into thermal energy and radiating it away  star or planet debate?<br />
  8. 8. Brown Dwarfs<br />Hard to detect because they are very faint and cool (2500 K  dull, muddy-red/brown color); mainly emitting at infrared wavelengths.<br />
  9. 9. Evolution on the Main Sequence<br />A normal main sequence star supports its weight by fusing H into He.<br />Chemical composition in its core changes and the star evolves.<br />Core contracts and temperature and density increase  outer layers expand and cool (larger, brighter, cooler).<br />When a star begins its life, it settles on the lower edge of a band called the zero-age main sequence (ZAMS).<br />
  10. 10. Evolution on the Main Sequence<br />How long a star spends on the main sequence is dependent upon its mass.<br />Massive stars  lots of fuel, but they use it rapidly and live short lives.<br />A 25 Mwill exhaust its H and die in ~ 7 million years.<br />Recall the Sun can fuse H into He for ~ 10 billion years.<br />Red dwarfs, which will be discussed later, have low masses and use their fuel so slowly they should survive for hundreds of billions of years.<br />Since the universe is estimated to be only about 14 billion years old, red dwarfs must still be in their infancy.<br />
  11. 11.
  12. 12. Trouble Looms for Earth<br />When the Sun began its life approximately 5 BYA, it had only about 70% of its present luminosity.<br />By the time it leaves the main sequence in another 5 billion years, it will have twice its present luminosity.<br />Average temperatures on Earth are estimated to climb 100°F and above.<br />Polar ice caps melt, oceans evaporate, atmosphere will vanish into space.<br />
  13. 13. Life Expectancy of Stars<br />The amount of fuel a star has is proportional to its mass; the rate at which it burns its fuel is proportional to its luminosity  you could make a first estimate of the star’s life expectancy by dividing its mass by its luminosity.<br />Recall the mass-luminosity relation: L = M3.5<br />Therefore: T = M/L = M/M3.5<br />Star Life Expectancy Formula<br />T = 1/M2.5<br />T = life expectancy, in solar lifetimes<br />M = mass, in solar masses<br />
  14. 14. Life Expectancy of Stars<br />For example, how long can a 4 solar-mass star live, relative to the Sun?<br />1/32 solar lifetimes<br />~<br />10 billion/32 <br />~ <br />310 million years<br />
  15. 15. Post-Main Sequence Evolution<br />When the H in a star’s core is completely converted into He, “Hydrogen Burning” (fusion of H into He) stops in the core.<br />H burning continues in a shell around the core.<br />The He core contracts and heats up, producing more energy than it needs to balance its own gravity.<br />Outer layers expand and cool producing ared giant.<br />
  16. 16.
  17. 17. Degenerate Matter<br />He core continues to contract  gravity squeezes it tighter and it becomes very small.<br />When a gas is so dense that its electrons are not free to change their energy, astronomers call it degenerate matter.<br />
  18. 18. Red Giant Evolution<br />As temperatures continue to increase in the He core, approaching 100,000,000 K, He nuclei begin to fuse together to make Carbon.<br />Three He nuclei are needed to make a C nucleus, and a He nucleus is called an alpha particle, therefore astronomers refer to He fusion as the triple-alpha process:<br />Onset of He fusion occurs very rapidly in an event known as a Helium flash.<br />4He + 4He 8Be + g<br />8Be + 4He 12C + g<br />
  19. 19. Fusion Into Heavier Elements<br />Fusion into elements heavier than C and O require very high temperatures; occurs only in very massive stars ( > 8 M).<br />
  20. 20. Death of Low-Mass Stars<br />Stars < 0.4 M are known as red dwarfs, and can survive a long time.<br />Small mass  little weight to support.<br />H and He well mixed throughout.<br />No phase of “shell burning” to expansion of red giant.<br />Slow conversion of H into He; not hot enough to ignite He fusion leads to slow death contracting into white dwarf (black dwarf eventual fate).<br />
  21. 21. Death of Sun-like Stars<br />Medium-mass stars like the Sun, roughly between 0.4 – 4 M, fuse H and later He, but cannot get hot enough to ignite C, the next fuel in the sequence.<br />Interiors contract while their envelopes expand, turning them into red giants.<br />Develops a core rich in C and O.<br />Not hot enough to fuse C  can’t resist weight so it contracts; energy released makes the star’s surrounding envelope expand and cool further.<br />Such stars can lose large amounts of mass from their surfaces.<br />
  22. 22. Death of Sun-like Stars<br />The “final breath” of medium-mass stars like our Sun, may come when the outward pressure of an aging giant expels its outer atmosphere in repeated surges in what is known as a planetary nebula.<br />Despite the name, it has nothing to do with planets.<br />Most scientists believe the Sun will form a planetary nebula, but it is still debated.<br />STAGE 1<br />STAGE 2<br />
  23. 23. Dumbbell Nebula<br />
  24. 24. Ring Nebula<br />
  25. 25. Eskimo Nebula<br />
  26. 26. Cat’s Eye Nebula<br />
  27. 27. Helix Nebula<br />
  28. 28. Hourglass Nebula<br />
  29. 29. Ant Nebula<br />
  30. 30. White Dwarfs<br />Among the most common stars.<br />Sirius B is an example of a white dwarf star.<br />Interior mainly C and O nuclei floating among a whirling storm of degenerate electrons (crystal formation?)<br />Doesn’t generate any nuclear energy  simply degenerate material.<br />Instead of calling a white dwarf a star, you can refer to it as a compact object.<br />
  31. 31. White Dwarfs<br />Very dense  1 tsp ~ 16 tons<br />Chunk of white dwarf material the size of a beach ball would outweigh an ocean liner.<br />Because white dwarfs contain a tremendous amount of heat, they need billions of years to radiate the heat through its small surface area.<br />Eventually, such objects may become cold and dark, known as black dwarfs.<br />Our galaxy is not old enough to contain black dwarfs.<br />The coolest white dwarfs in our galaxy are just a bit cooler than the Sun.<br />
  32. 32.
  33. 33. Chandrasekhar Limit<br />Math equations predict if you add mass to a white dwarf, its radius would shrink due to an increase in its gravity, causing it to squeeze tighter together.<br />If you added enough to raise its total to about 1.4 M, its radius would shrink to zero.<br />This is called the Chandrasekhar Limitafter Subrahmanyan Chandrasekhar.<br />Therefore, a star’s collapsing core that is more massive than 1.4 M cannot become a white dwarf.<br />Theoretical models show an 8 M star should be able to reduce its mass to 1.4 M before it collapses.<br />
  34. 34. Evolution of Binary Stars<br />Binary stars can sometimes interact by transferring mass from one star to another.<br />The gravitational fields of the two stars, combined with the rotation of the binary system, define a dumbbell-shaped volume around the pair of stars called the Roche lobes.<br />
  35. 35. Nova Explosions<br />An explosion on the surface of a white dwarf in a binary system is called a nova.<br />Occurs when mass transfers from a normal star through the Inner Lagrangian point into an accretion disk around a white dwarf.<br />Mass transfer resumes shortly after  new layer of fuel begins to accumulate.<br />
  36. 36. Death of Massive Stars<br />High-mass stars > 8 M live short, violent lives and have much too much mass to die as white dwarfs. <br />Evolution similar to lower-mass stars (consume H in their cores and expand into supergiants).<br />Heavy element fusion ends with an Iron (Fe) core being produced; ultimately collapses triggering an explosion destroying the star known as a supernova.<br />
  37. 37. Crab Nebula<br />
  38. 38. Types of Supernovae<br />Supernovae are rare and only a few have been seen in our galaxy along with some other galaxies.<br />Astronomers have noticed 2 main types:<br />Type I supernovae have spectra containing no Hydrogen lines.<br />Type II supernovaehave spectra containing Hydrogen lines.<br />
  39. 39. Neutron Stars<br />When a star > 8 M explodes (supernova), its central core usually collapses into a compact object more massive than the Chandrasekhar Limit(1.4 M) not able to form a white dwarf.<br />The pressure becomes so high at this point that electrons and protons combine to form stable neutrons throughout the compact object now termed a neutron star.<br />p + e-n + ne<br />
  40. 40. Neutron Stars<br />
  41. 41. Neutron Stars<br />A piece of neutron star material the size of a sugar cube would have a mass ~ 100 million tons!<br />The principal of conservation of angular momentum predicts that neutron stars should spin rapidly.<br />All stars rotate because they form from swirling clouds of interstellar matter.<br />The 1967 detection of regularly spaced pulses in the output of a radio telescope led to the discovery of pulsars.<br />
  42. 42. Pulsars<br />Pulsars are thought to be spinning neutron stars.<br />Emit winds and jets of high-energy particles; slows rotation slightly.<br />
  43. 43. Lighthouse Model of Pulsars<br />A pulsar’s magnetic field has a dipole, similar to that of Earth’s.<br />Radiation emitted mainly along the poles.<br />
  44. 44. Black Holes<br />Similar to how collapsing cores of white dwarfs have a mass limit (Chandrasekhar Limit), there is a mass limit to neutron stars (3 M).<br />If the core of a star contains > 3 M  when it collapses, no force can stop it.<br />If an object collapses to zero radius, its density and gravity become infinite in a point known as the singularity.<br />Both space and time will stop.<br />If the contracting core becomes small enough, the escape velocity in the region of space around it is so large not even light can escape and is known as a black hole.<br />
  45. 45. Black Holes<br />Since light cannot even escape, the inside of a black holes is totally beyond the view of an outside observer.<br />The event horizon is the boundary between the isolated volume of space-time and the rest of the universe.<br />The radius of the event horizon is called the Schwarzschild radius.<br />Limiting radius where escape velocity reaches the speed of light (c).<br />Named after Karl Schwarzschild<br />Schwarzschild Radius Formula<br />Rs = 2GM/c2<br />G = universal gravitational constant ~ 6.67 x 10-11 m3/kgs2<br />M = mass, in kg<br />c = speed of light ~ 3 x 108 m/s<br />
  46. 46. Schwarzschild Radius<br />
  47. 47. General Relativity Effects<br />At a distance, the gravitational fields of a black hole and a star of the same mass are virtually identical.<br />At smaller distances however, the much deeper gravitational potential will become noticeable.<br />
  48. 48. General Relativity Effects<br />An astronaut descending down towards the event horizonof a black holewill be stretched vertically and squeezed laterally in an effect known as spaghettification.<br />
  49. 49. Black Hole Candidates<br />
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