The Lifetime of Stars

1. The Lifetime of Stars • Once a star has reached the main sequence stage of it life, it derives its energy from the fusion of hydrogen to helium • Stars remain on the main sequence for a long time and most of their lifetime • The left hand edge of the H-R diagram is called the zero-age main sequence • Zero age is the time when the star matures enough to reach the main sequence • As the hydrogen is used up, the mass of the star does not change but its luminosity and temperature do change Lecture 20

2. Lifetime on the Main Sequence • The amount of time a star spends on the main sequence depends on its mass • Large star are much more luminous and have shorter lifetimes • Small stars are much less luminous and live much longer Lecture 20

3. From Main Sequence to Red Giant • When all the hydrogen in the core is fused, the star must contract • Hydrogen falling into the core releases gravitational energy heating the hydrogen just outside the core and causes it to ignite • The helium begins to burn providing even more heat • The outer layers of the star heat up and expand • Rapid departure from main sequence and creation of a red giant Lecture 20

4. Comparing a Supergiant to the Sun Lecture 20

5. Models for the Evolution of Red Giants • Computer models for the evolution of red giants with different masses and compositions on the H-R Diagram Lecture 20

6. Star Clusters Globular cluster M15 • It is reasonable to assume that stars located very close together were formed at about the same time • Star clusters • Globular clusters • Old stars • Open Clusters • Young stars • Stellar associations • Young stars Lecture 20

7. Globular Clusters Omega Centauri, the largest globular cluster in the Milky Way • Globular clusters were given their name because of their appearance • They have 104 to 106 stars • The brightest stars are giants that are pale yellow in color • About 150 globular clusters are known in our galaxy located in a spherical halo surrounding the flat disk of the galaxy • They are found very far away from the Sun at distance of 65,000 LY from the galactic plane Lecture 20

8. Open Clusters Open star cluster M6, the Butterfly Cluster • Open clusters are found in the disk of our galaxy associate with interstellar matter • Open cluster contain far fewer stars than globular clusters • There are many thousands of open clusters but many are invisible because of dust clouds • Open clusters often have a few brilliant stars Lecture 20

9. Stellar Associations • Stellar associations have very young stars and are often obscured by interstellar dust • Most are hidden from our view • The association typically has 5 to 50 hot, bright O and B spectral class stars less than a million years old Lecture 20

10. Model for Young Clusters Model predictions for young clusters Measurements for young cluster NGC 2264 Lecture 20

11. Model for Older Clusters Model for older clusters Measurements for globular cluster 47 Tucane Lecture 20

12. Helium Burning • When the star uses up its hydrogen, it burn helium through the triple alpha process to form carbon • Helium has 2 p, 2 n, stable • Carbon has 6 p, 6 n (3 He), stable • Beryllium-8 is not stable (4 p, 4 n) • When the helium burning process starts in low mass stars, the entire core ignites in a helium flash • Following the helium flash, the star readjusts more toward the main sequence • When the helium is consumed, carbon burning can only take place in large stars • In stars like our Sun, death is near Lecture 20

13. Mass Loss • When stars become giants, they begin to lose mass into space • By the time a star reaches the helium flash, it will have lost 25% of its mass • The outer layers of the star are stripped and planetary nebula are formed Animation of the formation of the planetary nebula called the Helix Nebula Lecture 20

14. Nucleosynthesis • All elements heavier than helium are made in stars • The elements in globular clusters are depleted in heavier elements • They were formed from material that had not been processed in stars as much as younger stars • Newer stars have 1% - 4% heavier elements • Old stars have 1/10 to 1/100 as many heavier elements as the Sun • The first generation of stars could not have formed planets like Earth that are rich in silicon and iron Lecture 20

15. The Death of Low Mass Stars • Let’s start with stars smaller than 1.4 Msun • After the small star burns all its hydrogen and then all its helium, it has an energy crisis • Because the star is small, it cannot ignite the remaining material in its core and so the star collapses and forms a white dwarf • White dwarfs are so dense that the electrons are degenerate • So compressed that they are nearly on top of each other • Electrons cannot be on top of each other • Fermions, Pauli exclusion principle • Cannot collapse further Lecture 20

16. White Dwarfs • White dwarfs are stabilized against further collapse • Calculations show the larger the mass the smaller the radius • Stars with mass greater than 1.4 Msun have zero radius • Chandrasekhar limit • For larger stars, the force of degeneracy cannot prevent the collapse of the star • Black hole!! Lecture 20

17. Evolutionary Track of a Sun-like Star • Start the calculation when the star becomes a red giant (A) • The star loses mass and its core begins to collapse • The star heats up (B) as it collapses • The luminosity remains constant until it begins to shrink significantly and then it begins to dim • The star is now a white dwarf (c) and will continue to radiate its energy into space Lecture 20

18. The Ultimate Fate of White Dwarfs • The onset of hydrogen fusion constitutes the birth of a star and the exhaustion of of all fusion fuel mean the death of the star • The only energy source of the white dwarf is residual heat which radiates into space leaving a black dwarf • This black dwarf is composed mostly of carbon and oxygen • In its final stages the black dwarf becomes a monumental diamond! Lecture 20

19. Evolution of Massive Stars • Stars with masses less that 7.5 Msun will lose mass as they age and end up as white dwarfs • Stars are known with masses as large as 150 Msun • Massive stars continue to fuse elements after the hydrogen and helium are fused • Helium to carbon • Carbon to neon • Neon to silicon • Silicon to iron • It all stops at iron because iron is the most well bound nucleus Lecture 20

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21. Collapse into a Ball of Neutrons • A massive star builds up a white dwarf in its center where no nuclear reactions are taking place • For a large star this center is made of iron • Fusion takes place outside this core producing more heavy nuclei that fall into the core • A higher mass means smaller radius so the core contracts • The core density surpasses that supported by degenerate electrons and passes over to degenerate neutrons • Neutron star! Lecture 20

22. Collapse and Explosion • The collapse of the core to a neutron star takes place catastrophically • The core goes from Earth size to 20 km in 1 second • When the core reaches nuclear density the collapse is halted abruptly and a shock wave bounces back through the star blowing off the outer layers of the star • Supernova! • Supernovae have been observed in history • 1006 • 1054, Crab nebula • 1181 • 1572, Tycho Brahe • 1605, Johannes Kepler • 1987, Supernova 1987A Lecture 20

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24. Material Ejected by Supernova • Most of the material of the star is ejected into space • This material is very important because the processed nuclear material are recycled into space • In addition, the supernova produces a flood of neutrons that can be absorbed by iron and other nuclei to build up all the heavier elements • Iron is Z=26 and elements up to Z=92 occur naturally • Supernova are the source of high energy cosmic rays that have contributed to mutation and evolution • However, you would not want to be near a supernova when it explodes • Anything within 100 LY would be disastrous Lecture 20

25. Supernova 1987A • First observed February 24, 1987 in the Large Magellanic Cloud • Thought to be 10 million years old with a mass of 20 Msun 160,000 LY away • When it became a red giant, material was ejected • Helium fusion last about 1 million years forming a core of carbon and oxygen • When the helium was exhausted, carbon and oxygen burning began and the star became a blue supergiant • When the carbon was exhausted, burning to heavier elements only lasted for a few years • Once iron was created, collapse occurred Lecture 20

26. Collapse of SN 1987A • Collapse occurred in a few 0.1 of a second blowing off the outer layers of the star Picture taken in 1994 Picture taken in 1997 • Pictures show collision of material from supernova colliding with material ejected from red giant stage Lecture 20

27. SN 1987A • Up until 40 days after the explosion, the light produced originated from the explosion • After that time, the radioactive decay of the produced heavy elements kept the star bright • 56Ni to 56Co to 56Fe, 6 days and 77 days • Most of the energy emitted by SN 1987A was in the form of neutrinos • 11 neutrinos were seen in Japan and 8 in the US over a span of 13 seconds • These neutrinos were detected after passing through the Earth Lecture 20

28. The Discovery of Neutron Stars • Neutron stars are giant nuclei, 1057 nucleons • The radius of a neutron star is about 10 km • They were first discovered in 1967 when Bell and Hewish found an intense, regularly varying radio source that repeated every 1.33728 seconds • Pulsars • Soon after several more were found • A pulsar was found in the center of the Crab Nebula • Supernova of 1054 Lecture 20

29. Model of Pulsar • Pulsars are spinning neutron stars • Their collapse has made them spin very rapidly • Conservation of angular momentum • Radiation comes from the north and south magnetic poles Lecture 20

30. The Evolution of Pulsars • The pulsar gives off immense amounts of energy and must slow down over time • Old pulsars are slower than new pulsars • The pulsar in the Crab Nebular has been observed to be slowing down over the past 34 years • Pulsars are not always observed in the center of a nebular left over from a supernova • Pulsars live 100 times longer than the time it takes for a nebula resulting from a supernova to disperse • Pulsars may also be ejected from the region of the supernova by the explosion Lecture 20

31. Binary Systems • As many as half of all stars exist in binary systems • If one of the stars is white dwarf, material from other star can accumulate on that star causing a reignition of the hydrogen • Nova • Thousands have been observed, all in binary systems • The white dwarf survives • If the white dwarf is large, the explosion may destroy the white dwarf • Supernova type I Lecture 20

32. Neutron Stars with Companions • A binary system can survive the explosion of one of the stars • An ordinary star can be paired with a neutron star • It is possible for the neutron star to accrete material from its partner and become a gamma-ray emitter or an x-ray burster • These bursters can rotate 1000 times a second (compared with pulsars of 30 per second) • Another fate of star is the black hole which is the subject of our next lecture Lecture 20