1. Lifecycle Lifecycle of a main sequence G star Most time is spent on the main-sequence (normal star)

2. Stellar Lifetimes From the luminosity, we can determine the rate of energy release, and thus rate of fuel consumption Given the mass (amount of fuel to burn) we can obtain the lifetime Large hot blue stars: ~ 20 million years The Sun: 10 billion years Small cool red dwarfs: trillions of years  The hotter, the shorter the life!

3. Mass Matters Larger masses –higher surface temperatures –higher luminosities –take less time to form –have shorter main sequence lifetimes Smaller masses –lower surface temperatures –lower luminosities –take longer to form –have longer main sequence lifetimes

4. Mass and the Main Sequence The position of a star in the main sequence is determined by its mass  All we need to know to predict luminosity and temperature! Both radius and luminosity increase with mass

5. Main Sequence Lifetimes Mass (in solar masses) Luminosity Lifetime 10 Suns 10,000 Suns 10 Million yrs 4 Suns 100 Suns 2 Billion yrs 1 Sun 1 Sun 10 Billion yrs ½ Sun 0.01 Sun 500 Billion yrs

6. Is the theory correct? Two Clues from two Types of Star Clusters  Open Cluster Globular Cluster 

7. Star Clusters Group of stars formed from fragments of the same collapsing cloud Same age and composition; only mass distinguishes them Two Types: –Open clusters (young  birth of stars) –Globular clusters (old  death of stars)

8. What do Open Clusters tell us? Hypothesis: Many stars are being born from a interstellar gas cloud at the same time Evidence: We see “associations” of stars of same age  Open Clusters

9. Why Do Stars Leave the Main Sequence? Running out of fuel

10. Stage 8: Hydrogen Shell Burning Cooler core  imbalance between pressure and gravity  core shrinks hydrogen shell generates energy too fast  outer layers heat up  star expands Luminosity increases Duration ~ 100 million years Size ~ several Suns

11. Stage 9: The Red Giant Stage Luminosity huge (~ 100 Suns) Surface Temperature lower Core Temperature higher Size ~ 70 Suns (orbit of Mercury)

12. The Helium Flash and Stage 10 The core becomes hot and dense enough to overcome the barrier to fusing helium into carbon Initial explosion followed by steady (but rapid) fusion of helium into carbon Lasts: 50 million years Temperature: 200 million K (core) to 5000 K (surface) Size ~ 10  the Sun

13. Stage 11 Helium burning continues Carbon “ash” at the core forms, and the star becomes a Red Supergiant Duration: 10 thousand years Central Temperature: 250 million K Size > orbit of Mars

14. Deep Sky Objects: Globular Clusters Classic example: Great Hercules Cluster (M13) Spherical clusters may contain millions of stars Old stars Great tool to study stellar life cycle

15. Observing Stellar Evolution by studying Globular Cluster HR diagrams Plot stars in globular clusters in Hertzsprung-Russell diagram Different clusters have different age Observe stellar evolution by looking at stars of same age but different mass Deduce age of cluster by noticing which stars have left main sequence already

16. Catching Stellar Evolution “red-handed” Main-sequence turnoff

17. Type of Death depends on Mass Light stars like the Sun end up as White Dwarfs Massive stars (more than 8 solar masses) end up as Neutron Stars Black HolesVery massive stars (more than 25 solar masses) end up as Black Holes

18. Reason for Death depends on Mass Light stars blow out their outer layers to form a Planetary Nebula The core of a massive star (more than 8 solar masses) collapses, triggering the explosion of a Supernova SupernovaAlso the core of a very massive stars (more than 25 solar masses) collapses, triggering the explosion Supernova

19. Light Stars: Stage 12 - A Planetary Nebula forms Inner carbon core becomes “dead” – it is out of fuel Some helium and carbon burning continues in outer shells The outer envelope of the star becomes cool and opaque solar radiation pushes it outward from the star A planetary nebula is formed Duration: 100,000 years Central Temperature: 300  10 6 K Surface Temperature: 100,000 K Size: 0.1  Sun

20. Deep Sky Objects: Planetary Nebulae Classic Example: Ring nebula in Lyra (M57) Remains of a dead, exploded star We see gas expanding in a sphere In the middle is the dead star, a “White Dwarf”

21. Stage 13: White Dwarf Core radiates only by stored heat, not by nuclear reactions core continues to cool and contract Size ~ Earth Density: a million times that of Earth – 1 cubic cm has 1000 kg of mass!

22. Stage 14: Black Dwarf Impossible to see in a telescope About the size of Earth Temperature very low  almost no radiation  black!

23. More Massive Stars (M > 8M Sun ) The core contracts until its temperature is high enough to fuse carbon into oxygen Elements consumed in core new elements form while previous elements continue to burn in outer layers

24. Evolution of More Massive Stars At each stage the temperature increases  reaction gets faster Last stage: fusion of iron does not release energy, it absorbs energy  cools the core  “fire extinguisher”  core collapses Region of instability: Variable Stars

25. Supernovae – Death of massive Stars As the core collapses, it overshoots and “bounces” A shock wave travels through the star and blows off the outer layers, including the heavy elements – a supernova A million times brighter than a nova!! The actual explosion takes less than a second

26. Supernova Observation

27. Type I vs Type II Supernovae

28. Type I – “Carbon Detonation” Implosion of a white dwarf after it accretes a certain amount of matter, reaching about 1.4 solar masses Very predictable; used as a standard candle –Estimate distances to galaxies where they occur

29. Type II – “Core Collapse” Implosion of a massive star Expect one in our galaxy about every hundred years Six in the last thousand years; none since 1604

30. Supernova Remnants Crab Nebula From Vela Supernova

31. SN1987A Lightcurve

32. Formation of the Elements Light elements (hydrogen, helium) formed in Big Bang Heavier elements formed by nuclear fusion in stars and thrown into space by supernovae –Condense into new stars and planets –Elements heavier than iron form during supernovae explosions Evidence: –Theory predicts the observed elemental abundance in the universe very well –Spectra of supernovae show the presence of unstable isotopes like Nickel-56 –Older globular clusters are deficient in heavy elements