Stellar Evolution

1. 0 Stellar Evolution

2. The Birthplace of Stars The space between the stars is not completely empty. Thin clouds of hydrogen and helium, seeded with the “dust” from dying stars, form in interstellar space.

3. Dark Clouds gather

4. Molecular Clouds Sometimes (especially in spiral arms), the gas is compressed enough that the dust is thick and gravity can collapse knots in these “molecular” clouds to make new stars.

5. Formation of stars • First what is formed is a protostar • Protostars with mass 0.08MS do not develop the pressure and temperature necessary to initiate nuclear fusion reaction and will contract to become a brown dwarf. • Protostars with mass greater than 100MS develop a pressure a pressure much higher than the gravitational pressure that disrupts the evolution of the star • When the star is formed, it joins the main sequence the star, it location depending on the mass of the star.

6. 0 Evolution on the Main Sequence (I) Main-Sequence stars live by fusing H to He. MS evolution Zero-Age Main Sequence (ZAMS) Finite supply of H => finite life time.

7. 0 Evolution on the Main Sequence (II)

8. 0 Evolution off the Main Sequence: Expansion into a Red Giant • H in the core completely converted into He – radiation reduces and core begins to collapse due to gravity resulting in an increase in temperature • This leads to “H burning” in the outer shell around the core • Thus the core continues shrink while the outer shell expands and cools • This is the RED GIANT phase

9. 0 Expansion onto the Giant Branch Expansion and surface cooling during the phaseof an inactive He core and a H- burning shell Sun will expand beyond Earth’s orbit!

10. 0 Red Giant Evolution He-coregets denser and hotteruntil thenext stage of nuclear burningcan beginin the core: 4 H → He He fusion: 3 4He → 12C “Triple-Alpha Process” Fusion of Helium into Carbon He This is followed by 4He +12C→ 16O

11. Evolution after Red Giant Phase – Low mass stars • For mass less than 4MS the star becomes unstable. The star loses the outer envelope of the star of gases exposing the inner core of oxygen and carbon  Galactic nebula. Eventually this core cools to become white dwarf • If the mass of white dwarf is less than 1.4MS , it is able to support itself due to electron degeneracy pressure and remain stable • When the white dwarf mass exceeds this limit (Chandrasekhar’s limit), then it collapses further due to gravity to become neutron star

12. Evolution after Red Giant Phase – High mass stars • For mass greater than 4MS , fusion in the core continues resulting in the formation of Ne, Si, and finally Fe. • No more thermonuclear reaction happen, and gravity takes over collapsing the core. This collapse is an IMPLOSION termed as the type II Supernova

13. Evolution after Red Giant Phase – High mass stars • During the Supernova collapse, the protons and neutrons are crushed to form neutrons. Eventually the remnant of this collapse is a neutron star • When the mass of the neutron star exceeds 2-3 times the mass of Sun (this limit is not precisely estimated), neutron degeneracy pressure does not allow stability. The neutron star collapses further to become a Black hole. This is known as the Oppenheimer-Volkoff limit

15. 0 Summary of Post-Main-Sequence Evolution of Stars Fusion proceeds to formation of Fe core. Evolution of 4 - 8 Msun stars is still uncertain. Fusion stops at formation of C,O core. M > 8 Msun Red dwarfs: He burning never ignites M < 4 Msun M < 0.4 Msun

16. 0 High-mass stars evolve off the main sequence (to become red giants) earlier than low-mass stars. => For a given age, low-mass stars are still on the MS, while high-mass stars are already red giants!

17. Red shift of light from galaxies • Due to expansion of universe, galaxies move away from each other. • This leads to red shift in the light received from these galaxies (Doppler Effect)

18. Red shift formula The relativistic red shift formula is (z is red shift parameter) At low speeds

19. Red shift formula - problem Estimate the speed of a galaxy, if the wavelength for the hydrogen line at 434nm is measured on earth as being 610nm.

20. Hubble’s law "The distance to objects beyond the Local Group is closely related to how fast they seem to be receding from us" “ v ” is the recessional speed of the galaxy, “ d” is the distance of the galaxy from us and “ H” is the Hubble parameter or Hubble constant. H = 71 km/s/Mpc = 22km/s/Mly

21. Hubble’s law - limitations • Can be applied to galaxies other than local cluster • Galaxies in local cluster may even show blue shift (Andromeda moves towards Milky Way) • Distance and speed of distant galaxies cannot be accurately estimated – this lead to uncertainties in the estimation of Hubble’s constant

22. Measurement of Hubble constant • Observing Cepheid variables in different galaxies • Observing Supernova explosions (READ FURTHER TO COMPLETE THIS TOPIC)

23. Age of the universe • Reciprocal of Hubble constant gives a rough estimate of the age of the universe (Here we make a assumption that H is really a constant) • Rough estimate of the age of universe is 10 to 20 billion years

24. Sample problem based on Hubble law

25. Solution to problem in previous slide

26. Smile please ….Astrophysics class is FINALLY OVER ….