The sun - past and future The Later Evolution of Low Mass Stars (< 8 solar masses) During 10 billion years the suns luminosity changes only by about a factor of two. After that though, changes become rapid http://apod.nasa.gov/apod/astropix.html What happens when the sun runs out of hydrogen in its center? He H The outer envelopes of He 10.9 x 10 9 yr H L T All surfaces become convective
Recall convective structure on the main sequence Evolution to Red Giant Stage cross hatching indicates where in the HRdiagram a star spends a long time. H-shell ignition M > 2.0 Type I Cepheids are shortlived-stages of massive stars as the cross the HR diagram on the way to becoming red giants.
Red Giant 1.3 Gy He core burning 100 My Helium Burning and Beyond 3
* lasting only a few minutes. Up to 100 billion solar luminosities in center. Adjustment time scale ~10 5 yr http://en.wikipedia.org/wiki/helium_flash Helium Burning The net result is: 4 He + 4 He 8 Be 8 Be + 4 He 12 C + Helium burning, often called the triple alpha process occurs above temperatures of 100,000,000 K. 8 Be is unstable and decays back into He in 2.6 10 16 secs, but in the stellar interior a small equilibrium of 8 Be exists. The 8 Be ground state has almost exactly the energy of two alpha particles. In the second step, 8 Be + 4 He has almost exactly the energy of an excited state of 12 C. This resonance greatly increases the chances of Helium fusing and was predicted by Fred Hoyle. As a side effect some Carbon fuses with Helium to form Oxygen: 3( 4 He) 12 C + 7.27 MeV or 5.8 x 10 17 erg g -1 The extra burning to oxygen, 12 C(,) 16 O raises this to 7.5 x 10 17 erg g -1, or about 10% of what hydrogen burning gave. Because helium burning produces less energy and because the luminosities are actually greater, helium burning is a shorter stage in the life of a star than the main sequence. 12 C + 4 He 16 O +
The Sun Horizonal Branch Star For stars of one solar mass and less, lower mass HB stars are hotter (bluer) than higher mass ones The globular cluster M10. The bright yellow and orange stars are red giants burning hydrogen in a shell, but the bright blue stars are horizontal branch stars, burning helium in their centers. Both kinds of stars are more massive and brighter than the low mass main sequence stars in M10.
Globular Cluster M5 http://www.dur.ac.uk/ian.smail/gccm/gccm_intro.html M TO = turn off mass; HB = horizonal branch; Gap is a region of atmospheric instability
The Seven Ages of the Sun AGB STARS Main sequence 10.9 Gy First red giant 1.3 Gy Helium burning 100 My Second red giant 20 My Unstable pulsation 400 Ky Planetary nebula 10 Ky White dwarf forever Cutaway drawing of the interior structure of an Asymptotic Giant Branch or AGB star. Hydrogen an helium burning shells are both active, though not necessarily both at the same time. The He and H burning regions are much thinner than this diagram suggests. The outer layers are convective. The C-O core is degenerate and transports its radiation by conduction. http://en.wikipedia.org/wiki/ring_nebula AGB stars are known to lose mass at a prodigious rate during their final stages, around 10-5 - 10-4 solar masses per year. This obviously cannot persist for much over 100,000 years. The mass loss is driven in part by the pulsational instability of the thin helium shell. These pulses grow more violent with time. Also, and probably more importantly, the outer layers of the star get so large and cool owing to the high luminosity, that they form dust. The dust increases the opacity and material is blown away at speeds ~ 10 30 km s -1 The evolution is terminated as the outer layers of the star are blown away. The Ring Negula in Lyra (M57) 700 pc; magnitute 8.8
NGC 2440 White dwarf ejecting envelope. One of the hottest white dwarfs known is in the center of the picture About 200,000 K and 250 times the sun s luminosity http://homepage.oma.be/gsteene/poster.html Note the consequences for nucleosynthesis here. Blinking eye Hourglass The outer layers of the star contain hydrogen and helium to be sure, but also nitrogen from CNO processing and C and O from helium burning. It is thought that stars in this mass range are responsible for producing most of the nitrogen and maybe 60 80% of the carbon in the universe. Egg Cats Eye The rest of carbon and most other elements comes from massive stars. Kissing squid Red Rectangle
Additional Nucleosynthesis The s-process. years 44.5 days This is called the slow process of neutron addition or the s-process. (There is also a r-process) Beginning of the s-process 63 Cu 64 Cu 12.7 h The s-process of neutron addition 60 Ni 61 Ni 62 Ni 63 Ni 100 y Z 59 Co 60 Co 61 Co 5.27 y 56 Fe 57 Fe 58 Fe 59 Fe 60 Fe N 44.5 d = (n,) = (e - ) Each neutron capture takes you one step to the right in this diagram. Each decay of a neutron to a proton inside the nucleus moves you up a left diagonal.
WHITE DWARFS P c = MASS RADIUS RELATION FOR WHITE DWARFS GM 2R = 1.00 1013 (Y e ) 5/3 GM 3M GM 2R 4 R 3 2R 3M = 1.00 10 13 4 R 3 3G 8 M 2 3 R 4 = 1.00 10 13 8 5/3 M 5/3 R 5 2/3 M 1/3 3 = 1.0010 13 1 8 GR R = 3.6310 19 / M 1/3 R = 2.9 10 8 M M 1/3 cm if supported by non-relativistic electron degeneracy pressure 5/3 1 2 5/3 White dwarfs are known with temperatures ranging from 4000 K to 200,000 K
IK Pegasi A Class A star P = 21.7 days The sun IK Pegasi B T e = 35,500 K THE CHANDRASEKHAR MASS As M gets larger and the radius decreases, the density rises Eventually at greater than about 10 7 g cm 3 electrons in the central part of the white dwarf start to move close to the speed of light. As the mass continues to grow, a larger fraction of the star is supported by relativistic electron degeneracy pressure. Consider the limit: P R deg = 1.24 10 15 ( Y e ) 4/3 = GM 2R As usual examine the constant density case for guidance Nb. R drops out 3M 4 R 3 1.24 10 15 4/3 3M Y e 4 R 3 1/3 = GM 2R = P central / M 2/3 = 1.24 10 15 4/3 3 1/3 2 Y e 4 G M 2/3 = 2.3 10 22 4/3 Y e M = 3.5 10 33 Y e 2 gm = 1.75 Y e 2 M Actually M = 5.7 Y e 2 M = 1.4 M if Y e = 0.5
Aside: This result extends beyond white dwarfs. There can be no stable star whose pressure depends on its density to the 4/3 power EVOLUTION OF WHITE DWARF STARS Crystallization in white dwarfs When the interior temperature declines to ~5000 K, the carbon and oxygen start to crystallize into a lattice. This crystallization releases energy and provides a source of luminosity that slows the cooling. Hansen et al (2007) NGC 6397 - globular cluster The number counts pile up. For a WD of constant mass, R = constant
http://en.wikipedia.org/wiki/white_dwarf The coolest, faintest white dwarfs still have a surface temperature of ~4000 K. The universe is not old enough for black dwarfs to have formed yet. 0.08 M 0.50 M Critical Masses Contracting protostars below this mass do not ignite hydrogen burning on the main sequence. They become brown dwarfs or planets. Stars below this mass are completely convective on the main sequence do not ignite helium burning E.g., 0.59 solar mass WD - like the sun will make - takes about 1.5 billion years to cool to 7140 K and another 1.8 billion years to cool to 5550 K. 2.0 M Stars below this mass (and above.5) experience the helium core flash Stars above this mass are powered by the CNO cycle (below by the pp-cycles) Stars above this mass have convective cores on the main sequence (and radiative surfaces) 8 M Stars below this mass do not ignite carbon burning. They end their lives as planetary nebulae and white dwarfs. Stars above this mass make supernovae. ~ 150 M Population I stars much above this mass pulse apart on the main sequence. No heavier stars exist.