The Sun and the Stars

1. The Sun and the Stars The Sun and the Stars

2. The Sun and the Stars Stellar evolution i) Stellar birth HST image of The Eagle nebula, a stellar nursery Stars appear to be born in groups – why?

3. The Sun and the Stars Consider giant molecular cloud (H2 region), with mass M, radius R, no. of particles N, of average mass m, at temperature T The potential energy of the cloud U, is given by The total kinetic energy (KE) of the particles in the cloud is, Alternatively, this can be written in terms of the Jeans density, J, where J is given by For collapse, So for collapse require low temperatures, and large masses So, Where MJis the Jeans mass

4. The Sun and the Stars • Molecular cloud (H2 region) collapses under its own self-gravity • (free-fall  internal pressure zero, no collisions) • NB fragmentation likely. • [Exact trigger unknown - possibly density inhomogeneities • shocks (SN), winds etc.] • As cloud collapses, PE converted into KE+ radiation • in roughly equal proportion • Collapse continues provided: • KE dissipated • Radiation escapes • If these conditions satisfied, gas remains cool, pressure remains low, collapsing material is a Protostar Collapse continues until density of core high enough that   1, core optically thick. Radiation trapped, core heats up, collapse stalls. Hydrostatic equilibrium established. Star is now a Pre-Main Sequence star

5. Cloud Collapse

6. The Sun and the Stars Stellar birth cont’d • Because T low, opacity high, PMS star is fully convective • Heat transported rapidly to surface, R is large, therefore L is large (30xLsun, point B on next page) PMS contracts slowly, core heats up, but T(sfce)constant • L decreases slowly (point C). As core Temp rises opacity decreases, radiation becomes increasingly dominant transport mechanism and moves outward slowly from core. Once radn transport exceeds convection, sharp kink to left on H-R diagram (point D) Eventually T core high enough (few million K) that nuclear fusion dominates energy production. Hydrostatic equilibrium now maintained by fusion, star stops contracting. Star is now a Zero Age Main Sequence Star (ZAMS) E. Star radiative in core, convective in outer layers. Takes ~20 million years to reach ZAMS from initial collapse. A solar mass star will spend approx 10 billion years on main sequence quietly converting H  He in the core via the PP-chain.

7. The Sun and the Stars Evolutionary track for a PMS star A core has developed B PMS fully convective (L~30xLsun) C core contracts without change in Tsfce D radiative transport dominates E T> few million K (TNR) contraction halted ZAMS star It takes approximately 20 million years from the initial collapse for star to join main sequence

8. Star Formation

9. Star Formation

10. Star Formation

11. Star Formation

12. Stellar Lifecycle

13. The Sun and the Stars Mass-luminosity relation Eddington 1924 - for stars on the main sequence, a graph of mass versus luminosity follows a power-law, with slope   3 • In fact =2.3 for dim red stars, and breaks to =4 for more luminous stars, at a mass of 0.43 Msun. • This break in slope reflects • differences in stellar interiors • changes in opacity with temperature

14. The Sun and the Stars Stellar lifetimes M-L relationship for main-sequence stars Stellar lifetime depends on its mass M, and the rate at which it consumes fuel, ie its’ luminosity L. Therefore a stars lifetime t, relative to the suns lifetime tsun, So,moremassive stars have shorter lifetimes!!

15. The Sun and the Stars Stellar evolution I – evolution of 1 solar mass star (pop I) After 10 billion years, most of Hydrogen in core exhausted  core mostly He. during this time, T (core) has increased slightly and star has expanded slightly, luminosity increases A-B Once hydrogen in core used up thermonuclear reactions in core cease. Drop of pressure in core causes core to contract. Surrounding H is pulled into core region and raised in temperature. Burning occurs in H-shell around He core. Burnt H added to core, whose density increases B-C Core contracts when too much material has been added to it. Energy generation in shell accelerates. Envelope must store more energy, and so it expands  radius of star increases, surface temperature decreases. (moves to right on H-R diagram) D E B C A

16. The Sun and the Stars Stellar evolution I – evolution of 1 solar mass star ~0.65Msun C-D Lower T  higher opacity, energy carried by convection. Radius of star increases, surface temperature decreases further. Heat transport dominated by convection, v efficient. Luminosity increases rapidly star moves up Red Giant Branch – winds remove a fraction of star’s atmosphere Core continues to contract When T reaches 100 million K , Helium burning occurs (triple-alpha process) D E B C A Lighter elements are rare because they quickly combine with H to produce He nuclei, eg.

17. The Sun and the Stars D Heat spreads rapidly through core. Triple-alpha reaction rate increases due to increased temperature, increases energy and hence temperature…. thermal runaway (Helium flash – heat spreads throughout core in few minutes, 60-80% He burnt at this stage) Core pressure increases, core expands and cools. He core burning temporarily ceases D-E Outer envelope and core contracts under gravity  luminosity decreases, but surface temperature increases Star moves down and to left on HR diagram. When core temperature sufficiently high core helium reignites E Star burns He in core and H in layer around core (Subgiant) Once core converted to C, core contracts again, burning layer of He around core, forces star to expand, star again becomes a giant. Star moves up Asymptotic Giant Branch (AGB). Triple alpha-reaction v. sensitive to temperature, star becomes unstable – thermal pulses. D E B C A

18. The Sun and the Stars Thermal pulses – core contracts, causes burning shell around core to heat up, heats outer layers which expand and therefore cool, energy generation drops, core contracts…cycle repeats Thermal pulses (every few thousand years) cause luminosity to vary by up to 50% on timescales of a few years. Energy transported rapidly to surface by convection Star develops super wind removing outer layers, exposing core Wind driven material form Planetary nebula (ionised by core). Core cools as a white dwarf.

19. Evolutionary phases of a solar mass star, post main-sequence

20. End of Main Sequence

21. The Sun and the Stars Example planetary nebulae – note WD at centre of nebulae

22. Typical stars are young, in galactic spiral arms where gas and dust found Typically reside in open star clusters ~2% of mass elements heavier than H or He (ISM enriched by supernovae) If M* a little > M energy generation is by CNO cycle Sun is population I Population I Stars

23. First stars to be formed in Universe Have only 0.01% heavy elements Typically found in galactic bulge and globular clusters Similar sequence of evolution but occupy different region of H-R diagram during core He burning Significant temperature changes, heating and then cooling Population II Stars