1. The Death of a Low Mass Star n Evolution of a sun-like star post helium- flash –The star moves onto the horizontal branch of the Hertzprung-Russell diagram –Helium burning produces carbon and oxygen “ash” –Eventually, the helium concentration falls too low to sustain burning in the core

2. Post Core Helium Burning n Similar sequence of events to the end of hydrogen burning –Core contraction and heating –Degenerate carbon/oxygen core forms –Helium shell burning commences

3. Post Core Helium Burning n External Appearance –The star moves off the horizontal branch and ascends the red giant region again, becoming even larger and more luminous –The star is now an Asymptotic Giant and is on the Asymptotic Giant Branch

4. Asyomptotic Giants –Location on the Hertzprung- Russell Diagram 2,500 10 -2 1 10 2 10 4 10 6 Luminosity (L  ) 40,000 20,000 10,000 5,000 Temperature (K) Zero age main sequence Termination of core hydrogen burning 1 M  2 M  Core helium burning ceases Asymptotic Giant Branch

5. Asymptotic Giants n Appearance and Structure AGB sun dia. ~ 1.5AU L ~ 10000 Orbit of Mars dia. ~ 1x Earth Degenerate C/O core Helium burning shell Dormant hydrogen shell

6. Asymptotic Giants n Material Redistribution –Convection layers may reach to the core –Carbon and oxygen brought to the surface –In consequence, molecular absorption bands often seen in the spectra of AGB stars –Soot coccoons may also form around such carbon stars

7. Late Evolution –As helium is consumed, the core contracts and heats up. –The hydrogen shell may re-ignite, producing more helium which re-fuels the temporarily depleted shell –Helium shell burning re-ignites in a helium shell flash, leading to a short-lived spike of luminosity - a Thermal Pulse –Luminosity rises by ~ 2

8. Late Evolution n Such thermal pulses may occur a number of times: 3x10 5 years

9. Late Evolution n AGB stars produce strong stellar winds –Typical mass loss ~ 10 -4 solar masses per year n 10 3 x a “normal” red giant n 10 10 x the sun –Combined with the thermal pulses, such winds drive off the outer layers of the star –As much as 40% of a star’s mass may be lost in this way

10. Late Evolution n A number of shells of material now surround the dying star Central star in opaque cocoon Concentric shells Note: the phase shown here is very brief - ~ 1000 years See http://oposite.stsci.edu/pubinfo/PR/1998/11/b.html for details

11. The Final Stages n Ultimately, the hot carbon/oxygen core is exposed –Core surface temperature ~ 100,000K –Sufficient UV produced to ionise and excite the outer layers n The spectrum is now characterised by emission lines

12. Planetary Nebulae –The emitted gases now glow in the radiation of the exposed core, forming a Planetary Nebula Exposed core Fluorescing gas –Speed of gas ~ 10 kms -1 –Diameter ~1 ly

13. Planetary Nebulae n Planetary nebulae often appear as rings –actually spherical –looking through a greater depth of material at the edges Core of “dead” star Partner star

14. Planetary Nebulae n A disc of material around a star may allow a bipolar nebula to form

15. Planetary Nebulae n The planetary nebula phase is relatively short lived –The nebulae in the previous slides are estimated to be only a few thousand years old –The material rapidly disperses, leaving the central core

16. White Dwarfs n Sun-like stars never achieve the core temperatures and densities to ignite carbon and oxygen –After the planetary nebula has dissipated, the hot core is left n Degenerate matter n Mass ~ 1 solar mass n about the size of the Earth n about 100,000 K surface temperature n <10 -2 solar luminosities

17. White Dwarfs –No further nuclear reactions take place n Luminosity due to contained heat only n No further contraction takes place n Electron degeneracy pressure supports the star –Cooling occurs over many billions of years 2,500 10 -2 1 10 2 10 4 10 6 Luminosity (L  ) 40,000 20,000 10,000 5,000 Temperature (K) Cooling curve of a 1/4 solar mass white dwarf

18. White Dwarfs n Bizarre properties: –All as a consequence of the properties of degenerate matter –Higher mass white dwarfs are smaller n hence dimmer –Maximum mass ~1.4 solar masses n the Chandrasekhar Limit –These properties will be explored in a future lecture

19. The Death of a High Mass Star n High mass stars behave very differently –Higher core temperatures and densities imply burning beyond oxygen –Final stages often violent, leaving remnants even more bizarre than white dwarfs –To be discussed in the next lecture