PHYS The Main Sequence of the HR Diagram During hydrogen burning the star is in the Main Sequence. The more massive the star, the brighter and hotter it is. Star Death: a star like our Sun (a) Hydrogen burning stops in the core (b) Core collapses under gravity and heats up (c) Hydrogen burns in shell around core, star becomes a red giant (d) Core collapses further to become a degenerate electron gas (e) Core hot enough for helium to burn for a short time to give carbon (helium flash, internal, not seen) only takes minutes!

PHYS Star Death - continued... (f) Stability returns, helium burns in the core and hydrogen burns in the shell around it. Eventually all the helium is converted to carbon and the core shuts down (g) The star now becomes unstable and thermonuclear explosions occur in the shell (thermal pulses). Convective gases carry heavier elements outwards (h) The star develops a very strong superwind which rips off the outer layers leaving the hot core behind. Now called a planetary nebula (i) Core becomes a white dwarf star made up mostly of carbon. This very gradually cools.

PHYS Star Death: a star 5 times the mass of our Sun Greater temperature in centre, so hydrogen burns by CNO cycle and is used up quickly Star’s Main Sequence life is shorter Helium core does not become degenerate; helium burns before core is dense enough As core turns to carbon, helium burns in a shell around it. Star expands to a red giant Carbon core becomes degenerate Carbon flash - outer layers blown off leaving a white dwarf about the size of the Earth Note that a smaller fraction of the star’s original mass remains. In theory, a white dwarf will cool very slowly to become a black dwarf.

PHYS Star Death: a star of more than 15 solar masses Still greater temperature in centre Hydrogen burns by CNO cycle Main Sequence life is even shorter Helium core does not become degenerate; helium fuses before core is dense enough Carbon core burns to give oxygen Oxygen core burns to give silicon Silicon core burns to give iron When iron core reaches 1.4 solar masses, it collapses in seconds, to form a neutron core energetic neutrino particles ( ) escape Outer layers fall in and bounce off the core Shocked outer layers blown off, leaving a neutron star

PHYS Novae - Mild Explosions Star becomes up to a million times brighter in days and then dims in hundreds of days Photosphere expands to a few hundred solar radii - supergiant Photosphere relaxes back into star Shell of material thrown off is about one ten thousandth of a solar mass Possible model for a nova: Binary star system - a Main Sequence star with a less massive companion MS star dies first resulting in a white dwarf When companion becomes a red giant, material flows between Roche lobes to form an accretion disc around the white dwarf Matter spirals onto white dwarf’s surface. Hydrogen forms a new envelope of fuel

PHYS Novae - continued... Envelope thickens and its base gets hotter Hydrogen fusion occurs explosively Star becomes a red giant Material is blown off at hundreds of kilometres per second. See Zeilik p. 376 or Kuhn & Koupelis p. 439 Supernovae Can peak at a luminosity 10,000,000,000 times greater than that of the Sun. They come in one of two varieties: TYPE I In a binary system of a red giant and a white dwarf (WD). Material flows onto WD as in a nova, but WD is close to the Chandrasekhar mass limit of 1.4 solar masses.

PHYS Supernovae - continued... The degenerate electron gas collapses under gravity. Material heats up further. Carbon fuses to nickel, cobalt and iron. Star is probably blown to bits. TYPE II A star of more than 15 solar masses collapses - see OHP 34 Supernova 1987A A 20 solar mass blue supergiant star exploded in the Large Magellanic Cloud (our nearest neighbour galaxy)  Type II supernova First saw neutrinos from the formation of the neutron core. These carry away most of the energy million million per square metre hit the Earth. ~20 were detected.

PHYS Supernova Remnants SN1987A: initial spectra had hydrogen lines from the expanding envelope. Doppler measurements gave a speed of 17,000 kilometres per second. Later there were emission lines from heavier elements formed in the explosion. The supernova remnant is now brightening again as a shock wave from the explosion hits a spherical shell of matter blown out from the star about 20,000 years earlier. Figs. Z17.19 & K15-8 The Crab Nebula: is the remnant of a supernova explosion recorded by Chinese astronomers in AD The rapidly spinning neutron star which was left after the explosion is a pulsar - we receive radio pulses from it 30 times per second. Figs. Z17.33 & K15-14