Summary of Stellar Evolution

Evolution of Sun-like stars off main sequence

Sequence of fusion in stars, dependence on mass, change in composition

Properties of white dwarfs

Helium Flash

Contrast evolution of high and low mass stars

Binary modification of stars through mass transfer

Stellar Explosions

Supernova remnant in the nearby galaxy, the Large Magellanic Cloud (LMC)

Binary White Dwarfs

Binary white dwarfs can form accretion disk

Mass transfer from one star to another, forms an accretion disk

The mass does not land directly on the binary star

Accretion Disk

Due to rotation of the binary system, the material (mostly hydrogen and helium) does not land directly on the star, but forms a disk

Matter in the disk, slowly spirals inward due to friction (viscosity) with other particles in disk

The temperature of disk heats up as friction continues

When material reaches surface, H builds up, eventually temperature increases and H ignites and starts fusing - sudden increase in light- nova

Nova

Inner part of disk becomes so hot that it emits X-rays, UV, and visible light

These bursts of light can be recurrent and depend on turbulence of disk

Nova Persei, brightened by 40,000 times in 1901

Fig. 21.1 and 21.2

Type I Supernova

Material gradually accumulates on White Dwarf, as nuclear explosions do not eject all the new material from surface

Star reaches the Chandrasekhar limit (1.4 M_M), overcomes electron degenerate pressure and starts collapsing

Carbon starts fusing everywhere, thermonuclear detonation

Fig. 21.9a

Type I and II Supernovae

Two distinct types of light curves

Two distinct types of spectra

Type I-H poor

Type II- H rich

Fig. 21.8

Type II Supernovae

Type II Sne are massive stars that undergo core collapse

Fig. 21.9b

Shell Model of Fusion

Fusion occurs in layers, with Hydrogen forming a fusing shell that is farthest from the core

Fig. 21.5

P-P Chain

4 p become Helium-4

Energy results from He-4 having less mass than 4 p

Fig. 16.27

Helium Fusion

On left, 4 protons fuse to firm He-4

On right, 3 He-4 fuse to form Carbon 12

Fig. 21.14 and 21.15

Carbon Fusion

Carbon 12 (C-12) can fuse to form higher elements

Bottom reaction is helium capture

Fig. 21.16

Why Iron Core?

Iron-56 is one of the most stable elements

Fig. 21.6

Summary of Stellar Explosions

What is a nova

What is a Supernova?

Core collapse

Core detonation

Difference between Type I and Type II SN

Stellar nucleosynthesis

Helium capture

Neutron capture

Stellar recycling