HW 3- due 14 February

R+D: 20-5, 21-3, 21-9, 21-12

P: 21-6, 21-7, 22-2, 22-7

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

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 Ib SN2002ap

Quiz 7 - 12 February

It takes less and less time to fuse heavier and heavier elements inside a high mass star.

True

False

Type II Supernovae

Type II Sne are massive stars that undergo core collapse

Fig. 21.9b

Supernova Remnant

Radio emission from front edge of forward shock

Reverse shock exites ejecta which emits in the X-ray

Optical emision occurs behind the forward shock as the nuclei recapture their electrons and emit visible light

Energy in a Supernova Explosion

Problem 21-7

Compare energy released in a SN to the energy released in the Sun's lifetime

The Crab Supernova Remnant

The Crab SNR

Explosion in 1054 AD

Optical emission lines give shock velocity

Can measure movement of knots and filaments

How can you use this info to get the Crab's distance?

Fig. 21.10

The Crab Pulsar

Neutron Star

Pulsed emission from Radio to X-rays

Fig. In Disc. 21-2

SN 1987A Discovery Image

21.7

SN 1987A Observations

Light Curve on left

Remnant of SN 1987A

Figs. in Disc. 21-1

Multi-Wavelength SNR

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

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

CNO cycle

6 steps in CNO cycle

¹²C + ¹ H --> ¹³N + energy

¹³N -->¹³C + positron + neutrino

¹³C + ¹ H --> ¹⁴N + energy

¹⁴C + ¹ H --> ¹⁵O + energy

¹⁵O -->¹⁵N + positron + neutrino

¹⁵N + ¹ H --> ¹²C + ⁴He

Sum of the above: ¹²C + 4(¹ H) --> ¹²C + ⁴He

CNO cycle only dominates at high core temps

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

Helium Capture

¹²C + ⁴ He

¹⁶O + ¹⁶ O

¹⁶O + ⁴ He

Compare (2) and (3)

Which fusion reaction is more likely

Alpha Process

High energy photons break apart heavy nuclei into He nuclei (at high temps)

He nucleus is also called an alpha particle

Then He capture occurs

Fig. 21-17

Elemental Abundance

Most H and He is primordial-made in the big bang

Fig. 21.13

Iron core

⁵⁶Ni is very unstable

⁵⁶Ni decays very rapidly to ⁵⁶Co

Then ⁵⁶Co decays to stable ⁵⁶Fe

Leads to buildup of iron in stellar core

Why Iron Core?

Iron-56 is one of the most stable elements

Fig. 21.6

P-P Chain

4 p become Helium-4

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

Fig. 16.27

Energy is Mass

Mass of 4 protons = M (4p) = 4( mass of a proton)

= 4(1.672630x10^-27 kg) = 6.6943x10^-27 kg

Mass of helium-4 = 6.6466x10^-27 kg

M (4p)-mass (⁴ He) = 0.048x10^-27 kg

E = mc² = 0.048x10^-27 kg (3x10⁸ m/s)² = 4.3x10^-12 J

J=kg m/s²

1kg 6.4x10¹⁴ J

Heaviest Nuclei

s-process (s-slow ~1 year)

Nuclei capture free neutrons and become a higher atomic number isotope

Eventually, isotope gets too many neutrons and becomes unstable and decays to another element with same atomic number

Zi, Pb, Cu, Ag

r-process (r-rapid), elements heavier than Fe

Occurs during SN explosion

So many free neutrons during explosion that neutron capture is easy and very fast, before decay can happen

Stellar Nucleosynthesis

Stellar Nucleosynthesis-The process by which higher atomic number elements are created

Explains abundance of elements

Fig. 21.16

Light Curves

Type I light curve

Fig. 21.18

Stellar Recycling

Explain how stars recycle stellar material

Fig. 21.19

Example: Brightness of SN

Problem 21-3 and 21-4

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