1) Review: main sequence lives of low mass stars

Red Giant formation2) Hotter, more Massive

stars CNO cycle nucleosynthesis – creation

of the heavier elements3) Supernova4) Split class: “the life

and times of Albert Einstein” (Planetarium)

Today’s Lecture: purpose & goals

Supernova 1987a

Einstein Memorial, National Academy of Sciences, Washington D.C.

2005 is the Einstein Centennial!

Stellar Midlife/Death – large stars “live fast, die young”

AST 1002Planets, Stars and Galaxies

Review

tMS = 1/M2.5 x 1010 years

Stellar Lifetimes

Most luminous stars live shortest lives also, largest (most massive) stars 80-100M Blue giants few tens

of thousands of years Very few recent young still here.

Less luminous stars live longer which are the less massive,

smaller stars 0.08-0.12M Red dwarf stars

over a trillion years ALL still here!! (age of the

universe 15 billion years) Formation of Red Giants

remember:

Stefan-Boltzmann Law: L = 4R2 . T4

These are the same star at different stages of life!!

Review2: Lives of Less Massive Stars

Low mass stars follow a pattern: protostar main sequence star

stable when gravity and pressure balance.

star dies when hydrogen used up at core.

Small stars no mixing @core degenerate helium ashe.

red giant hydrogen shell and helium core

burning shrinks core which gets hotter; expands

outer surface which gets cooler high luminosity Red giant

Planetary nebula Blow off outer surface (stellar wind) leaving behind bare core

white dwarf (no fusion, very hot, very small low luminosity)

Binary stars accretion onto white dwarf novae (only fuse surface,

recurrent), or type Ia supernovae (collapse and fusion of entire 1.4M

white dwarf)

Hotter Stars More massive stars:

Gravity is stronger! so core is denser, with higher

pressure These stars are hotter! Hotter stars burn faster! More interesting stuff

happens… supernovae (again… but

different) neutron stars black holes variable stars

Massive Core Burning Core burns differently in massive

stars Core is denser, and hotter over a

larger region Burns faster!

Convection stirs core mixes elements

Converts all of core to helium Not just the very center Core not degenerate

Then core starts burning helium Hydrogen burning shell appears

Start of multiple-layer burning in star Still basically a main sequence star

(process stars while hydrogen still burning at core)

CNO Cycle – More Burning

Carbon-Nitrogen-Oxygen (CNO)

12C + 4 1H 12C + 4He

occurs if carbon and hydrogen are together and hot enough (>15,000,000K)

needs star to be at least ~1.5 solar masses to be main power source

Nucleosynthesis If temperatures get high

enough, additional fusion reactions are available

H He C O Ne Si Fe Hotter stars produce

heavier elements Each fusion stage

produces energy each stage goes faster than

previous Iron (Fe) does not burn!

needs energy into reaction rather than giving up energy

VERY unstable run-away process!!

e.g. Betelgeuse, eta Carina

A Giant Onion

Degenerate Fe core

During its life, a massive star burns each step progressively outward

Shells form, w/innermost shells burning heavier elements

Fe Si O Ne

Elements between He and Fe created in latter stages of massive stars life.

Fe cannot fuse and give off energy the next step will be a catastrophe Core collapse and

Type-II Supernova

C He H

Supernovae – Type II Type II

At some point the pressure in the core is unable to balance the gravitation- al compression, and the

iron core collapses, leaving a void.

The outer layers of the star crash onto the core, crushing it further and heating it to extreme temperatures.

Core Collapse The collapse of the

degenerate iron core fuses every proton with an electron to form a neutron and a neutrino.

Supernovae – Type II,…

Neutron Star As the outer layers expand the

neutron core continues to be crushed.

Left behind an unimaginably dense object a neutron starneutron star. (>1.4

All elements heavier than Fe are created only in supernovae!

Your gold ring is a souvenir of a supernova!! M)

Black Hole For the most massive stars,

the neutron star is unstable and is predicted to collapse to

a black holeblack hole. (>3.0 M)

All!!

SN1987A 1987 Supernova

In 1987 a fantastic milestone occurred in astronomy. For the first time in history scientists were able to record the neutrino shockwave from a Type II supernova.

The supernova occurred in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way, that lies about 160,000 light years from us.

Neutron Stars and Pulsars Neutron Stars: calculated properties

Radius:Radius: about 10 km10 km. Density:Density: from 101044 g/cm g/cm33  at

the surface to about 10101515 g/ g/ cmcm33 at the center. This is about a billion times denser than a white dwarf!

Structure:Structure: the star is a very smooth, rapidly spinning, rigid metallic shell filled with a neutron super-fluid, a fluid that moves without resistance.

very strong magnetic strong magnetic fields!fields!

observable as pulsars. pulsars.

Chandrashekar limit:

e.g. Crab nebula

Heavierbecomes a black hole

Nucleosynthesis Some typical times and

temperatures and rates of reactions

Stage9 solar masses

Temperature (K)

H burning

20 million years

(3-10)x107

He burning

2 million years

(1-7.5)x108

C burning

380 years (0.8-1.4)x109

Ne burning

1.1 years (1.4-1.7)x109

O burning

8 months (1.8-2.8)x109

Si burning

4 days (2.8-4)x109

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Betelgeuse, eta Carina In massive stars, like Betelgeuse Betelgeuse

and eta Catrina eta Catrina, nuclear synthesis of heavier and heavier elements occurs up to the element ironiron.

The reaction that creates iron is endothermicendothermic (it cools the surroundings), which decreases the core’s pressure.

This process is very unstable!!

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Supernovae – Type II,… Core Collapse

The outgoing neutrinos carry energy outward but do not push against further graviational collapse, accelerating it.

Neutrino Shockwave The mass of the collapsed

core is of order 10103030 kg kg. Each proton has a mass of

approximately 1010-27-27 kg kg. So the number of protons

converted to neutrons is about 10103030 / 10 / 10-27-27 = 10105757, equal to the number of neutrinos that rush outwards from the collapsed core at the speed of light.

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