The Death of Stars - II.

! How can we use H-R diagrams to measure the age of star clusters (and hence the age of our Universe)?

! Why do high and low mass stars evolve differently? How are heavy elements such as iron and oxygen made in our Universe? What is the stellar evolution cycle?

! Know the physics of how high mass stars evolve into neutron stars (fusion, gravity, collapse, expansion etc.)

! How do evolving high mass stars move around the H-R diagram (when expanding, helium/carbon burning etc?)

! What is neutron degeneracy pressure? What is the Chandrasekhar Limit (1.4 solar masses)? Can a neutron star exceed 3 solar masses? How big is a neutron star?

! What are type Ia and core collapse (type II) supernovae?

Learning Objectives

Low-mass stars Massive stars

Live Fast, Die Young

Guess The Cluster’s Age! We can estimate the age

of a cluster of starsfrom its oldest Main Sequence star! Stars in clusters are

born at the same time! Stars leave the Main

Sequence as they age! Massive stars age faster

than low mass stars! The Main Sequence turnoff is the

point where a cluster’s Main Sequence ends

The Hydrogen Runs Out! Similar to lower-mass stars

in the first few stages! When the hydrogen

supply runs out the star’s core starts to contract

! Hydrogen shell burning (around the large helium core) begins

! The outer atmosphere expands quickly becoming a red supergiant

Core collapses

The Supergiant Phase! The outer atmosphere of the star grows larger

! More than 5 AU in size!! The surface of the star

cools because it is so far from the hot core

! The star’s core contracts and heats up

! Eventually, the core is hot enough to fuse helium into heavier elements

! The star contracts and heats back up, becoming a blue supergiant

! Up to this point, the lives of high-mass stars are very similar to the lives of low-mass stars

! But more mass = more forceful gravitational contraction

! When helium fusion stops, and the inert helium core starts to collapse, the force due to gravity creates really high temperatures

! For approximately an 8 M⊙ star, orgreater, the temperatures are high enough to

ignite the carbon left over after helium burning

The Supergiant Phase

Stage Temperature DurationH fusion 40 million K 7 million yrHe fusion 200 million K 500,000 yrC fusion 600 million K 600 yr

Ne fusion 1.2 billion K 1 yrO fusion 1.5 billion K 6 monthsSi fusion 2.7 billion K 1 day

Values for a ~20M⊙ star

Iron – The End of the Road! Supergiants “burn” heavier and heavier atoms in

the fusion process! Creates shells of different elements inside the star! Each stage is faster than the last! The process stops at iron

Main sequence

Red supergiant

Blue supergiantCarbon ignition

Supernova

Evolutionary Path of a High-Mass Star

Helium ignition

Electron-degenerate matter 1 ton per cubic cm

p

pe

e

pe

Matter in the core of a normal star

pe

pe

pe

pe

pe

pe

pe

pe

pe

pe

pe

pe

SQUEEZE

n nn n

n nn n

n n n n

n nn nn n

Neutron-degenerate matter 100 million tons per cubic cm

SQUEEZEν

νν

Neutrinos are produced as electrons are forced into nuclei

When Electron Degeneracy Just Isn’t Enough

If the core is 1.4 solar masses or more (called the Chandrasekhar Limit) then a Type II Core Collapse supernova occurs

There’s More Than One Way to Make a Supernova

! If a white dwarf in a binary system steals enough matter, it can go over the Chandrasekhar Limit. A “Type 1A” supernova

! The white dwarf collapses under its own gravity

! Carbon and oxygen fuse into iron and nickel

! The star rips itself apart in a thermonuclear explosion!The white dwarf star is destroyed

Supernova! During the collapse, part of the core

rebounds, producing a shock wave!The material is so dense it absorbs

even the neutrinos that are produced!The neutrinos give the shock a “kick”!Rips the outer layers of the star apart

! The star explodes in a supernova! This releases a tremendous amount

of energy!99% of the energy is in the form of

neutrinos

Bright as a Galaxy! Supernovae are

bright!A star’s

brightness increases by a factor of 10,000

!This is almost as bright as an entire galaxy! Combined light of

100 billion stars

Light from one supernova

Before Feb. 23, 1987

Supernova 1987A

Supernova 1987A in 1994

Stellar Evolution Cycle! Stars form out of the interstellar medium! They manufacture helium, carbon, nitrogen and

more in their interiors by nuclear fusion! Heavier elements (iron, lead, uranium, etc..) are

made by supernovae! Stars pass these processed materials back to the

interstellar medium when they die! The processed materials are included in the gas

and dust out of which the next generation of stars and planets will form

! We’re the death of stars. Without stellar processing, there’d, e.g., be no calcium to make your bones

Supernova Leftovers! What’s left of a star’s core after a Type II supernova?! A neutron star

!About 1.4 – 2 M⊙!Very small diameter – around 20 km!Composed of a sea of neutrons

! Supported by neutron degeneracy pressure! A teaspoon of neutron star material on Earth

would weigh almost 1 billion tons!Surface gravity – 200 billion times that on Earth!Escape velocity – of half the speed of light

20 km

Neutron star

Size of a Neutron Star

Optical - ESO X-rays - Chandra

Crab Nebula – Remnant of the Supernova of 1054

! The maximum neutron star mass! is about 3.0 MSun

! Beyond this mass, neutron degeneracy cannot stop gravity

! There is nothing left to stop total collapse

! A black hole…

When Neutron Degeneracy Just Isn’t Enough

Next Time

Black Holes, or the Monster at the Center of the Galaxy