the death of stars - 2faraday.uwyo.edu/~admyers/astr1050/handouts/the... · we’re the death of...
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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