Transcript Lecture 16

How do High Mass Stars Die?
Class Announcements
• Homework is due today.
• Email to me the title of your choice of class
project TODAY.
• Friday there is a class midterm test and a
homework due.
• NEW HOME WORK
– Complete the lecture tutorial ‘Motion of Extrasolar
Planets’, hand in on Tuesday 2nd August at the start of
class.
Life Stages of High-Mass Stars
• By high mass stars we mean stars greater than
8Msun
• Late life stages of high-mass stars are similar to
those of low-mass stars:
—Hydrogen core fusion (main sequence)
—Hydrogen shell fusion (subgiant to supergiant)
—Helium core fusion (supergiant)
How do high-mass stars make the
elements necessary for life?
Big Bang made 75% H, 25% He—stars make everything else.
Helium fusion can make carbon in low-mass stars.
Helium Capture
•
High core temperatures allow helium to
fuse with heavier elements.
Helium capture builds C into O, Ne, Mg …
Advanced Nuclear Burning
•
Core temperatures in stars with >8MSun
allow fusion of elements as heavy as iron.
Advanced reactions in stars make elements such as Si, S, Ca, and Fe.
Multiple Shell Burning
• Advanced nuclear
burning proceeds in
a series of nested
shells.
Iron is a dead end
for fusion because
nuclear reactions
involving iron do
not release energy
(Fe has lowest
mass per nuclear
particle.)
Fusion of nuclei
heavier than iron
absorb energy and
would cause the
core to cool and
collapse).
Evidence
for helium
capture:
Higher
abundances
of elements
with even
numbers of
protons
How does a high-mass star die?
Iron builds up in the
core until electron
degeneracy pressure
can no longer resist
gravity.
The core then
suddenly collapses,
creating a supernova
explosion.
The Death Sequence of a High-Mass Star
Supernova Explosion
• Core electron
degeneracy pressure
goes away because
electrons combine
with protons,
making neutrons
and neutrinos.
• Neutrons collapse to
the center, forming a
neutron star.
Energy and neutrons released in a supernova explosion enable elements
heavier than iron to form, including Au and U.
Supernova Remnant
• Energy released by
the collapse of the
core drives outer
layers into space.
• The Crab Nebula is
the remnant of the
supernova seen in
A.D. 1054.
Supernova 1987A
•
The closest supernova in the last four
centuries was seen in 1987.
How does a star’s mass determine its life story?
Role of Mass
• A star’s mass determines its entire life story
because it determines its core temperature.
• High-mass stars have short lives, eventually
becoming hot enough to make iron, and end in
supernova explosions.
• Low-mass stars have long lives, never become hot
enough to fuse carbon nuclei, and end as white
dwarfs.
Life Stages of Low-Mass Star
1. Main Sequence: H fuses to He in core
2. Red Giant: H fuses to He in shell around He core
3. Helium Core Fusion:
He fuses to C in core while H fuses to He in shell
4. Double Shell Fusion:
H and He both fuse in shells
5. Planetary Nebula: leaves white dwarf behind
Not to scale!
Reasons for Life Stages
• Core shrinks and heats until it’s hot enough for fusion.
• Nuclei with larger charge require higher temperature for fusion.
• Core thermostat is broken while core is not hot enough for fusion
(shell burning).
• Core fusion can’t happen if degeneracy pressure keeps core from
shrinking.
Life Stages of High-Mass Star
1. Main Sequence: H fuses to He in core
2. Red Supergiant: H fuses to He in shell around He core
3. Helium Core Fusion: He fuses to C in core while H
fuses to He in shell
4. Multiple Shell Fusion: many elements fuse in shells
5. Supernova leaves neutron star behind and creates all
elements heavier than Iron.
How are the lives of stars with close
companions (close binaries) different?
Thought Question
The binary star Algol consists of a 3.7MSun mainsequence star and a 0.8MSun subgiant star.
What’s strange about this pairing?
How did it come about?
Stars in Algol are
close enough that
matter can flow from
the subgiant onto the
main-sequence star.
The star that is now a
subgiant was
originally more
massive.
As it reached the end
of its life and started
to grow, it began to
transfer mass to its
companion (mass
exchange).
Now the companion
star is more massive.
How does the life of a high-mass star differ
from the Sun’s life?
A. It forms much faster.
B. It lives a shorter time on the main sequence.
C. As a red giant or supergiant, it makes elements heavier
than carbon.
D. When it dies, it explodes in a tremendous supernova
explosion.
E. All of the above
What is different about nuclear reactions of
elements lighter than iron or heavier than
iron?
A. Lighter elements give off energy when they fuse,
heating the stars core and keeping gravity from
crushing it.
B. Heavier elements take in energy if they fuse, taking
away heat from the core and leading to a collapse.
C. A and B
What remnant does a supernova leave?
A.
B.
C.
D.
White dwarf
Neutron star
Black hole
B or C
Why are supernovas important to galactic
ecology?
A. They recycle material.
B. They create new elements and blow them out into
space, and a new generation of stars can be made from
them.
C. They destroy elements, letting each new generation of
stars begin anew.
The binary star Algol has a 3.7 solar mass main
sequence star and a 0.8 solar mass red giant. How
could that be?
A. In this system the lower mass star must have evolved
faster than the higher mass one.
B. The red giant might be made of some different
elements, so it evolved faster.
C. The lower mass star used to be a more massive main
sequence star, but when it became a giant some of its
mass went onto the other star.
Suppose the universe contained only lowmass stars. Would elements heavier than
carbon exist?
A. Yes, all stars create heavier elements than carbon when
they become a supernova.
B. Yes, but there would be far fewer heavier elements
because high-mass stars form elements like iron far more
prolifically than low-mass stars.
C. No, the core temperatures of low-mass stars are too low to
fuse other nuclei to carbon, so it would be the heaviest
element.
D. No, heavy elements created at the cores of low-mass stars
would be locked away for billions of years.
E. No, fission reactions would break down all elements
heavier than carbon.
If you could look inside the Sun today, would you
find that its core contains a much higher proportion
of helium and a lower proportion of hydrogen than
it did when the Sun was first born?
A. Yes, because the Sun is about halfway through its
hydrogen-burning life, so it has turned about half its core
hydrogen into helium.
B. No, the proportion of helium only increases near the end
of the Sun’s life.
C. No, the proportion of helium in the Sun will always be
the same as when it first formed.
D. No, the lighter helium will rise to the surface and the
proportion of hydrogen in the core will remain the same.
• Finish yesterday’s lecture tutorial,
Luminosity Temperature and Size.
• Then work on the lecture tutorial section
Analyzing Spectra.