Download Foundation 1 - Discovering Astronomy

Chapter 12
The Deaths of Stars
What do you think?
• Will the Sun explode? If so, what is the
explosion called?
• Where did carbon, silicon, oxygen, iron,
uranium, and other heavy elements on Earth
come from?
• What is a pulsar?
• What is a nova?
Low-mass stars expand into the giant
phase twice before becoming planetary
nebulae
Stages in the evolution of low-mass
stars beyond the helium flash:
•
•
•
•
Movement to horizontal branch
Core helium fusion
Asymptotic GIANT branch (AGB)
Planetary nebula formation
Low-mass stars expand into the
supergiant phase before expanding
into planetary nebulae
The burned-out core of a lowmass star becomes a white dwarf
white dwarf
Sirius and its white dwarf companion
white dwarf
The burned-out core of a lowmass star becomes a white dwarf
• Stable stars are supported by
– gas pressure
– radiation pressure
– electron degeneracy pressure
•
•
•
•
•
Star loses hydrostatic equilibrium
Gravitational contraction of the core
Temporary, nuclear fusion-based stability
Surrounding planetary nebula disperses
Remaining core is WHITE DWARF
The starting
MASS
determines the
exact pathway
Mass-loss causes
the end-state, a
planetary nebula
and a white
dwarf, to have
substantially less
mass than the
original red
supergiant.
What’s a nova?
• A nova is a relatively gentle explosion of
hydrogen gas on the surface of a white
dwarf in a binary star system.
• It occurs when the white dwarf steals mass
from its companion and the external layers
quickly ignite and shine brightly.
• This process does not damage the white
dwarf and it can repeat.
Yeah, but what about the
really BIG stars?
A series of different types of fusion
reactions in high-mass stars lead to
luminous supergiants
A series of different types of fusion
reactions in high-mass stars lead to
luminous supergiants
• When helium fusion ceases in the core, gravitational
compression increases the core’s temperature above 600
million K at which carbon can fuse into neon and
magnesium.
• When the core reaches 1.5 billion K, oxygen begins fusing
into silicon, phosphorous, sulfur, and others
• At 2.7 billion K, silicon begins fusing into iron
• This process immediately stops with the creation of iron
which can not fuse into larger elements and a catastrophic
implosion of the entire star initiates.
High-mass stars die violently by blowing
themselves apart in supernova explosions
Remnants of supernova explosions can
be detected for millennia afterward
The most famous
“before and after” picture
Supernova 1987 A
Supernova 1987A
offers a close-up look
at a massive star’s
death
Consider the change in brightness with
time for some supernovae ….
There are at
least two
distinctly
different
types of
brightness
fall-off
observed.
Accreting white dwarfs in close binary
systems can also explode as supernovae
white dwarf
White dwarfs in close binary systems can
rapidly gain mass from a companion and
create powerful explosions
white dwarf
White dwarfs in close binary systems
can create powerful explosions if it
exceeds 1.4 solar masses (Chandrasekar limit)
before
after
Called a TYPE I supernova
After an initial brightening, there is a
slow drop-off in brightness
Let’s again consider the end state of
very large stars
The cores of may Type II
supernovae become neutron stars
• When stars between 4 and 9 times the mass
of the Sun explode as supernovae, their
remnant cores are highly compressed
clumps of neutrons called neutron stars.
• These tiny stars are much smaller than
planet Earth -- in fact, are about the
diameter of a large city.
• Spinning neutron stars are called pulsars.
Neutron Star
Pulsars
• first detected in 1967 by Cambridge University
graduate student Jocelyn Bell
• Radio source with an regular on-off-on cycle of
exactly 1.3373011 seconds
Pulsars
• first detected in 1967 by Cambridge University
graduate student Jocelyn Bell
• Radio source with an regular on-off-on cycle of
exactly 1.3373011 seconds
• Some scientists speculated that this was evidence
of an alien civilization’s communication system
and dubbed the source LGM
Little Green Men
• Today, we know pulsars are rapidly spinning
neutron stars.
THE LIGHT
HOUSE
MODEL
A rotating
magnetic field
explains the
pulses from a
neutron star
Pulsating Xray sources
are neutron
stars in close
binary
systems
Other neutron stars in binary systems emit
powerful jets of gas
Neutron stars in binary systems can also
emit powerful isolated bursts of X-rays
X-ray bursters probably arise from mass
transfer in binary star systems where one
star is a neutron star rather than a white
dwarf. A helium layer 1km thick would be
enough to cause a flash across the surface
that emits X-rays
Recently discovered
gamma-ray
bursters, which
happen over
fractions of
seconds, might have
a similar origin.
What did you think?
• Will the Sun explode? If so, what is the explosion called?
The Sun will explode as a planetary nebula in about five billion
years.
• Where did carbon, silicon, oxygen, iron, uranium, and other
heavy elements on Earth come from?
These elements are created by supernovae.
• What is a pulsar?
A pulsar is a rotating neutron star in which the magnetic field does
not pass through the rotation axis.
• What is a nova?
A nova is a relatively gentle explosion of hydrogen gas on the surface
of a white dwarf in a binary star system.
Self-Check
1: List the stages in the evolution of low-mass stars beyond the helium
flash.
2: List the stages in the evolution of high-mass stars beyond the initial red
giant or supergiant stage.
3: Name the objects that represent the end phases of evolution for mainsequence stars and indicate the mass range for each.
4: Compare and contrast the physical and observable properties of Type I
and Type II supernovae.
5: Describe the properties of gas clouds that are produced by late stages of
stellar evolution and indicate from which type of stars they are formed.
6: Review the observational evidence that links pulsars with neutron stars.
7: Compare and contrast pulsars with X-ray sources that pulsate.
8: Compare and contrast the physical processes that occur in supernovae
with those in novae and bursters.