Chapter 12The 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 low-mass star becomes a white dwarf
white dwarf
white dwarf
Sirius and its white dwarf companion
The burned-out core of a low-mass 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.
white dwarf
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 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 X-ray 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-Check1: 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 main-sequence 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.
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