Download - 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?
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
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.
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.
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
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
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.
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.
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.