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 low- mass star becomes a 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

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) beforeafter 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 seconds

Pulsars first detected in 1967 by Cambridge University graduate student Jocelyn Bell Radio source with an regular on-off-on cycle of exactly 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.

T HE L IGHT H OUSE M ODEL 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-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 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.