1 Stellar Evolution The Lives and Deaths of Stars Chapter 12 Evolution

2 Objectives 1. Explain why stars evolve off the main sequence. 2. Summarize the evolutionary stages followed by a Sun-like star once it leaves the main sequence, and describe the resulting remnant.

3 Objectives 3. Explain how white dwarfs in binary systems can become explosively active. 4. Contrast the evolutionary histories of high-mass and low-mass stars. 5. Describe the two types of supernova, and explain how each is produced.

4 Objectives 6. Explain the origin of elements heavier than helium, and discuss the significance of theses elements for the study of stellar evolution. 7. Discuss the observations that help verify the theory of stellar evolution.

Leaving the Main Sequence OOOOn the main sequence, a star slowly fuses hydrogen into helium in its core. This process is called: core-hydrogen burning AAAA star’s equilibrium is the result of a balance between gravity and pressure. gravity-in/pressure-out _________________________________ ________________________________

6 Leaving the Main Sequence  As the hydrogen in the core is consumed, leaving ______, the balance between these opposing forces changes. The end of a star’s life is near.  What happens now depends on the star’s _____. Low mass stars die _____, while high mass stars die _____________.  The dividing line is about _ solar masses. helium massgently catastrophically 8

Evolution of a Sun-like Star  In order to fuse hydrogen into helium the temperature must be ____.  It is only above this temperature that hydrogen nuclei _______ have enough speed to overcome the repulsion of the electron’s ______________ force.  The helium has a stronger magnetic field so _____ temperatures are required for helium fusion to occur. The Red Giant Branch 2 protons 107K (proton) electromagnetic higher

8 Evolution of a Sun-like Star  Without nuclear burning, the outward push ________ the helium inner core is weakened.  However the inward pull of gravity remains _______, creating structural changes in the star.  When the hydrogen becomes depleted enough (after about ________ years), the helium core begins to contract. The Red Giant Branch (pressure) constant 10 billion

9 Evolution of a Sun-like Star TTTThe shrinking helium core generates that increases the outer layers temperature enough to begin fusing ________ to ______. TTTThis is known as: hydrogen-shell burning. TTTThe star’s response to the disappearance to the nuclear fire at its center is to get _______. The Red Giant Branch heat hydrogenhelium brighter ___________________

10 Evolution of a Sun-like Star  The horizontal track from the main sequence (stage 7) to stage 8 is called the subgiant branch.  The nearly vertical branch leading to stage 9 is called the red giant branch. The Red Giant Branch

11 Evolution of a Sun-like Star BBBBy stage 9 its luminosity is many hundred times the solar value. Its radius is around 100 solar radii (the size of Mercury’s orbit) IIIIn contrast, its helium core is only about 1/1000 the size of the _________. TTTThis is just slightly larger than _______ diameter. The Red Giant Branch entire star Earth’s ______________________________________.

12 Evolution of a Sun-like Star  The central temperature reaches the 10 8 K which is required for ______ to fuse into ______. This is a very violent event.  Once burning starts, the core cannot respond fast enough to the rapidly changing conditions. Its temperature ____ sharply in a runaway explosion called the __________. Helium Fusion helium carbon rises helium flash

13 Evolution of a Sun-like Star  At stage 10 the star is now stably burning ______ in its inner core and fusing ________ in outer layers.  This location on the H-R diagram is the _______________. Helium Fusion helium hydrogen horizontal branch

14 Evolution of a Sun-like Star TTTThe second trip to the red giant region has the luminosity and radius _____ than the first trip (stage 9). TTTThe star is now a: Red supergiant The Carbon Core greater _____________

15 Concept Check Why does a star get brighter as it runs out of fuel in the core? Because the nonburning inner core, unsupported by fusion, begins to shrink, releasing gravitational energy, heating overlying layers, and causing them to burn more vigorously.

The Death of a Low-Mass Star  At this point the inner carbon core is _____, but the outer layers continue to burn hydrogen and helium.  The _________ envelope continues to expand and ____.  Eventually the increasing radiation including the energy released as electrons recombine to form atoms. Planetary Nebulae dead nonburning cool stage 12

17 The Death of a Low-Mass Star  The envelope becomes ________ and is ejected into space at a speed of a few tens of kilometers per second.  This forms an expanding, cooling shell of matter called a ______________.  The term planetary nebula is misleading because they have nothing to do with _______. Planetary Nebulae unstable planetary nebula planets

18 The Death of a Low-Mass Star  The carbon core continues to evolve, once was concealed by the atmosphere of the red giant star is now visible as a __________.  Shining only by stored heat, not by ______________, this small star has a white-hot surface when it becomes visible.  It appears dim because it is _____. White Dwarfs nuclear reactions white dwarf small stage 13

19 The Death of a Low-Mass Star TTTThe white dwarf continues to ____ and ___ with time, following the white, yellow, red line. EEEEventually becoming cold, dense, and burned out… black dwarf White Dwarfs cool dim stage 14 a _________

20 The Death of a Low-Mass Star  ___________ can become explosively active, this highly luminous event is called a ____.  This is what we see when a white dwarf undergoes a violent explosion on its surface, resulting in a rapid, _________ increase in _________. Novae nova luminosity White dwarfs temporary

21 The Death of a Low-Mass Star  In a ____________ the white dwarf’s gravity can pull from the companion star.  The material forms an ____________ around the white dwarf.  As it builds up the gas becomes more dense and hotter.  Eventually the temperature exceeds 10 7 K and hydrogen is fused to ______. Novae binary system (material) accretion disk helium

22 Concept Check Will the Sun ever become a nova? No because it is not a member of a binary system.

Evolution of High-Mass Stars  At the cool periphery of the core, hydrogen fuses to helium.  In the intermediate layers, shells of ______, ______ and ______ burn to form heavier nuclei.  Deeper down neon, magnesium, silicon and others are produced by nuclear fusion in the layers overlying the __________ inner core. Heavy Element Fusion heliumcarbonoxygen nonburning

24 Evolution of high-Mass Stars  As each element is burned to ________, the core contracts, heats up and the cycle starts over.  In contrast to low mass stars this happens much _____ in high mass stars. Heavy Element Fusion faster depletion

25 Evolution of high-Mass Stars  In a star ___ times more massive than the Sun hydrogen burns in __ million years, helium for _ million years, carbon for _____ years, oxygen for _ year and silicon for __week. It’s iron core grows less than _ day. Heavy Element Fusion 1,

26 Evolution of high-Mass Stars  The core shrinks and the density continues to rise, the protons and electrons are crushed together, creating ________ and releasing neutrinos.  The shrinking core shrinks past the point of __________ and like a ball thrown against a wall it _________. Death of a High-Mass Star equilibrium rebounds neutrons

27 Evolution of high-Mass Stars TTTThis only takes about _ second. DDDDriven by the rebounding core, an enormously energetic _________ that sweeps outward through the star blasting all the overlying layers away. TTTThe star explodes in one of the most energetic events known: core-collapse supernova Death of a High-Mass Star into space shock wave 1 ____________________

28 Concept Check Why does the iron core of a high-mass star collapse? Because iron does not fuse to produce energy. As a result, not further nuclear burning is possible, and the core’s equilibrium cannot be restored.

Supernova Explosions TTTType 1 Supernovae - A white dwarf accretes so much mass that it cannot support its own weight. The star collapses and the increased temperature is enough to fuse carbon. TTTThe resulting explosion is a… type 1 supernova carbon-detonation supernova Novae and Supernovae ______________ or a ________________________

30 Supernova Explosions  – Explosive death of a star in which its highly evolved iron core rapidly implodes and then explodes, destroying the surrounding star in the process.  Unlike type 1 supernova, type 2 supernovae are ________ rich and have a distinctive __________. Novae and Supernovae light curve hydrogen plateau Type 2 supernovae

31 Supernova Explosions  The detonation of a carbon white dwarf, the descendent of a low-mass star, is a type 1 supernova and has very little hydrogen.  The type 2 supernova is the core collapse supernova. Novae and Supernovae

32 Supernova Explosions  The most important aspect of supernovae is their role in creating, and ________ the heavy elements out of which _________ is made.  Astronomers now realize that all the hydrogen and most of the helium are _________. Everything else formed later, through ______________. Supernovae and Heavy Elements everything dispersing primordial stellar evolution

33 Concept Check Why are supernovae important to life on Earth? Because they are responsible for creating and dispersing all the heavy elements out of which we are made.

Observing Stellar Evolution in Star Clusters  Star clusters provide excellent test sites for the theory of _____________. Every star in a given cluster formed at the same time, from the same interstellar cloud, with virtually the same __________.  Only the _____ varies from star to star with in the cluster. mass stellar evolution composition

35 Observing Stellar Evolution in Star Clusters  After 100 million years, stars brighter than ________ have left the main sequence and a few more supergiants have become visible.  The main sequence turnoff is the point at which the high luminosity begins as the star approaches the ____ branch. (this is around stage 8) (this is around stage 8) Type B5 giant ________________________________

36 The End All pictures and graphs taken from Chaisson/McMillan