Life and Death of a Star. Chapters 20 and 21

Transcription

1 Life and Death of a Star Chapters 20 and 21

2 90 % of a stars life Most stars spend most of their lives on the main sequence. A star like the Sun, for example, after spending a few tens of millions of years in formation (stages 1 6), resides on or near the main sequence (stage 7) for 10 billion years before evolving into something else. Most red dwarfs will burn for a trillion years Most O and B stars burned out long ago

3 Leaving the Main Sequence On the main sequence, a star slowly fuses hydrogen into helium in its core. This process is called core hydrogen burning. A star's equilibrium during the main-sequence phase is the result of a balance between gravity and pressure, in which pressure's outward push exactly counteracts gravity's inward pull As the hydrogen in the core is consumed, the balance starts to shift, and both the star's internal structure and its outward appearance begin to change

4 Once a star begins to evolve away from the main sequence, its days are numbered. The post main-sequence stages of stellar evolution the end of a star's life depend critically on the star's mass. Low-mass stars die gently, whereas high-mass stars die catastrophically. The dividing line between these two very different outcomes lies around eight times the mass of the Sun

5 The Helium Core After approximately 10 billion years of steady core hydrogen burning, a Sun-like star begins to run out of fuel. The star's helium content increases fastest at the center, where temperatures are highest and the burning is fastest.

6 Because helium nuclei, with two protons each, carry a greater positive charge, their electromagnetic repulsion is larger, and even higher temperatures are needed to cause them to fuse at least 10^8 K. The shrinkage of the helium core releases gravitational energy, driving up the central temperature and heating the overlying layers. The higher temperatures now well over 10^7 K (but still less than 10^8 K) cause hydrogen nuclei to fuse even more rapidly than before. Hydrogen is burning at a furious rate in a shell surrounding the nonburning inner core of helium "ash" in the center. This phase is usually known as the hydrogen-shell burning phase.

7 The helium core is unbalanced and shrinking. The rest of the core is also unbalanced, fusing hydrogen into helium at an ever-increasing rate. The gas pressure exerted by this enhanced hydrogen burning increases, forcing the star's non-burning outer layers to increase in radius. Not even gravity can stop them. While the core is shrinking and heating up, the overlying layers are expanding and cooling. The star is on its way to becoming a red giant. The change from normal main-sequence star to elderly red giant takes about 100 million years.

8 The star first evolves to the right on the diagram, its surface temperature dropping while its luminosity increases only slightly. The star's roughly horizontal path from its main-sequence location (stage 7) to stage 8 on the figure is called the subgiant branch. By stage 8, the star's radius has increased to about three times the radius of the Sun. The surface temperature at stage 8 has fallen to the point at which much of the interior is opaque to the radiation from within. Beyond this point, convection carries the core's enormous energy output to the surface.

9 One consequence is that the star's surface temperature remains nearly constant between stages 8 and 9. The nearly vertical path followed by the star between stages 8 and 9 is known as the red-giant branch of the H R diagram. By stage 9, the giant's luminosity is many hundreds of times the solar value, and its radius is around 100 solar radii.

10 Helium Burning and Helium Flash Stars with Masses greater than 2 MSUN as Helium burning begins, the heated core heats and expands, slowing the helium burn Stars with masses less than 2 MSUN as Helium burning begins, heated core will not expand initially. Why? The cores of these stars are supported by electron degeneracy resulting from the Pauli Exclusion Principle, (instead of thermal pressure) Pauli Exclusion Principle You can't have two things (electrons) in the same place at the same time. Applied at the atomic scale

11 No expansion -> no cooling -> runaway helium burning -> helium flash (hours) Finally core expands and cools Now stable helium burning begins and star resides on the Horizontal Branch a "helium main sequence" of sorts, where stars remain for a time Can we observe this evolution in real stars?

12 Look at the H-R diagram for globular cluster M80 We do see the stars in various stages of evolution, just as the theory predicts

13 Helium Flash (re-stated) After H burning stops in core Core is all helium and collapses due to loss of internal pressure (no hydrostatic equilibrium) Collapse stopped by electron degeneracy: density(10^8 kg/m3) note: water = 10^3 kg/m3 Electron degeneracy results from the Pauli Exclusion Principle: No two electrons can occupy the same space (very tightly packed, but there is a limit to the packing) core heats to 100 x 10^6 K Triple-alpha begins 3 He -> 12 C core heats more, temperature rises no expansion-no cooling triple alpha rate increases even more -> Helium Flash

14 Evolution of a Low-mass Star (about 1 x M SUN ) Collapse to a star in few x 10^7 yrs main sequence life lasts 10^10 yrs Helium core collapses and star expands into Red Giant Helium flash- core reaches 10^8K Helium core burns into carbon and oxygen and eventually helium shell burning with a carbon core formed at the center of the star

15 Planetary Nebula Dying low-mass star ejects its outer layers of gas. As the star expands, and helium shell burning proceeds, a series of helium-shell flashes cause the stars radius to pulsate These pulsations help push the outer envelope of a star to greater and greater distances These gases are ionized by star and glow. (They look a bit like planets through small telescopes, but have nothing to do with planets) Expand for about 50,000 years before fading. Very common (20,000 to 50,000 in the Milky Way)

16 Planetary Nebula is ejected (25-60% of mass ejected) expands for about 50,000 yrs before fading core collapses into white dwarf at center of planetary nebula

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18 Evolution Following Planetary Nebula Phase (solar mass star) core is all carbon very dense - degenerate carbon ignition never occurs temperature never reaches 600 x 106K after 75,000 years white dwarf cooling to black dwarf

19 Deaths of Stars Low-mass (lightweight) Stars (< 3 x MSUN) Planetary Nebula White Dwarfs Type I Supernova High-mass (heavyweight) Stars Type II Supernova Neutron star and Supernova remnant black hole and accretion disk

20 White Dwarf After a low-mass star burns its core into carbon and oxygen, heat generation stops and the core collapses again. The core will heat further as gravitational energy is released, but the collapse stops before the core is hot enough to burn carbon and oxygen because of the Pauli Exclusion Principle and Electron Degeneracy. The density in the core will not collapse further and supports gravitational weight of star.

21 Observation of White Dwarfs by the Hubble Space Telescope There should be many white dwarfs in globular clusters globular clusters are old: > 10 billion years this is enough time for the lighter stars to evolve into white dwarfs

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23 Solar Mass Stars H-core burning begins H-core burning ceases H-shell burning Helium flash - He-core burn begins He-core burn/ H-shell burns core burning stops / shell burning continues expulsion of outer layers planetary nebula with central star all fusion stops white dwarf - slowly cooling

24 Lower Mass Stars (less than a Solar Mass) Many have not had time to age off the main sequence (example: 0.74 Msun "lives" 20 billion years as main sequence star - longer than age of universe) If mass < 0.08 Msun Never gets hot enough in core (8 x 10^6 K) to start nuclear reactions "BROWN DWARF"

25 Evolution of Stars More Massive than the Sun High mass stars age more rapidly no helium flash if M > 2.5 x MSUN If M > 8 x MSUN Core temperature rises to 600 million K (6 x10^8 K) Carbon fusion is ignited Fusion of even heavier elements occurs, as star gets hotter and hotter

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