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11 Stellar Evolution Where do gold earrings come from?
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11 Goals Where do stars come from? How do stars evolve? How does mass affect what happens? How do stars die?
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11 The Stuff Between Stars Space isn’t empty. Interstellar Medium – The gas and dust between the stars. All the interstellar gas and dust in a volume the size of the Earth only yields enough matter to make a pair of dice.
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11 Dust Space is dirty. Dust blocks or scatters some light. Result: black clouds and patterns against the background sky.
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11 The IR Universe Orion - visible Orion – by IRAS
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11 The Trifid Nebula – Gillian Schnider (Johnston, May 2003) Interstellar Gas
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11 Dust obscuring H emission nebula Horsehead Nebula – copyright Arne Henden
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11 HII Regions For light: atoms must be excited. Energy comes from very hot stars. Orion Nebula – copyright Robert Gendler
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11 Cold Dark Clouds If dust clouds block light, then inside thick dust clouds there should be no light at all. Without light, there is little energy. With little energy, gas inside is very, very cold. Inside molecules form.
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11 Gravity vs. Pressure Stars and other interstellar material are in a perpetual battle between forces pulling in (gravity) and forces pushing out (pressure). Gravity comes from the mass of the cloud or star. Pressure comes from the motion of the atoms or molecules. –Think of hot air balloons. –The hotter the air, the bigger the balloon.
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11 Star Formation Remember: Cold interstellar clouds: No heat = no velocity = no outward pressure. Gravity wins. Gas begins to contract. HOTTER COOLER
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11 How to Make a Star 1 2 3
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11 1. The Interstellar Cloud Cold clouds can be tens of parsecs across. Thousands of times the mass of the Sun. Temperatures 10 – 100 K. In such a cloud: –Something makes a region denser than normal. –Force of gravity draws material to denser region. –Gravitational collapse begins.
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11 Orion Nebula – copyright Robert Gendler
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11 Visible and IR image of protostars in the Orion Nebula.
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11 Concept Test A new star reaches the main sequence when inward gravitational collapse is: a.Halted by degeneracy pressure in the core. b.Halted when the atoms are pushed up against one another and contraction stops. c.Finally balanced by outward thermal pressure from nuclear reactions. d.Finally balanced by radiation emitted in the photosphere. e.none of the above.
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11 An H-R Life-Track
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11 The Main Sequence For the Sun: –While it took 40 – 50 million years to get here, the new star will spend the next 10 billion years as a main sequence star. Bigger Stars: –Everything goes quicker. Smaller Stars: –Everything longer.
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11 Now what? The mass of the star that is formed will determine the rest of its life! Recall: the more massive the star, the more pressure in the core. The more pressure, the more fusion. More fusion: –More energy produced –Hotter –Shorter life span
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11 Concept Test Which Cluster is the oldest? A B C D
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11 Open Clusters These are the new stars. Small groups of young stars. Slowly drifting apart. Jewel Box – copyright MichaelBessell
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11 Concept Test Order the clusters from youngest to oldest. a. DBCA b. ACBD c. DCBA d. ADBC
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11 The Main Sequence A star is a delicate balance between the force of gravity pulling in, and pressure pushing out. Stars on the main sequence fuse hydrogen in their core to produce thermal pressure. Longest phase of a star’s life.
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11 What then? When the hydrogen in the core is almost consumed the balance between gravity thermal pressure pushing out and gravity pushing in is disturbed. The structure and appearance of the star changes dramatically. What happens then, depends on the star’s mass. Two cases: –Low-mass (< 8 x mass of Sun) –High-mass (> 8 x mass of Sun)
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11 Low-Mass Stars Where are low-mass stars? Longer lived or shorter than high-mass stars?
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11 Helium Ash Heavier elements, sink to the “bottom.” After 10 billion years, core is “choked” with helium “ash”. H He continues in shell around non-burning core.
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11 The Red Giant Branch Without fusion pressure in core: –Helium core collapses (no counter to gravity) –Density in core increases. 3He C + Energy in core 4H He + Energy in shell Extra energy results in extra pressure. Star expands. The star gets bigger while its outside gets cooler.
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11 The Onion Sun Red Giant Stars Layers of: –Non-fusing H –Fusing H –Fusing He –Non-fusing C “ash”
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11 …And the Solar System? A few million years from now: –Sun becomes slightly brighter –Ocean’s begin to evaporate –“Hot House” Earth A few billion years from now: –Sun swells up –Engulfs the inner Solar System –Certain death for terrestrial planets –Possible “spring” on the Jovian ocean-moons!
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11 Red Supergiant What happens when the Sun runs out of helium in its core? Same as before. Core shrinks, surface expands. Radius ~ 3 AU!
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11 Death Core is contracting and heating. –Surface is cooling and expanding. Will it finally become hot enough in core for Carbon to fuse? For the Sun: No. Gravity keeps contracting the core: 1000 kg/cm 3 ! What stops it? Electron degeneracy pressure!
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11 Electron Degeneracy Pressure from motion of atoms
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11 Electron Degeneracy Pressure from electron shells
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11 Where are we now? Core dead – nothing happening. Shells – burning H and He, but soon stop too. Outside atmosphere of star still cooling and expanding. …and expanding Force of radiation from burning shells blows the atmosphere away.
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11 NGC3242 – HST – Bruce Balick
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11 M57 – Ring Nebula
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11 M27 – Dumbbell Nebula – copyright VLT, ESO
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11 Cat’s Eye
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11 Eskimo Nebula
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11 Hourglass Nebula
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11 NGC2440 – HST – Bruce Balick White Dwarf Mass of Sun Radius of Earth Hot as Sun’s core A million times denser than lead Slowly cool off
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11 High-Mass Stars Think back to the first carbon core. How they get from main sequence to the carbon core stage is a little different. Now however, there is enough mass that it becomes hot enough to fuse carbon? Hot enough to eventually fuse lots of elements.
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11 The Iron Core 4H He + Energy 3He C + Energy C + He O + Energy The ash of one reaction, becomes the fuel of the next. Fusion takes place in the core as long as the end result also yields energy. This energy causes pressure which counters gravity. But Iron doesn’t fuse.
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11 Core-Collapse Iron core – no outward pressure. Gravity wins! Star collapses rapidly! Electron degeneracy can’t stop it. Atomic structure can’t stop it. Electrons and protons crushed together to produce neutrons. Neutrons pushed together by force of gravity.
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11 Supernova
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11 The result of the catastrophic collapse is the rebound and explosion of the core. From start of collapse to now: 1 second! Matter thrown back into the interstellar medium. Matter rushing outwards, fuses with matter rushing inwards. Every element after Fe is made in the instant of a supernova!
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11 M1 – Crab Nebula – copyright VLT
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11 Veil Nebula – Lua Gregory (English ’05)
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11 NGC 4526 – 6 Million parsecs away
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11 Concept Test Which of the following lists, in the correct order, a possible evolutionary path for a star? a.Red Giant, Neutron Star, White Dwarf, Nothing b.Red Giant, White Dwarf, Black Hole c.Red Giant, Supernova, Planetary Nebula, Neutron Star d.Red Giant, Planetary Nebula, White Dwarf e.Red Giant, Planetary Nebula, Black Hole
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11 Homework #13 For Wednesday a video For Friday read Chapter 10 Do Chapter 10 Quiz
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