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
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11 The Stuff Between Stars Space isnt 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 stars 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
stars 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 Oceans 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 Cats 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 Suns core A million times denser than lead Slowly cool
off
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11
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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 doesnt fuse.
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11 Core-Collapse Iron core no outward pressure. Gravity wins!
Star collapses rapidly! Electron degeneracy cant stop it. Atomic
structure cant 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
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11
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11
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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
