Formation and Structure of Stars

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Formation and Structure of Stars

• Chapter 9
• Mr. Saks
• Astronomy

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Existence

• Stars exist because of gravity
• Gravity makes clouds of gas contract
• Stars spend long lives generating nuclear energy
• This nuclear energy at the center balances their
  gravity
• In the end they die because they deplete their
  fuel source and suffocate under their own gravity

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Star Formation
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Birth of Stars

• In order to understand star formation we must
  know
• Correlation between young stars and clouds of gas
• Correlation between large clouds of gas
  illuminated by the hottest and brightest of the
  new stars
• Do stars form in such clouds?
• To study stars we must know the clouds

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Interstellar Medium II

• Interstellar Medium is the gas and dust between
  stars
• 75 of mass is hydrogen
• 25 mass is helium
• Traces of carbon, nitrogen, oxygen, calcium,
  sodium, and heavier atoms
• About 1 is microscopic dust (cigarette smoke)
• Mostly carbon and silicates mixed or coated with
  frozen water
• Distance between dust grains about 150m

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Interstellar Medium

• Nebula is any cloud of gas and dust
• Latin for cloud
• Emission nebula is a nebula hot enough to ionize
  nearby gas producing an emission spectrum
• Strong Balmer lines signifying high H content
• Reflection Nebula are nebula that appear blue
  because light of nearby stars are scattered by
  dust in the nebula
• Looks blue because blue is scattered easier than
  red

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Interstellar Medium III

• Some clouds make their presence known by blocking
  star light behind them
• A less dense cloud will partially block star
  light but allow some through
• Since blue scatters easier the light will appear
  red in color
• Interstellar Reddening

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Interstellar Medium IV

• A lot of interstellar medium is observed in
  nonvisible wavelength
• Dust, although cold, will radiate energy in the
  far infrared
• Infrared Cirrus faint, whispy network of dusty
  clouds covering entire sky
• Shows that the interstellar medium is in constant
  motion
• X-ray shows areas of very hot gas produced by
  exploding stars
• UV provides distribution of gas and its chemical
  composition and temperature
• Radio waves allow us to see atoms in space
  linking together to make molecules

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Formation of Stars from Interstellar Medium

• Gravity is a force that wants to squeeze things
  together
• Heat is an expansion process resisting gravity
• Molecular Clouds a dense interstellar gas cloud
  in which atoms are able to link together to form
  molecules such as H2 or CO.
• This H2 is usually found in cool gases of about
  10 K.
• Stars form in these clouds when the densest part
  of the clouds become unstable and contract under
  their own gravity

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Star Formation

• Some collapse due to collision with shock waves
  (sonic boom)
• Supernova explosions
• Birth of hot stars
• collision of two interstellar clouds

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Stars forming Stars

• Giant molecular clouds we see stars forming in
  repeating cycles
• High mass stars form more stars
• Radiation or supernovae explosions cause waves
• Waves compress surrounding gas
• Compression triggers formation of new stars

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Cloud Collapsing

• Cloud collapses under gravity
• Falling atoms pick up speed
• Increased speed means increased energy
• Temperature is the speed at which atoms are
  moving
• Atoms move faster, temperature increases
• Contracting interstellar cloud heats gas by
  converting gravitational energy into thermal
  energy

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Association

• Collapsing cloud fragments and produces 10 to
  1000 stars
• Star Cluster are stars held together in a stable
  group by their gravity
• Association is a group of stars that are not
  gravitationally bound to one another
• They eventually drift apart in a few million
  years
• Youngest associations are rich in young stars
  such as O and B stars

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Formation of Protostar

• Protostar is an object that will eventually
  become a star
• As material falls toward center, picks up speed,
  picks up temperature
• This action transforms a cold gas into a warm
  protostar buried deep in the dusty gas
• Evolutionary Track to how a star changes in
  luminosity and temperature
• Can be tracked on HR diagram

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Forming a Protostar

• Protostar original free fall contraction is
  slowed by increasing density and pressure
• Hot gas resists gravity
• Can contract only as fast as it can radiate
  energy into space
• Must contract to get hot enough for nuclear
  fusion
• Gravity is converted to thermal energy
• ½ to heat the protostar
• ½ is released to space
• As temperature increases gas become ionized with
  free electrons
• Becomes hot enough for nuclear reactions
• This explosion halts its collapse and nuclear
  explosion blows away its cocoon of gas and dust
• Star stabilizes and becomes a main sequence star

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Time to form

• How long this process takes depend on mass of
  star
• More massive stronger gravity faster
  contracts
• Our Sun (average) took about 30 million years to
  get to main sequence
• Stars 15 times our sun can do it in 160,000 yr
• HMWK 1

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Observations of Star Formation

• Birth Line Point at which a protostar becomes
  visible
• only when a protostar is hot enough to drive away
  its enveloping cloud of gas and dust do we see it
  from Earth
• Can be seen earlier in infrared
• To detect star formation we must use infrared,
  radio, and X-ray

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Evidence of Forming Stars

• Looking into a Nebula
• T Tauri stars fluctuate irregular with brightness
• Bright in infrared and suggest surrounded by gas
  and dust and some cases by dust disks
• Doppler shows gas flowing away from T Tauri stars
  located near birth line
• T Tauri shows that stars do blow away their outer
  dust cocoons before entering main sequence

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Evidence of Forming Stars

• Herbig-Haro objects are small neublae that
  fluctuate in brightness
• Produced by jets from newborn stars exciting the
  interstellar medium
• Bipolar Flows are high-energy jets in opposite
  directions causing a swirling disk

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Evidence of Forming Stars

• Bok Globules small, dark cloud only 1 ly in
  diameter that contains 10 to 1000 solar masses of
  gas and dust
• Related to star formation

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EGGs are small globules of dense gas and dust.
Some are contracting under their own density to
form protostars
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Fusion in Stars

• CNO Cycle
• For stars more massive than the Sun
• Hydrogen-fusion process using carbon, nitrogen,
  and oxygen as stepping stone
• Begins with a carbon nucleus, transforms if first
  into a nitrogen nucleus, then to oxygen nucleus,
  and then back to a carbon nucleus
• The carbon is unchanged in the end
• But along the way four hydrogen nuclei are fused
  to make a helium nucleus plus energy

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Heavy-Element Fusion

• Later stages in the life of a star
• Exhausted the hydrogen fuel
• To fuse helium need temp of 100 million K
• To fuse carbon need 600 million K
• Triple-alpha process is the fusion of He
• Hard fusion because Be is so volatile that it
  often breaks apart back into He before it can
  combine to make Carbon

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Heavier Element Fusion

• Temperature above 600,000,000 K Carbon Fuses
• Complicated because protons, neutrons, or a
  helium nucleus cause different reactions
• Higher temperatures will turn Al, Mg, and Si into
  heavier atoms

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Pressure Temperature Thermostat

• Stars have a built-in thermostat to keep
  reactions from reacting out of control
• Relation between gas pressure and temperature
• Nuclear reactions generate just enough energy to
  counter gravity
• To much reaction too much energy blow star
  apart lowering temperature slowing nuclear
  energy generation
• To little reaction too little energy star
  contract slightly increasing central
  temperature increasing nuclear energy
  generation
• Always trying to balance the energy out

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Stellar Structure

• Laws of Mass and Energy
• Conservation of mass is the total mass of star
  must equal the sum of the masses of its shells
• Conservation of Energy
• Amount of energy flowing out of the top layer in
  the star must be equal to the amount of energy
  coming in at the bottom plus whatever energy is
  generated within the layers

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Hydrostatic Equilibrium

• Hydrostatic Equilibrium in a star is a balance
  between weight and pressure
• Knowing the temperature of the outer shells and
  the composition of the star we can make
  predictions on the temperature and pressure
  inside the star with this idea of Hydrostatic
  Equilibrium

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Energy Transport

• Energy transport tells us that energy flows from
  areas of hot regions to cooler regions by
  conduction, convection, or radiation
• Conduction is the transfer of energy by direct
  contact
• Convection is the transfer of energy by
  temperature and density differences
• Radiation is the transfer of energy by wave
  motion
• Cool and dense gas allows less radiation to get
  through (opaque)
• Hot and thin gas, the photons can get through
  easier
• Opacity is the resistance to flow of radiation in
  a gas
• Depends strongly on its temperature

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Four Laws of Stellar Structure

• Conservation of Mass
• Conservation of Energy
• Hydrostatic Equilibrium
• Energy Transport

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Main Sequence Stars

• Mass-Luminosity Relation
• More massive the more luminosity
• Less massive the less luminosity
• Hydrostatic Equilibrium tells us pressure must
  balance weight
• Bigger star more pressure more weight
• Brown Dwarfs objects that are about 12 times the
  size of Earth are not all that big and thus not
  all that luminous
• Forming star that is not massive enough to start
  hydrogen fusion
• This lower end main sequence star adds evidence
  to star formation
• Emit strongly in infrared

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Life of Main Sequence Star

• Star does not enter main sequence until fusion
  begins (muy importante)
• Main sequence stars are balanced by Gravity and
  nuclear fusion
• As hydrogen is depleted, He is created which
  has less outward pressure
• Gravity begins to squeeze the star raising core
  temperature
• Increased temp increases reactions which in turn
  increases energy which causes the star to expand
• This expansion causes outer surface to cool from
  being away from heat source

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Main Sequence

• As stars change their fuel their size,
  luminosity, and energy change
• Their position on HR diagram begins to climb up
  and to the right
• By upper edge almost all H is gone

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Our Sun

• The Sun 5 billion years ago was only about 70 as
  luminous as it is now
• In 5 billion more years it will have twice its
  present luminosity
• Long before this
• Sun will raise Earths average temperature
• Ice caps will melt
• Climate will change
• Life will not survive
• We only have a billion years or so to prepare

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Main Sequence Life

• Star spends about 90 of its life in main
  sequence
• More massive the star the less time on main
  sequence because it uses fuel to quickly
• 25 solar masses will exhaust fuel and die in 7
  million years
• Sun about 10 billion years
• Red Dwarfs about 100 billion years
• Which kind of stars do you think we find more of
  in our sky?

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Orion Nebula

• Found as fuzzy wisp on Orions sword
• Center has four brilliant blue-white stars known
  as Trapezium
• For years in visible light it looked like nothing
  really
• A ball of gas and dust

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Orions Nebula Evidence of Young Stars

• Contains lots of O and B stars which in turn
  appear bright
• Contains a lot of T Tauri stars which are known
  to be young
• Left shoulder of Orion are stars about 12 myo
• Right shoulder stars are 8 myo
• The Trapezium are no more than 2 myo
• Current theory states that stars from West
  shoulder triggered star formation in the belt
  which triggered star formation in Great Nebula
• HOW?

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Four Points of Orion Nebula

• First the nebula we see is only small part of
  vast, dusty, molecular cloud
• Second we see it only because it is hot ionized
  gas and is driven outward by thermal energy
• Third infrared observations reveal clear
  evidence of active star formation deeper in
  molecular cloud
• Fourth Many stars in nebula are surrounded by
  disk of gas and dust
• Disks like these do not last long around new
  stars and are evidence of young stars