Life cycle of a star

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Life cycle of a star
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Birth of Stars

• All stars are born in nebulae, huge clouds of
  dust and gas
• These collapse under gravitational forces,
  forming protostars
• These young stars undergo further collapse,
  forming main sequence stars once fusion starts
• Protostars that are too small (lt0.08 Msun) never
  ignite and become brown dwarves

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Dumbbell Nebula
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Large Ant Nebula
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Horsehead Nebula
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Protostars to StarsGravitational Collapse

• Gravity pulls matter into the center of the
  protostar
• The force of gravity accelerates the incoming
  particles
• As particles collide they give up their kinetic
  energy as heat
• If the star is massive enough, 30 million years
  later core temperatures reach 1.0 X 107 K, hot
  enough for fusion

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Protostars to StarsFusion Begins

• Electrons are stripped off to form a plasma at
  high temperatures
• Density increases, nuclei are 10-15 m apart
  (touching)
• Strong nuclear force overcomes electrostatic
  repulsion between nuclei
• Fusion begins

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A Star is Born
500 light years away

• Fusion of small nuclei releases energy
• Fused nuclei weigh a little less than the nuclei
  they were formed from
• Mass defect becomes energy
• Emc2
• Thermal pressure due to increasing temperature
  balances gravitational contraction
• A stable star forms

New stars
Protostars
http//www.nasa.gov/images/content/107455main_regi
on_88_lgweb.jpg
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How Do We Know What a Star is Made of?

• Disperse the light from the star
• Look at atomic emission or absorption spectra

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The Big Idea

• A stars life cycle will depend on its mass
• The bigger its mass, the shorter its life
• The bigger its mass, the hotter it is
• Stars less than 0.08 Msun never ignite!
• Stars greater than 100 Msun cant resist thermal
  pressure so they explode
• The hotter it is, the faster it will convert H to
  He

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Classifying Main Sequence Stars

• Three sizes
• Sun-like stars
• up to 1.5 X the mass of the sun
• Huge stars
• 1.5-3 X the mass of the sun
• Giant stars
• More than 3X the mass of the sun

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Relative Star Sizes
http//www.essex1.com/people/speer/main.html
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Classifying Stars

• Stars differ in
• Mass
• Color
• Related to their surface temperature
• Blue is hotter than yellow which is hotter than
  red
• Brightness
• Related to how much energy the star is producing
  and how far away the star is

http//images.usatoday.com/tech/_photos/2005/03/10
/stars-main.jpg
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Star Color Depends on Surface Temperature

• Peak of curve gives you the color
• The hotter the surface, the more energy at all
  wavelengths

visible light
www.warren-wilson.edu/.../WebCamPub/Webcam.htm
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New Vocabulary

• Apparent brightness
• How bright the star looks to our eyes
• Luminosity
• Total amount of light energy a star emits
• Usually given relative to the sun as Lsun1
• Most luminous are 106 Lsun

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Young Starsare Main Sequence Stars

• These stars' energy comes from nuclear fusion,
  converting H to He
• 90 of all stars are main sequence stars
• For these stars, the hotter they are, the
  brighter they are
• The hotter they are, the quicker they burn out
• The sun is a typical main sequence star
• Formed 5 billion years ago
• Likely to last another 5 billion years

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Forming Red Giants and Supergiants

• Eventually the H in the core is all converted to
  He
• Thermal pressure decreases, star collapses inward
• Star reheats as it collapses, igniting He fusion
  to C in the core at higher temperature
• Outer shell blows up in size, but cools off
• 100X bigger diameter
• Temperature is about 4000 K
• Star moves off Main Sequence in H-R Diagram

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Dying Sun-Like Stars

• Fuel for fusion runs out
• Outer shell blows out forming planetary nebula
• Star collapses and reheats
• Collapse is halted by degeneracy pressure
• Electrons cant be shoved too close together
• White dwarf is formed

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White Dwarves

• Very hot because of gravitational collapse
• No energy production so can only cool by
  radiating energy
• Eventually becomes a black dwarf
• 80 of mass of original star, but now the size of
  earth

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Life Cycle of Massive Stars
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The Fate of Red Supergiants

• Massive stars burn a succession of elements as
  fuel
• When He runs out, big enough to collapse and get
  hot enough to fuse C to heavier nuclei, cycling
  up to Fe

http//imagine.gsfc.nasa.gov/docs/teachers/lifecyc
les/life_cycles_v2.1.ppt282,21,The End of the
Line for Massive Stars
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The Fate of Red Supergiants

• Fusion beyond Fe requires addition of extra
  energy, so star cools
• Collapse is incredibly fast, then Supernova
  explosion over in a few minutes
• All heavier elements are formed

Crab Nebulaleftover of supernova in 1054 AD
http//www.etacarinae.net/Spaceart/crabhighres.jpg
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Before and After
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Elements from Supernovae

• Elements are spewed out into space during the
  explosion
• We can identify these elements from the
  wavelengths of their X-ray emissions

All X-ray Energies
Silicon
http//imagine.gsfc.nasa.gov/docs/teachers/lifecyc
les/life_cycles_v2.1.ppt291,25,Elements from
Supernovae
Calcium
Iron
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Whats Left Neutron Stars

• Neutron stars are really dense and small
• Electrons and protons fuse to form neutrons
• 1.4 - 3 Msun compressed into a ball with radius
  of 10 km (smaller than Warren!)
• A thimbleful would weigh more than 100 million
  tons on earth
• Neutron stars are really hot
• Surface is 1,200,000 F

http//observe.arc.nasa.gov/nasa/space/stellardeat
h/stellardeath_3ci.html
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Neutron Stars are Pulsars

• Neutron stars can sometimes be detected as
  pulsars
• Conservation of angular momentum makes them spin
  up to 100 X second
• Huge magnetic fields focus X-ray or light
  emission into cones
• Lighthouse effect

http//imagine.gsfc.nasa.gov/Images/basic/xray/pul
sar.gif
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Black Holes

• More massive than neutron stars
• So dense that nothing entering its gravitational
  field ever escapes, not even light!
• The edge of a black hole, the point of no return,
  is referred to as event horizon
• Most galaxies are now thought to have black holes
  with masses of 1.2 billion suns at their centers

Core of galaxy with accretion disk of 1.2 X 109
Msun
http//science.howstuffworks.com/black-hole3.htm
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Summarizing It AllThe Hertzsprung-Russell (HR)
Diagram
More luminous
Cooler but much brighter means these are much
bigger
Sun
Hotter but very tiny
dim
blue
red
http//www.courses.psu.edu/astro/astro010_pjm25/he
rtzsprung2.html
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Determining the Age of a Cluster

• All stars in a cluster are born at approximately
  the same time
• Lifetime on main sequence depends on the mass of
  the star
• Most massive stars move off the main sequence
  soonest
• Look at largest stars left on main sequence to
  estimate age