Stars and Stellar Evolution

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Stars and Stellar Evolution

• Unit 6 Astronomy

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What are stars?

• Stars spheres of very hot gas
• Nearest star to Earth is the sun
• Constellations group of stars named for a
  mythological characters
• 88

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Characteristics of Stars

• Star color and temperature
• Color can give us a clue to stars temperature
• Very hot (above 30,000 K) blue
• Cooler red
• In-between (5000-6000 K) yellow

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Characteristics of Stars

• Binary stars and stellar mass
• Binary star stars that orbit each other (Pair)
• Because of gravity
• 50 of all stars
• Can calculate mass of star
• Equal mass --gt center of mass halfway between
  stars
• Size of orbits known --gt masses can be calculated

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Distances to Stars

• Light-year distance light travels in a year
  (9.5 trillion kilometers)
• Parallax slight shifting in the apparent
  position of a nearby star due to the orbital
  motion of the Earth
• Photographs (comparisons) --gt angle
• Nearest largest angles distant too small to
  measure
• Only a few thousand stars are known

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How bright is that star?
-1.4

• Brightness magnitude
• Apparent magnitude a stars brightness as it
  appears from Earth
• How big it is
• How hot it is
• How far away it is
• Larger number dimmer

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How bright is that star?

• Absolute magnitude how bright a star actually
  is
• Magnitude of star if I was a distance of 32.6
  light-years
• Ex Sun apparent magnitude -26.7, absolute
  magnitude 5
• More negative brighter, more positive dimmer

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Hertzsprung-Russell Diagram

• H-R diagram shows the relationship between the
  absolute magnitude and temperature of stars
• Can also help us infer distance, life span mass
• Stars are plotted according to their temperature
  and absolute magnitude
• Interpret stellar evolution
• Birth, age, death

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H-R diagram

• Bright stars are near the top and dimmer stars
  are near the bottom
• About 90 are main-sequence stars
• Hottest brightest
• Coolest dimmest

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H-R diagram

• Brightness of main-sequence stars are related to
  mass
• Hottest blue stars are 50 times more massive than
  the sun
• Coolest red stars and only 1/10 as massive
• Main-sequence stars appear in decreasing order
• Hotter, more massive blue stars --gt cooler, less
  massive red stars

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H-R diagram
Betelgeuse

• Red giants
• Above and to right of main-sequence stars
• Size --gt compare them with stars of known size
  that have same temperature
• Supergiants bigger
• Ex Betelgeuse

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H-R diagram

• White dwarfs
• Lower-central part
• Fainter than main-sequence stars of same
  temperature

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Variable Stars

• Stars might fluctuate in brightness
• Cepheid variables brighter and fainter in
  regular pattern
• Nova sudden brightening of a star
• Outer layer ejected at high speed
• Returns to original brightness
• Binary systems

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How does the H-R diagram predict stellar
evolution?

• Illustrate changes that take place in a star in
  its lifetime
• Position on H-R diagram
• Represents color and absolute magnitude at
  various stages of evolution

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Stellar Evolution

• How stars are born, age and die
• Study stars of different ages

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Life of a Star
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Star Birth

• Born in nebula dark, cool, interstellar clouds
  of gas and dust
• Milky Way 92 hydrogen, 7 helium
• Dense --gt contracts --gt gravity squeezes
  particles toward center --gt energy converted into
  heat energy

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Protostar Stage

• Protostar a developing star not yet hot enough
  to engage in nuclear fusion
• Contraction continues --gt collapse causes the
  core to heat much more intensely than the outer
  layer
• When is a star born?
• Core of protostar reaches about 10 million K --gt
  nuclear fusion of hydrogen starts

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Balanced Forces

• Hydrogen fusion
• Gases increase motion --gt increase in outward gas
  pressure
• Outward pressure from fusion balances inward
  force of gravity
• Becomes main-sequence star (stable)

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Main-sequence stage

• Balanced between forces of gravity (trying to
  squeeze into smaller space) and gas pressure
  (trying to expand it)
• Hydrogen fusion for few billion years
• Hot, massive blue stars deplete fuel in only few
  million years
• Least massive main sequence remain stable for
  hundreds of billions of years
• Yellow star (sun) 10 billion years
• 90 of life as main-sequence star
• Runs out of hydrogen fuel in core --gt dies

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Red Giant Stage

• Zone of hydrogen fusion moves outward --gt helium
  core
• All hydrogen in core is used up (no fusion in
  core) --gt still taking place in outer shell
• Not enough pressure to support itself against
  force of gravity --gt core contracts
• Core gets hotter --gt hydrogen fusion in outer
  shell increases --gt expands outer layer --gt giant
  body
• Surface cools --gt red
• Core keeps heating up and converts helium to
  carbon to produce energy

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Burnout and Death of Stars

• Low-mass stars
• 1/2 mass of sun
• Consume fuel slowly --gt main sequence for up to
  100 billion years
• Consume all their hydrogen --gt collapse into
  white dwarfs
• Eventually ends up as a black dwarf

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Burnout and Death of Stars

• Medium-mass stars
• Similar to sun
• Turn into red giants --gt once fuel is gone,
  collapse as white dwarfs --gt eventually black
  dwarf

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Burnout and Death of Stars
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Burnout and Death of Stars

• Massive stars
• Shorter life spans
• End lives in supernovas becomes 1 million times
  brighter (rare)
• Consumes most of its fuel -gt gas pressure does
  not balance gravitational pull --gt collapses --gt
  huge implosion --gt shock wave moves out and
  destroys the star (outer shell blasted into
  space)

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Formation and Destruction of Stars
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Stellar Remnants

• All stars collapse into one of the three white
  dwarf, neutron star, or black hole
• White dwarf remains of low-mass and medium-mass
  stars
• Extremely small with high densities
• Surface becomes very hot
• No energy --gt becomes cooler and dimmer
• Last stage of white dwarf black dwarf (small,
  cold body)
• Smallest most massive
• Collapse of larger stars
• Largest least massive
• Collapse of less massive stars

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Stellar Remnants

• Neutron star smaller and more massive than
  white dwarfs
• Remnants of supernovas
• Composed entirely of neutrons

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Stellar Remnants

• Supernovae outer layer of star is ejected --gt
  collapse into hot neutron star
• Pulsar emits short bursts of radio energy
• Remains of supernova

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Stellar Remnants

• Black hole a massive star that has collapsed to
  such a small volume that its gravity prevents the
  escape of everything
• Cannot be seen
• Evidence of matter being rapidly swept into an
  area
• Animation

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Where did the elements of the universe come from?

• After the universe became cool enough for atoms
  to form, they began to clump together into clouds
  of gas
• First stars made up of mostly hydrogen with a
  small amount of helium
• Heavier elements like iron and silicon not yet
  made inside stars
• More and more stars formed, became main-sequence,
  grew old, and died
• More and more matter was fused into heavier
  elements and expelled back into interstellar
  space by supernovas and dying red giant stars
• Eventually our sun and its planets formed from
  this interstellar gas and dust

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Citations

• TLC Elementary School The Moon and Beyond.
  Discovery Channel School. 2004.unitedstreaming.
• Science Investigations Earth Science
  Investigating Astronomy. Discovery Channel
  School. 2004. unitedstreaming.