Stars and Galaxies

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Stars and Galaxies

• Origin of Universe

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Life and Death of Stars

• On a clear night you can see about 6000 stars
• Stars are huge spheres of very hot gas that emit
  light and other radiation
• The Greeks grouped stars into constellations such
  as Orion
• Stars are located at different distances from
  Earth
• The unit light-year is the distance that light
  travels in one year 9.5 trillion kilometers

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Characteristics of Stars

• Stars are driven by nuclear fusion reactions
• Stars are made of Hydrogen and Helium and is held
  together by a huge gravitational force
• The pressure in the center of a star can be more
  than a billion times the pressure at the surface
  of Earth

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Characteristics of Stars

• The temp is hotter than 15 million Kelvins
• Density is more than 13 times the density of lead
• Fusion takes place in the core and turns Hydrogen
  to Helium, when this occurs leftover energy from
  the reaction escapes as light

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Movement of Energy

• Energy moves slowly through the layers of a star
• Energy moves through the layers by a combination
  of radiation and convection

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Movement of Energy

• Convection the hot gas moves away from the
  center and cooler denser air moves toward the
  center
• Radiation energy is transferred to individual
  atoms, the atoms absorb it and transfer it to
  others and the atoms near the stars surface
  radiate energy into space

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Movement of Energy

• Energy from a nuclear fusion reaction may take
  millions of years to work its way through a star
• Once it leaves the star it travels at the speed
  of light through space

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Studying Stars

• The brightness of a star depends on the
  temperature, size, and distance from Earth
• The brightest star in our sky is Sirius in the
  constellation Canis Major
• It is relatively close to Earth and is 10,000K,
  our sun is 6000K

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Studying Stars

• We learn about stars by studying light
• Stars produce other wavelengths of light X-rays
  to Radio waves

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Studying Stars

• A stars color is related to its temperature
• Hotter objects glow with light that is more
  intense and has shorter wavelength ? closer to
  blue
• Cooler objects glow with longer wavelength ?
  closer to red
• Yellow corresponds to a temp of around 6000K ?
  the color of our sun

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Fate of Stars

• Stars are born, go through different stages and
  eventually die
• Nearly 90 of the stars in our galaxy are in
  midlife ? still converting hydrogen to helium
• Stars are born in nebula

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Fate of Stars

• Our sun formed from a cloud of gas and dust
• Once the cloud spun faster and faster and got
  smaller and smaller the cloud reached a temp of
  15 million K
• At this point the Hydrogen combine and form
  Helium
• This is when the star turns on

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Fate of Stars

• The star now has a balance of inward and outward
  forces and keeps it stable
• Our sun is currently producing Helium, there will
  be a point when the majority of the core will be
  He and it will run out of H, this is when the
  star begins to die
• Scientists believe that the sun will produce He
  for another 5 billion years
• http//haydenplanetarium.org/resources/ava/page/in
  dex.php?fileS0801starform

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Fate of Stars

• The sun will become a red giant before it dies
• As fusion slows the pressure of the sun will drop
  and contract which causes the suns temp to rise
• When this happens the suns outer layers will
  expand and the sun will become a red giant
• A red giant is a large reddish star late in its
  life cycle
• The outer part of the red giant will expand pass
  the orbit of Earth almost to Mars

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Fate of Stars

• After 100 million years the sun will run out of
  He and contract farther, which cause the outer
  layers to contract again
• At this point the core is not hot enough to make
  new elements, the outer layers will continue to
  expand and separate from the star
• The star now becomes a white dwarf
• White dwarf is a small, hot dim star that is the
  leftover center of an old star
• Our sun will now be about the size of Earth

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Life Cycle of Star

• Stage 1- Stars are born in a region of high
  density Nebula, and condenses into a huge globule
  of gas and dust and contracts under its own
  gravity.

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Life Cycle of Star

• Stage 2 - A region of condensing matter will
  begin to heat up and start to glow forming
  Protostars.
• If a protostar contains enough matter the central
  temperature reaches 15 million degrees centigrade.

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Life Cycle of Star

• Stage 3 - At this temperature, nuclear reactions
  in which hydrogen fuses to form helium can start.
• Stage 4 - The star begins to release energy,
  stopping it from contracting even more and causes
  it to shine. It is now a Main Sequence Star.
• Our Sun

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Life Cycle of Star

• Stage 5 - A star of one solar mass remains in
  main sequence for about 10 billion years, until
  all of the hydrogen has fused to form helium.
• Stage 6 - The helium core now starts to contract
  further and reactions begin to occur in a shell
  around the core.

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Life Cycle of Star

• Stage 7 - The core is hot enough for the helium
  to fuse to form carbon. The outer layers begin to
  expand, cool and shine less brightly. The
  expanding star is now called a Red Giant.

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Life Cycle of Star

• Stage 8 - The helium core runs out, and the outer
  layers drift of away from the core as a gaseous
  shell, this gas that surrounds the core is called
  a Planetary Nebula.

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Life Cycle of Star

• Stage 9 - The remaining core (thats 80 of the
  original star) is now in its final stages. The
  core becomes a White Dwarf the star eventually
  cools and dims. When it stops shining, the now
  dead star is called a Black Dwarf.

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Fate of Stars

• Supergiant stars will explode in supernovas
• Massive stars evolve faster then smaller ones,
  they also make heavier elements in the core due
  to hotter temperatures
• Once the star forms an iron core a violent death
  is imminent

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Fate of Stars

• Iron causes fusion to stop, when it stops there
  is no longer any outward pressure to balance the
  gravitational pull and the star collapse due to
  its own gravity
• Due to the rapid collapse the star rebound with a
  shock wave that violently blows the star outer
  layers away from the core

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Fate of Stars

• This results in a huge bright explosion called a
  Type II supernova
• A supernova is a gigantic explosion in which the
  massive star collapses and throws its outer
  layers into space
• Heavier elements such as gold and lead are formed
  during a supernova
• After a supernova either a neutron star or black
  hole forms

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Supernovae
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Life of Star

• After a supernova either a neutron star or black
  hole forms
• A star with a mass of 1.4 to 3 solar masses will
  become a neutron star
• Neutron stars are only a few km in diameter and
  are extremely massive
• A thimbleful of neutron star will weigh more than
  100 million tons on Earth

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Black Holes

• If the leftover star has a mass greater than 3
  solar masses a black hole will form
• A black hole is an object so massive and dense
  that not even light can escape from its gravity
• Black holes cannot be seen directly
• But can be detected by observing radiation of
  light and Xrays from objects that revolve
  rapidly around them

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Milky Way and Other Galaxies

• A galaxy is a collection of millions or billions
  of stars bound together by gravity
• The closet galaxy to ours is millions of light
  years away
• There may be 100 billion galaxies in our universe
• Galaxies contain millions or billions of stars
• They may also contain many different types of
  stars

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Galaxies
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Galaxies

• Gravity holds galaxies together in clusters
• Galaxies are not spread out evenly through the
  universe
• They are grouped in clusters by gravity
• The Milky Way galaxy and the Andromeda galaxy are
  two of the largest members in the Local group ?
  more than 30 galaxies and more are being
  discovered

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Galaxy Clusters

• The Coma Berenices cluster of galaxies contains
  more than 1000 galaxies

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Galaxies

• Clusters of galaxies then cluster together even
  more and form superclusters
• Superclusters can contain thousands of galaxies
  containing trillions of stars
• Superclusters can be as large as 100 million ly
  across, they are the largest structures in the
  universe ? according to man

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Types of Galaxies

• There are three main types of galaxies
• Spiral, elliptical, and irregular

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Types of Galaxies

• Our galaxy is the Milky Way ? spiral
• The milky way has spiral arms made up of gas and
  dust
• All the stars we see in the sky are apart of the
  milky way
• Extraterrestrial life is looked for in our galaxy
• Our solar system is located about half the
  distance from the edge ? 26,000 ly from the center

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Galaxies

• Our galaxy has a huge bulge in the center and is
  very dense and composed of many old stars
• Many scientist think that in the center is a
  supermassive black star
• Some spiral galaxies have interstellar matter
  located inbetween the stars in a galaxy, this
  allows for new stars to be made

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Galaxies

• Elliptical galaxies have no spiral arms
• They contain mostly old stars and have little
  interstellar matter
• They can be as large as 200,000 ly in diameter

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Galaxies

• All other galaxies are classified as irregular
  galaxies
• They have a lack of regular shape and do not have
  a well defined structure
• Irregular galaxies can have little interstellar
  matter or an abundant amount and are compose of
  young stars

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How Galaxies Evolve

• When scientist look at galaxies they are looking
  back in time
• Quasars may be infant galaxies
• A quasar is a quasi-stellar radio source very
  luminous objects that produce energy at high rate
  and that are thought to be the most distant
  objects in the universe

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Quasars
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Quasars

• Scientists believe that each quasar has a huge
  central black hole and a large disk of gas and
  dust around it
• Telescopes detecting radio waves show quasars
  embedded in faint galaxies

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Galaxies

• Galaxies change over time
• As galaxies consume their gas and dust, they
  become unable to make new stars
• They also change as a result of collisions
• As galaxies approach each other the gravitational
  pull changes their shape
• The collision of gas and dust sets off a rapid
  burst of new star formation

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Colliding Galaxies
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Origin of Universe

• Universe is the sum of all space, matter, and
  energy that exist that have existed in the past
  and will exist in the future
• We see the universe now as it was in the past
• It takes time for light to travel in space

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Origin of Universe

• The farther away an object is in space, the older
  the light when we see it
• Pluto is 5 ly away
• Most of the universe is made of empty space
• Space is a vacuum with no air or air pressure

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Origin of Universe

• What happened at the beginning?
• The universe is expanding
• Hubble based his conclusion on studying spectral
  lines given off by stars and galaxies
• He found these lines were always shifted toward
  the red end of the spectrum ? red shift

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Origin of Universe

• Red shift is an apparent shift toward longer
  wavelengths of light caused when a luminous
  object moves away from an observer
• When an object is moving toward us ? blue shift
  shift toward shorter wavelengths of light caused
  when a luminous object moves toward an observer

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Origin of Universe

• Expansion implies that the universe was once
  smaller
• Although galaxies are attracted to one another
  due to gravity, they are generally moving away
  from one another
• They are expanding somewhat like an explosion
• Scientists call this hypothetical situation the
  big bang
• Using estimates from the red shift and if the
  theory is correct, scientists have determined the
  universe to be 10-20 billion years old

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Big Bang

• The big bang theory states that the universe
  began with a gigantic explosion 10-20 billion
  years ago
• According to this theory, nothing existed before
  the big bang, no time, no space
• The explosion released all of the matter and
  energy that still exists in the universe today

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Big Bang

• Cosmic background radiation supports the big bang
  theory
• Due to cosmic background radiation left over from
  the big bang, the estimated temp of the universe
  is 2.7 K
• CBR is a steady but very dim signal from all over
  the sky

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Big Bang

• Radiation dominated the early universe
• According to the theory, the expansion of the
  universe caused it to cool allowing for protons,
  neutrons, and electrons to form from the
  radiation within a few seconds after the big bang

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Big Bang

• Process in stars lead to bigger atoms
• All other elements form in stars
• Nuclear fusion is stars produce H, He, Be, and C
  and even Iron
• Heavier elements form during supernovas such as
  Gold and Lead

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Future of Universe

• The future of the Universe is uncertain
• It is still expanding but may not do so forever
• Gravity also wants to pull the universe back in

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Future of Universe

• Three things may happen
• The universe may expand forever
• The expansion will gradually slow down, and will
  approach a limit in size
• It will stop expanding and fall back into itself

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Fate of Universe

• The fate of the universe depends on mass
• Which one of the 3 depends on the amount of mass
  in the universe
• If there is not enough gravity will not will and
  will expand forever
• If there is the right amount it will balance out
  and stay static
• If there is too much it will eventually collapse
  in on itself ? the big crunch

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Fate of Universe

• There is a debate over dark matter
• Much of the matter is what we can see
• But the interactions between stars suggest
  another type of undetectable matter called dark
  matter

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Fate of Universe

• Dark matter may be in planets, black holes, or
  brown dwarfs
• Brown dwarfs ? starlike objects that lack enough
  mass to begin fusion
• As much as 90 of the universe could be made of
  dark matter, what it is, where it is, is a
  mystery
• Just as the universe remains an ultimate mystery