Stars

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Stars
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How Many Stars?

• With the naked eye, we can see about 6,000 stars
  in the sky.
• However,
• Through a telescope, we can see more than 3
  billion stars.

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The Sun is a Star

• Therefore, like our sun, stars are balls of gases
  that give off tremendous electromagnetic energy.
• Electromagnetic energy is what causes
  electromagnetic waves gamma, x-rays, uv,
  visible, ir, micro and radio.
• Therefore, all stars give off these forms of
  radiation.

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Nuclear Fusion

• Like our sun, all stars use nuclear fusion.
• Nuclear fusion is combining light atomic nuclei
  into heavier atomic nuclei, to turn mass into
  energy.
• Example H to He.

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Color of Stars

• From Earth, the stars appear to be specks of
  white light.
• In truth they vary in color.
• Our sun is a yellow star.

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Mostly H and He

• Just like our sun, the most common elements in
  the stars are Hydrogen and Helium.
• What are the most common elements on Earth?

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The Temperatures of Stars

• The Surface temperature of a star is indicated by
  its color.
• The temperature of most stars ranges from 2,800C
  to 24,000C.

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Color and Temperature

• The hottest stars emit a blue light indicating
  that they are in the 35,000C to 50,000C range.
• The coolest stars are red stars. They have an
  average surface temperature of 3,000C.
• Yellow stars, such as the sun, have surface
  temperatures of about 5500C.

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Size of Stars

• Stars vary in size and mass.
• Dwarf stars are about the same size as Earth
  about 12,756 km.
• Our sun is 1,390,000 km.
• Some giant stars have diameters that are 1,000 X
  our sunabout 1 billion km.

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Mass of Stars

• Many stars have about the same mass as our sun.
• However, stars may be more or less massive.

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Stellar Motion

• Stars have two types of motion
• Actual can only be measured with a high powered
  telescope.
• Apparent is caused by the movement of the Earth,
  not the star.
• Apparent motion makes it looks like all the stars
  revolve around the North StarPolaris.
• However, this is only because Polaris is just
  above the North Pole.

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Do the Stars Change

• Stars located on the side of the sun opposite the
  Earth are obscured by the sun so we cant see
  them.
• As the Earth orbits the sun, different stars
  become visible.
• Since the Earth is moving eastward, the stars
  appear to move westward every night.

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Seasons and Stars

• As the seasons change, some stars seem to appear
  and some seem to disappear.
• Of course, what is really happening is the sun is
  blocking our view of some stars and allowing us
  to see others as we revolve around it.
• This is why in different seasons we can see
  different constellations.

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Circumpolar Stars

• Some stars are always visible in the night sky.
• However, which stars are always visible depends
  on which hemisphere you are in.
• In the Northern Hemisphere, the little dipper is
  always visible and appears to circle Polaris.
• If you lived at the North Pole you would always
  see the same stars.

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Actual Motion of the Stars

• Stars rotate on their axis.
• One star may revolve around another star.
• They may move away or toward our solar system.
• The Doppler Effect
• Stars moving toward our solar system show a blue
  shift.
• Stars moving away from our solar system show a
  red shift.

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Light Year

• The speed of light is 300,000 km/s.
• This means that light travels 9.46 trillion km in
  one year.
• Light from the sun takes 8 minutes to reach the
  Earth.
• Therefore, the sun is 8 light-minutes from the
  Earth.

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Proxima Centauri

• Proxima Centauri is the next closes star to the
  Earth after our sun.
• It is 4.2 light years from Earth which is nearly
  300,000 times the distance of the Earth from the
  sun.

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Polaris

• Polaris, the North Star, is 700 light years from
  Earth.
• When we look at Polaris we are not seeing it as
  it is now.
• We are seeing it as it was
• 700 years ago!

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The Evolution of Stars

• A typical star exists for billions of years.
• Since we dont have a billion years to study
  them, how can we know how they live, die and
  evolve?

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Luminosity

• The first step for astronomers is to plot the
  surface temperatures of a star against their
  luminosity.
• Luminosity is the total amount of energy a star
  gives off each second.

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The Hertzsprung-Russell Diagram

• Luminosity Shows a Pattern

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• Temperature is plotted from left (hottest) to
  right (coldest).
• Luminosity is plotted from top (brightest) to
  bottom (faintest).

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The Main Sequence Stars

• The temperature and luminosity for most stars
  falls within a band that runs diagonally through
  the middle of the diagram.

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What the H-R Diagram Tells Us

• The H-R diagram tells us that most stars start
  out very hot and bright blue.
• They end up red and cold.
• This means that our sun is an older middle aged
  star.
• If you were to compare it to human years, it
  would be around 50.
• Proxima Centuari, the next closest star is a red
  stararound 85.

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The Solar Nebula

• Recall that stars begin as solar nebulas, clouds
  of gas and dust.
• They are then condensed by gravity for millions
  of years forming a protostar.
• The protostar becomes so hot (10,000,000C) that
  all of the nuclei ionize into plasma.
• Nuclear fusion begins.

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The Main Sequence Stage

• The second and longest stage in the life of a
  star.
• Energy continues to be generated as Hydrogen
  fuses into Helium releasing enormous energy.
• A star with the mass of our sun stays on the main
  sequence for about 10 billion years.

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More Mass Shorter Life

• Stars that are more massive than our star burn
  Hydrogen faster.
• They only stay on the main sequence for about 10
  million years.
• Stars that are less massive than our sun may be
  able to stay on the sequence for hundreds of
  billions of years!

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How Long Will Our Sun Live

• Scientists estimate that over a period of almost
  5 billion years, the sun has only converted 5 of
  its original Hydrogen to Helium.
• In another 5 billion years it will have converted
  10.
• Scientists believe that once 10 of the Hydrogen
  fuel is gone, nuclear fusion will stop in the
  suns core.

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Off the Main Sequence

• Once fusion stops, the suns temperature and
  luminosity will change.
• It will no longer be on the main sequence.

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The Third Stage

• Once almost all of the Hydrogen atoms in the core
  have fused into Helium atoms, the star enters the
  third stage.
• Without Hydrogen fuel, the core contracts under
  the force of its own gravity.

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The Helium Core

• The Helium core gets hotter and hotter.
• It transfers energy from the core to the
  surrounding Hydrogen atmosphere.
• Hydrogen fusion takes place again but now it is
  not contained by the core.
• Instead it causes the star to expand rapidly.

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Red Giants

• The stars shell of gases grow cooler as it
  expands.
• This causes the reddish glow we see in the large
  stars that were once the size of our sun.

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Supergiants

• Main sequence stars with more mass than the sun
  will form bright red supergiants 100 times larger
  than the sun.
• Betelgeuse is an example of a supergiant.
• Remember that these stars are actually cool on
  the surface compared to other stars.

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The Final Stages

• The Helium atoms continue to fuse and cool.
• Eventually they become carbon and oxygen.
• Unlike the conversion from Hydrogen to Helium,
  Helium to carbon and oxygen does not gain energy,
  it depletes it.
• Eventually there is no energy left from fusion.

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Death of a Sunlike Star

• As the stars outer gases drift away, the
  remaining core heats these gases.
• A planetary nebula is formed around the dying
  star.

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White Dwarf

• As a planetary nebula disperses, gravity causes
  the remaining matter to collapse inward.
• The matter collapses until it cannot be
  compressed further.
• What is left is a hot, extremely dense core of
  matter called a white dwarf.
• These shine for billions of years before they
  completely cool.

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Black Dwarf

• When a white dwarf no longer gives off light, the
  star becomes a black dwarf.
• Astronomers do not believe that any black dwarves
  yet exist.

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Novas

• Some white dwarves are part of a binary star
  system.
• If a white dwarf revolves around a red giant, the
  gravity of the white dwarf may capture gases from
  the red giant.
• As these gases build up it may cause a huge
  explosion of energy and matter.
• This is normally a nova.

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Nova

• A nova may cause a star to become extremely
  bright.
• Within days, it fades to its normal brightness.
• These explosions do not disrupt the stability of
  the binary system and may occur several times.

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Supernova

• A white dwarf in a binary system may have such a
  tremendous explosion that it blows itself apart.
• This is called a supernova!
• Supernovas are thousands of times more violent
  than novas.
• Supernovas destroy the white dwarf and may
  destroy the red giant.

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Death of a Massive Star

• Stars that have masses of more than 8 times the
  mass of the sun may produce supernovas without
  needing a red giant to fuel them.
• Such supernovas can radiate energy equal to the
  output of 400 million suns.

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The Most Massive Stars

• In these stars, supernovas are a normal part of
  the lifecycle.
• After these stars become supergiants, they
  contract with such incredible force that nuclear
  fusion begins again.
• The carbon atoms fuse into oxygen, magnesium or
  silicon.

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Iron Core

• Fusion continues until the core is made of iron.
• Iron is so stable that no more fusion can occur.
• The core collapses from its own gravity.
• The energy released as the core collapses is
  transferred to the outer layers of the star
  causing a tremendous explosion.
• More energy is released in minutes than our sun
  released in its entire lifetime.

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Neutron Stars

• Stars that contain about 10 or more times the
  mass of the sun do not become white dwarfs.
• After a star explodes as a supernova,
• The core may contract into a small but incredibly
  dense ball of neutrons called a neutron star.

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Density of a Neutron Star

• A single teaspoon of matter from a neutron star
  would weigh more than 100 metric tons on Earth!

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Pulsars

• Some neutron stars emit a beam of radio waves
  that sweeps across space like a lighthouse light
  sweeps across water.
• We call these stars pulsars because we detect
  pulses of radio waves every time the beam sweeps
  the Earth.
• Each pulse represents one rotation of the star.

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Black Holes

• Some massive stars produce leftovers too massive
  to become stable neutron stars.
• If the remaining core of a star after a supernova
  contains more than 3 X the mass of the sun, the
  star may contract further than the neutron star.
• The core of the star is crushed leaving a black
  hole.
• Astronomers once thought that the gravity of a
  black hole is so great that nothing, not even
  light can escape it.
• Now they are not so sure

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Quasars

• Quasar means quasi-stellar radio source.
• Quasars are related to galaxies, not stars.
• They are located in the centers of galaxies very
  distant from Earth and they are very bright.
• They may be connected to giant black holes with
  masses billions of times the mass of our sun.
• Quasars are among the most distant objects
  observed from Earth.

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Dark Matter

• The kinds of matter that make the material world
  as we know it is only 4 of the Universe.
• 23 is made up of another type of matter that
  does not give off light.
• We know it is out there because we can detect its
  gravity.

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Dark Energy

• 73 of the Universe is composed of dark energy, a
  force scientists believe opposes gravity.
• This energy may be pushing galaxies apart.
• This means that not only is the universe
  expanding, but the rate of expansion is
  accelerating!