Stars and Galaxies

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Stars and Galaxies
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Multiple Star Systems

• Alpha Centauri is the star closest to the sun.
• It is 4.3 light-years from Earth.
• Alpha Centauri is a triple star system.

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Binary Star Systems

• One half of the Stars in the sky have companion
  stars.
• Most of these stars are double-star systems in
  which two stars revolve around each other.
• Algol was discovered by Arab shepherds thousands
  of years ago.

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Constellations

• Stars groups that form patterns
• Ursa Minor (Little Dipper or Little Bear)
• Ursa Major (Big Bear)
• Orion (the Hunter)
• Canis Major (Big Dog)
• Scorpius, Leo and Virgo

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Novas

• A nova is a star that suddenly increases in
  brightness up to 100 times in just a few hours or
  days.
• Most novas are members of binary-star systems. It
  is believed that gases from the companion star
  strike the nova.

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Star Clusters

• Open clusters are well organized and contain
  hundreds of stars. (Pleiades)
• Globular clusters are arranged in a spherical, or
  round, shape. (Hercules)
• Star clusters appear on Earth as a faint, white
  cloud.

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Nebulae

• A nebula is a massive cloud of dust and gas.
• Nebulae are the birthplace of new stars.

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Galaxies

• Galaxies contain various star groups.
• Most galaxies are spiral galaxies. Spiral
  galaxies are shaped like pinwheels.
• Galaxies that vary in shape from nearly spherical
  to flat disks are called elliptical galaxies.
  They contain very little dust and gas. They are
  usually older.
• Irregular galaxies have no definite shape.

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The Milky Way Galaxy

• The Milky Way Galaxy is a spiral galaxy.
• Most of the older stars in the Milky Way are
  found near the nucleus of the galaxy.
• The Milky Way is estimated to be 100,00
  light-years in diameter and about 15,00
  light-years thick.
• The Sun is located in one of the pinwheel arms.
• The stars rotate counter clockwise around the
  center. This takes 200 million years.

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Formation of the Universe

• Astronomers use telescopes and spectroscopes to
  study the stars.
• A spectroscope can break up the light from a
  distant star into colors.
• After the light is focused into a lens, a prism
  separates light into its different colors.
• This band of colors are a spectrum.
• By using a spectroscope, scientists can determine
  if an object is moving toward the Earth or away.

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The Red Shift

• When a star is approaching the Earth, the light
  waves will be compressed.
• The wavelengths are shorter and are
  characteristic of blue and violet light.
• If the star is moving away from Earth, the waves
  will be expanded. Longer wavelengths are
  characteristic of red light.
• The spectrum of a star moving away from Earth is
  shifted toward the red end. (the Red Shift)
• Astronomers have concluded that the Universe is
  expanding since all the galaxies showed a red
  shift.

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The Big-Bang Theory

• Astronomers believe that the expanding universe
  is the result of an enormous explosion known as
  the big bang.
• The explosion occurred 15-20 billion years ago.
• As the matter moved away from the explosion,
  gravity caused clusters to form.
• These clusters became the galaxies of the
  universe.

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Future for the Universe

• In an open universe, it will continue to expand
  until the stars die and cease to exist.
• In a closed universe, the gravitational
  attraction between the galaxies will cause the
  movement away from each other to slow down. Then
  gravity will begin to pull the galaxies back
  toward the center of the universe until another
  big bang occurs.

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Quasars

• If the universe is expanding, then the objects
  near the edge of the universe are the oldest.
• Quasars, the most distant objects, are 12
  billion-light years away.
• A quasar is a star like object that gives off
  radio waves.

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Characteristics of Stars

• Size
• Mass
• Color
• Temperature
• Brightness

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Size of Stars

• Neutron star
• White dwarf star
• Medium sized star
• Giant star
• Super giant star

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Composition of Stars

• Hydrogen (60-80)
• Helium
• Oxygen
• Neon
• Carbon
• Nitrogen

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Surface Temperature of Stars

• The surface temperature of a star can be
  determined by the color.
• Blue-white (hottest)
• White
• Yellow (Sun)
• Red-orange
• Red (coolest)

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Brightness of Stars

• The brightness of a star depends on
• Size
• Surface temperature
• Distance from the Earth (absolute and apparent
  magnitude)

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The Hertzsprung-Russell Diagram

• As the absolute magnitude increases, the
  temperature also increases.

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Measuring Star Distance

• One method of measuring the distance to stars is
  called parallax.
• Parallax refers to the apparent change in the
  position of a star in the sky.
• Measurement is based on the triangle formed by
  the Sun, Earth and star.
• This method cannot be used for a star more than
  100 light-years away.

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Why do stars shine?

• Within the core of a star, gravitational forces
  cause nuclear fusion.
• Hydrogen is fused to helium.
• Most of the mass is changed into energy.
• The rest is changed into heat and light.

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The Sun

• The sun is 150 million kilometers from the Earth.
• The sun is 4.6 billion years old.
• The volume of the is 1 million times greater than
  Earth.
• The suns density is only ¼ of the Earth because
  the sun is made only of gases.

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The Layers of the Sun

• Three layers make up the atmosphere and one layer
  makes up its interior.

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Atmosphere of the Sun

• Corona (hottest part 1,700,000 degrees C)
• Chromosphere (27,800 degrees C)
• Photosphere (surface of the sun 6000 degrees C)

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Core of the Sun

• This is the hottest part of the sun.
• Hydrogen and helium fuse.

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Activity on the Sun

• Prominences loops of gas originating in the
  chromosphere
• Solar flares bright bursts of light on the
  suns surface
• Solar wind a continuous stream of high-energy
  particles released into space in all directions
  from the suns corona (can interfere with radio
  signals and telephone communications)
• Sunspots cooler than the rest of the suns
  surface they appear as dark spots (their movement
  shows that the sun rotates)

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The Evolution of Stars

• New stars are born from the gases in a nebula.
• When hydrogen in the cloud reaches a certain
  temperature (15,000,000 degrees C), nuclear
  fusion begins. A protostar, or new star, is
  formed.
• The main factor that shapes the evolution of a
  star is the mass it began with.

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Medium-Sized Stars

• For the first few billion years, a star shines as
  nuclear fusion occurs in the core.
• When most of the hydrogen is gone, the helium
  core shrinks and heats up again.
• As the outer shell expands, it cools and its
  color reddens and become a red-giant.
• When all of the helium atoms are fused into
  carbon, the star begins to die.

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White Dwarfs

• When a red-giant begins to die, gravity causes
  the stars matter to collapse inward and become a
  white dwarf which is very dense.
• The amount of time it takes the white dwarf to
  die depends on the mass of the star when it first
  formed. The smaller the starting mass, the
  longer a star will live.

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Massive Stars

• At formation, massive stars have at least six
  times as much mass as our sun.
• The evolution is the same as a medium-sized star
  until they become red giants. Instead of becoming
  a white dwarf, the star begins to produce oxygen,
  nitrogen and iron.
• The iron atoms will not fuse and begin to absorb
  energy. The star will explode in a supernova
  which can light the sky for weeks.

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Supernova

• During a supernova explosion, the heat reaches
  such high temperatures that iron atoms can fuse
  to form new elements.
• These elements explode into space and form a new
  nebula.
• Chinese astronomers recorded a supernova in 1054
  that lit the night sky for 23 days and could be
  seen for 600 nights. (Crab Nebula)

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Neutron Stars

• A star that began 1.5 to 4 times as massive as
  the sun will end up as a neutron star after a
  supernova.
• A neutron star spins very rapidly.
• Some neutron stars give pulses of radio waves and
  are called pulsars.

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Black Holes

• Stars with 10 or more times the mass of the sun
  will have even shorter life spans that white
  dwarfs or neutron stars.
• After a supernova explosion in these large stars,
  the core that remains is so massive, that without
  the energy created by nuclear fusion to support
  it, the core is swallowed by its own gravity.
• The gravity of the core is so strong that light
  cannot escape a black hole.

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Detection of Black Holes

• Most black holes have companion stars.
• When the gases from the companion hole are pulled
  into the hole and heated, they give off a burst
  of X-rays.