Stars and Galaxies

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Chapter 2

• Stars and Galaxies

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Where are you?

• The Earth circles the sun
• The sun is one of billions of billions of stars.
• To measure distances between stars we a distance
  measurement called the Light- year
• 1 light-year is the distance light travels in one
  year.

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Light-Year

• Light moves at 300,000 km/sec
• Thats 186,000 mile/sec
• It would reach the sun in about 5 minutes
• How far would it go in a year?
• Nearest star is 4.3 light years away

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Binary Stars

• Most stars are found in pairs
• These stars revolve around each other
• If a dim star passes in front of a bright star,
  it will block its light.
• Called an eclipsing binary
• Algol dims every 69 hours
• The closest star -Alpha Centari is actually a
  triple star system

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Binary System
Side
Top
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Constellations

• Groups of stars that appear to stay together
• Zodiac
• Named after gods, animals, and heroes
• Stars are not necessarily near each other

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Nova

• A star getting suddenly brighter
• Occurs in a binary star system
• Gases from one star are pulled into the other.
• Causing a nuclear explosion.

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Clusters

• Smaller groups of stars within a galaxy
• Globular Clusters- Spherical shaped with may (up
  to 100,000 stars)
• Open clusters- less organized- with fewer stars
  ( hundreds )

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Nebula

• Gas and dust clouds in space.
• Most cant be seen
• If they reflect light from nearby stars they can
  be seen
• Probably the birthplace of new stars

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Ring Nebula
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Galaxies

• Huge collections of stars
• each may contain hundreds of billions of stars
• The major feature of the universe
• Maybe as many as 100 billion galaxies

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Types of Galaxies

• Elliptical - round, from flat disks to spheres -
  contain older stars
• Spiral- Flattened arms that spin around a center
• Irregular- no definite shape -less common

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Andromeda
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Large Magellanic Cloud
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M51
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M83
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Milky Way

• Our galaxy
• almost all the stars you can see in the sky
• 100,000 light-years across
• 15,000 top to bottom
• 100 to 200 billion stars

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Spectrum
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Prism

• White light is made up of all the colors of the
  visible spectrum.
• Passing it through a prism separate it.

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If the light is not white

• Stars give off different colors of light
• Passing this light through a prism does something
  different.
• How we know what stars are made of.

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• Spectra from stars will have lines missing

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Doppler Effect

• Change in wavelength caused by the apparent
  motion of the source.
• Cars moving by you
• Same things happen to light
• Light from objects coming toward you is
  compressed looks more blue
• Light from objects away looks more red

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Red Shift

• Light from galaxies moving away

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Blue Shift

• Light from galaxies moving toward us

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A big surprise

• No Galaxies showed blue shift
• All galaxies showed red shift.
• Which means
• All galaxies were moving away
• The universe is expanding

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The Big Bang Theory

• The universe started with a concentrated area of
  matter and energy.
• 15-20 billion years ago
• Then it exploded and has been expanding ever
  since
• Faster moving stuff traveled farther
• Explained red shift

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Big Bang Theory

• Predicts energy should be evenly distributed
• Astronomers did find it
• Called background radiation
• Evenly spread throughout the universe.

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Gravity

• Force of attraction
• All objects attract each other.
• Pulled matter into clumps
• These clumps became bigger
• became galaxies

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Open or closed?

• Two possible results of big bang.
• Open universe will continue expanding
• Stars will eventually lose all energy
• end of universe is emptiness.
• In a few hundred billion years

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Closed Universe

• Gravity will eventually pull all the galaxies
  back together.
• Eventually all matter will come back together at
  the center of the galaxy
• Blue shift
• Packed into a area as small as a period.
• Then another big bang
• Every 80 to 100 million years.

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Quasars

• Quasi - stellar radio source
• Quasi- means something like
• stellar means star
• Most distant objects in the universe -12 billion
  light years
• Give off tremendous energy as x-rays and radio
  waves
• as much as 100 galaxies

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Quasars

• 1 sec, enough for 1 billion years electricity for
  Earth
• At the edge of the universe
• At the very beginning of the universe

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Another Tool

• Spectroscope
• Breaks the light of a star up into its colors
• Called a spectrum
• Kind of spectrum tells scientists
• what the star is made of
• which way and how fast it is moving

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Stars

• Are formed by the same forces
• Have different
• Size
• Composition
• Temperature
• Color
• Mass
• Brightness

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Size

• 5 main categories
• Medium sized - like our sun
• from 1/10 size of sun to 10 times its size
• Giant stars- 10 to 100 times bigger than the sun
• Supergiant stars- 100 to 1000 times bigger than
  the sun

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Size

• White dwarfs- smaller than 1/10 the size of the
  sun
• Neutron stars - smallest stars - about 16 km in
  diameter

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Composition

• Determined with a spectroscope
• by the colors of light it gives off
• The lightest element Hydrogen makes up 60 - 80
  of a star
• Helium is second most
• 96-99 is hydrogen and helium
• rest is other elements -

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Temperature

• Color also indicates temperature
• hottest surface 50000 C
• coolest -3000C

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Brightness

• Magnitude - measure of brightness
• Apparent magnitude - how bright it looks from
  earth
• Absolute magnitude - how bright it really is
• Variable stars - brightness changes from time to
  time
• Cephid variables - pulsating variables- change
  both brightness and size

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Hertzsprung-Russell diagram

• Found that as temperature increased, so did
  absolute magnitude
• 90 of stars followed this pattern
• Called main sequence stars
• Other 10 were once main sequence stars but have
  changed over time

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Absolute Magnitude
50000
20000
10000
6600
6000
5000
3000
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Distance to stars

• One method is parallax
• Apparent change in position as the earth goes
  around the sun

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Measure the angle to the star
Wait half a year
Measure the angle to the star
Triangle tells distance
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Distance to stars

• Parallax works only to 100 light-years
• More than 100 light-years they use a complicated
  formula based on apparent and absolute magnitude.
• More than 7 million light-years they use the red
  shift

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Why Stars Shine

• Stars are powered by nuclear fusion
• Hydrogen atoms join to form helium
• Happens because gravity pulls the atoms in the
  core so close together
• The sun turns 600 billion kilograms of hydrogen
  to 595.8 kilograms of helium every second
• The 4.2 billion kilograms of mass are turned to
  energy -light, heat, UV, x-rays
• E mc2

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The Sun

• An average star
• Over 1 million earths would fit inside
• 1/4 the density of the Earth
• made of 4 layers

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• Corona- Outermost layer
• Temp-1,700,000ºC
• Few particles

• Chromosphere-
• middle of atmosphere
• Temp-27,800ºC
• 1000s of km thick

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Corona
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Chromosphere
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• Photosphere-
• Temp-6000ºC
• 550 km thick
• Surface of the sun

Core-
1,000,000ºC
15,000,000ºC
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Activity on the Sun

• Storms on the sun
• Prominences- Loops or arches of gas that rise
  from the chromosphere
• Solar Flares- Bright bursts of light, huge
  amounts of energy released
• Sunspots- Dark areas on the suns surface
• in the lower atmosphere
• Motion shows the rotation of the sun
• Interferes with radio

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Solar Prominence
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Solar Wind

• Continuous stream of high energy particles.
• Can also interfere with radio and TV

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Star Life Cycles

• Stars change over time
• New stars form from nebulae
• Gravity pulls the dust and gas together
• Mostly hydrogen
• Forms a spinning cloud
• Hydrogen atoms hit each other and heat up

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Star Life Cycle

• When the temperature reaches 15,000,000 C fusion
  begins
• Makes a protostar - a new star
• What determines the life cycle of the star is how
  much mass it starts with.

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Medium-Sized stars

• Shine for a few billion years as hydrogen turns
  to helium.
• When hydrogen is used up, the core is almost all
  helium.
• Helium core shrinks and heats up
• Makes outside expand and cool
• Gives off red light
• Becomes red giant

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Medium-Sized Stars

• Helium in core turns to carbon
• Last of hydrogen gas drifts away to become a ring
  nebula or a planetary nebula.
• When last of helium is used up the core collapses
  and becomes a white dwarf
• Incredibly dense- a teaspoon will weigh tons

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How long

• It depends on the mass.
• The smaller a star starts out, the longer it
  takes
• From a few to 100 billion years for medium sized
  stars
• The sun will take about 10 billion years

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Massive Stars

• Start with at least 6 times the mass of the sun.
• Like medium stars up until they become red
  giants.
• The helium in the core becomes carbon, but it
  keeps getting hotter.
• Carbon atoms for heavier elements like oxygen and
  nitrogen and even iron

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Massive stars

• Cant go further than iron.
• Iron absorbs energy until it explodes in a
  supernova
• Temperatures up to 100,000,000,000C
• Then heavier atoms can form
• Explosion results in a new nebula,but with the
  new elements

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Neutron Stars

• If the star started out 6 to 30 times the mass of
  the sun, the core of the exploding star becomes a
  neutron star.
• As massive as the sun, but only 16 km across.
• Neutron stars spin rapidly and give off pulses of
  radio waves
• If these radio waves come in pulses it is called
  a pulsar

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Black holes

• If the star was bigger than 30 times the mass of
  the sun
• The left over core becomes so dense that light
  cant escape its gravity.
• Becomes a black hole.
• Grab any nearby matter and get bigger
• As matter falls in, it gives off x-rays.
• Thats how they find them