Stars, Galaxies, and the Universe

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Stars, Galaxies, and the Universe

• Chapter 21

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Tools of Modern Astronomy

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Constellations

• Since man first started observing the heavens, he
  has grouped the stars into patterns.
• Each culture has seen their own patterns.
• A constellation is a group of stars that, when
  seen from Earth, form a pattern. The stars in the
  sky are divided into 88 constellations.

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Changing Constellations

• Because each star is actually moving in its
  orbit, the patterns we see in the night sky will
  change over time.
• They are so far away that we would not notice a
  lot of change for a long time, like 100,000
  years!
• http//www.astro.virginia.edu/class/oconnell/astr1
  30/im/proper_motion_big_dipper.gif

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Objects are Further than they Seem!

• Even though they look close together. They are
  often very far apart. They just appear in the
  same direction in the sky.

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The Zodiac

• Each month a different part of the night sky is
  visible, depending on where the Earth is in its
  orbit.
• Thats why your birth sign depends on the month
  your were born.

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Electromagnetic Radiation

• Energy travels through the vacuum of space in the
  form of electromagnetic (EM) waves.
• So the only way scientists can gather data on
  objects outside our atmosphere is to look at EM
  waves.

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Electromagnetic Spectrum

• All the frequencies of EM waves arranged in order
  of increasing frequency.

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EM Waves

• Visible light is just one type of EM waves.
• Scientists build telescopes to gather in all
  kinds.
• Radio telescopes gather radio waves.
• Infrared telescopes would gather infra red waves
  to study, and so on.

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Visible Light Telescopes

• Refracting telescopes use lenses to gather light
  and focus it onto a small area. Galileo was
  first to build one of these to observe the
  heavens in 1609.
• Reflecting telescopes use lenses and mirrors to
  gather light. The larger the mirror, the more
  light the telescope can collect.

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Other Telescopes

• Other telescopes detect IR, UV, and even higher
  energy waves.
• Remember, the only difference in all EM waves is
  their frequency.
• We must design telescopes to see these other
  frequencies of light.

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Observatories

• Buildings that contain telescopes.
• Often put on top of mountains so there is less
  air to blur the image.
• Often far away from city lights.

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Satellite Telescopes

• Since much of the UV, X-ray and gamma ray
  radiation are blocked by the atmosphere,
  satellites make great telescopes for these
  frequencies.
• The Hubble Space Telescope is a reflecting
  telescope with a 2.4m mirror.

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Spectrographs

• Breaks light into colors (frequencies).
• Used to gather information about stars, their
  chemical composition, and temperatures.
• Each element has a unique spectrograph.
• This is how scientists can tell what stars and
  planets are made of.

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Characteristics of Stars

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Stars

• A very large ball of mostly hydrogen and helium
  gas that shines extremely brightly by nuclear
  fusion.
• A star is basically an element creating factory!
• Our universe is full of stars.
• Our Milky Way galaxy contains hundreds of
  millions of stars.
• Our universe contains billions of galaxies!

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The Universe

• All of space and everything in it.
• Most of the universe seems to be empty space.
• The observable universe is over 10 billion light
  years across.
• It could be much larger!
• About 13.7 billion years old.
• Formed at the Big Bang.

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Distances to Stars

• Stars are so far apart that it is not practical
  to use kilometers.
• Light travels at 300,000 km/s, or 9.5 trillion km
  a year!
• So we use light years to measure distances in
  space.
• A light year is the distance light travels in one
  year which is about 5.88 trillion miles.
• Light from the closest star to us takes 4.2 years
  to reach us, so Proxima Centari is 4.2 light
  years away.

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Measuring Distances to Stars

• Parallax is one method to measure distances to
  stars that are somewhat close.
• Parallax is the apparent change in position of an
  object when you look at it from different places.
• Scientists look at a star when the Earth is on
  one side of the sun. Six months later they again
  look at that star. They measure the apparent
  shift of the background stars. The greater the
  parallax shift, the closer the star is.
• This method is only good for nearby stars, less
  than 1000 light years away.

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Parallax
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Classifying Stars

• Stars are classified by temperature.
• The brightness of stars is affected by its size
  and temperature.
• Even though they are all spheres of glowing gas
  that are powered by fusion, they can be very
  different from one another.

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Color and Temperature

• A stars color indicates its temperature.
• Reddish color stars are fairly cool, about
  3200C.
• Our sun appears almost white and is about 5500C.
• Rigel is bluish white, and is over 15,000C.
• They are classified by temperature, which puts a
  star in a spectral class.

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Sizes of Stars

• From Earth, all the stars appear to be the same
  size.
• But they range from 10s of km to super giants
  that are the size of our solar system.

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Brightness of Stars

• Brightness (luminosity) is the amount of light a
  star gives off.
• How bright a star appears from Earth depends on
  how far the star is from Earth and the amount of
  light it emits.
• The actual brightness of a star depends on its
  size and temperature.

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Apparent Magnitude

• The brightness of a star as seen from Earth.
• You cannot tell how much light a star gives off
  by its apparent magnitude. A small dim star
  close by can look as bright as a really bright
  star much further away.
• If two stars are equally bright, one closer to us
  will appear brighter than one further away.
• In this system, the smaller the number the
  brighter the star!
• So negative numbers are brighter than positive
  numbers
• How bright it seems.

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Absolute Magnitude

• The brightness of a star as if it were a standard
  distance from us (10 parsecs32.6 light years).
• Much more difficult to compute, as you need to
  know how far the star is from you.
• This method allows us to compare how bright stars
  are as if they were all the same distance from
  us.
• This way dimmer stars would appear dimmer, and
  brighter stars would appear brighter.
• How bright the star really is.

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Hertzsprung-Russell Diagram

• Graph of stars brightness and temperature.
• This makes a pattern with 4 areas that show the
  main classes of stars.
• Actually shows the life cycle of stars.

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Lives of Stars

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Lives of Stars

• Even though stars are not alive, they are said to
  have a life cycle, much as people do.
• Stars are born, go through childhood, middle age,
  old age, and eventually die.

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A Star is Born

• Stars are born in nebulas, a large amount of gas
  and dust spread over an enormous volume.
• Gravity can pull some of this gas and dust
  together. This contracting cloud eventually
  forms a protostar.

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Protostars

• Proto means earliest.
• A star is formed when the contracting gas and
  dust become so hot that nuclear fusion begins.
• This releases enormous amounts of energy.

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Birth of a Star 2
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Lifetimes of Stars

• The mass of a star determines how long it will
  live.
• Small stars have less fuel, but burn it slowly,
  like a small economy car.
• Large stars have lots of fuel, but burn it like a
  gas guzzling SUV!

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Mass and Lifetimes

• Small stars can live up to 200 billion years.
• Medium size stars (our sun) can live for about 10
  billion years.
• A star 15 times more massive than our sun might
  only live for 10 million years, less than tenth
  of one percent of our stars life.

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Supernovas

• A dying giant or supergiant star can suddenly
  explode. Within hours, the star blazes millions
  of times brighter.
• The explosion is called a supernova. After a
  supernova, some of the material from the star
  expands far into space. This material may
  eventually become part of a nebula.
• The nebula can then contract by gravity to form
  a new star. Astronomers think the sun began as a
  nebula that contained material from an ancient
  supernova explosion.

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White Dwarfs

• Small and medium mass stars have fuel for 10
  billion or more years.
• After their red giant phase, these stars lose
  their outer shells and have only the hot, white
  core remaining.
• They are only about the size of the Earth, but
  still have almost all their mass.
• A spoonful of material would have the mass of a
  large truck!
• When they completely run down, they lose even the
  slight glow they had.
• They are now black dwarfs.

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Neutron Stars

• A dying giant or supergiant star can suddenly
  explode.
• This explosion is called a supernova.
• They get millions of times brighter as they
  rapidly consume their fuel.
• The heat and pressure from this explosion can
  fuse very large elements, which can eventually
  end up in new stars and solar systems.
• The material left behind is a star smaller than a
  white dwarf. They can contain many times the
  mass of our sun but be only a few 10s of
  kilometers in diameter.

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Neutron Stars/Pulsars

• After a supernova, the center of the collapsed
  star contracts.
• The particles inside the star are forced together
  to form neutrons.
• This super dense star is small but heavy.
• If it is spinning it will send out beams of
  radiation that radio telescopes pick up as
  pulses.
• These are called pulsars.

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Neutron Stars 2

• Collapsed core of a large star.
• Pressure is so great in core that the electrons
  get pushed into the protons, forming neutrons.
• Only 10s of kms across
• million times denser than water.

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Black Holes

• The most massive stars can become black holes.
• After they nova out, they could have 5 or more
  times the mass of our sun left.
• This mass will contract due to its gravity.
• Eventually, it contracts so much that its mass is
  in a small area (like zero!).
• Its gravity is so strong that even light cannot
  escape from it.
• The remains are now a black hole.

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Black Holes Observed

• Since nothing can escape from a black hole, we
  cannot see them directly.
• We can observe the effects as they pull matter
  into them.
• As the matter falls into the black hole, it goes
  faster and faster. The particles bump into each
  other and give off X-rays, a very high energy EM
  wave.
• These X-rays can show us where a black hole might
  be found.

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Stars Rebirth

• Stars are constantly going through their life
  cycles as the universe ages.
• As stars get old and supernova, the dust from the
  explosion will eventually gravitationally
  collapse, allowing new stars and systems to be
  born.

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Quasars

• Quasi means something like, so quasi-stellar
  objects are called quasars.
• They are exceptionally bright and distant, over
  12 billion light years away. They seem to be
  from a time near the beginning of our universe.
• It is believed that they are distant galaxies
  with giant black holes at their centers.

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Star Systems and Galaxies

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Star Systems

• While our system has only one star, over half all
  stars are in groups of two or more called star
  systems.
• Pairs of stars are called binary stars while
  those with three stars are called triple systems.
• The closest star (Proxima Centari) to us is
  believed to be a triple system.

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Exosolar Systems

• Scientists have recently found evidence that many
  other stars have planets.
• They look for the wobble a planet would induce in
  a star that it is orbiting.
• This method works well for large planets, but not
  at all for small, Earth sized ones.
• Soon astronomers hope to be able to directly view
  exosolar planets with the new generation of space
  telescopes being planned.

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Galaxies

• A galaxy is a huge group of stars, dust, gas, and
  other celestial bodies bound together by
  gravitational forces.
• While there are many types, we will study the
  three main types spiral, elliptical, and
  irregular.

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Spiral Galaxies

• Have spiral, pinwheel shapes.
• Our galaxy, The Milky Way, is a spiral galaxy.
• Resemble hurricanes viewed from above.
• Have a central, flat disk containing a dense
  cloud of interstellar matter and young star
  clusters (mostly on the arms)
• Also contain a central bulge (or nucleus)
  containing older stars

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Elliptical Galaxies

• Look like flattened balls.
• Are the largest galaxies we find
• Found in clusters of galaxies.

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Irregular Galaxies

• These galaxies have no defined shape.
• Can be formed when galaxies collided or come
  close to each other.
• Could be young galaxies that have not attained
  their shape.

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Milky Way Galaxy

• Our galaxy is a spiral galaxy with about 200-400
  billion stars. It is 100,000 t0 120,000 light
  years across.
• Our Sun is about 2/3rds of the way from the
  center on the Orion Arm.
• Our Sun takes between 225 and 250 million years
  to orbit the center of the Milky Way Galaxy one
  time. This is called a cosmic year.

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History of the Universe

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In the Beginning

• Around 13 billion years ago, the universe burst
  forth in a blaze of light that immediately
  separated into matter and energy.
• In the next three minutes all the particles of
  the universe were formed, 75 hydrogen, 25
  helium, and a tiny bit of a few of the heavier
  elements.
• The universe expanded and continued to cool.
• But there was no light, as the stars had not
  formed yet.

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The Earliest Universe

• As gravity started pulling matter into clumps,
  the clumps grew bigger and bigger.
• Eventually their interiors got hot enough to fuse
  hydrogen.
• The first stars were born.
• These early stars had short lives, only a few
  million years.
• They were able to fuse hydrogen into larger
  elements, up to iron.

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The Universe Lives On!

• These stars reached the end of their lives and
  exploded, scattering these new elements across
  space.
• This enormous dust cloud formed the nurseries for
  the 2nd generation stars.
• The largest of the exploding stars fused the
  largest elements, allowing the new stars to form
  systems rich in materials.
• Our own system benefited from an ancient star
  that became a supernova, as almost every atom on
  our planet and in our bodies was once in an
  ancient star.

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Our Solar System and the Big Bang

• Our solar system is only about 5 billion years
  old.
• So the universe existed 8-10 billion years before
  our solar system came along.

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We are Stardust!

• Nearby large stars went supernova over 5 billion
  years ago.
• This created elements larger than Hydrogen
  Helium.
• So all the atoms in our body, our world, our
  solar system, came from inside an ancient star.
• The exploding stars created an enormous cloud of
  gas and dust we call a nebula or stellar nursery.

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So Here We Are!

• So the Big Bang created the universe.
• Our solar system came along much later, from the
  remains of 2nd or 3rd generation stars.
• The larger elements created by the early stars
  enabled our solar system to have rocky planets
  and the elements needed for life!

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Steady State Universe

• The Steady State Theory was popular before the
  Big Bang came along.
• This theory stated that the universe has always
  been the same.
• The universe has no beginning, it has always been
  here.
• In order to explain the expansion, this theory
  says that new matter is constantly being added.

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The Big Bang

• Most accepted model for the creation of
  everything.
• In the creation of the universe, everything was
  compressed into an infinitesimally small point,
  in which all physical laws that we know of do not
  apply.
• No information from any "previous" stuff could
  have remained intact. Therefore, for all intents
  and purposes, the Big Bang is considered the
  beginning of everything, for we can never know if
  there was anything before it.

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Big Bang

• In the Big Bang Theory, the observable universe
  began with an instantaneously expanding point,
  roughly ten to twenty billion years ago.
• Since then, the universe has continued to expand,
  gradually increasing the distance between our
  Galaxy and other galaxies.

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The First Pillar of the Big Bang

• The universe is expanding.
• Galaxies are moving away from us in all
  directions.
• The Hubble Constant will determine if the
  universe will keep expanding or come back
  together in the Big Crunch.

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Second Pillar of the Big Bang

• The background radiation from the big bang should
  have cooled to about 3 Kelvins.
• Everywhere we point a radio telescope towards the
  edge of the universe, we detect the remains of
  the background radiation. It has cooled to about
  3Kelvin.

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Third Pillar of The Big Bang

• The Big Bang math predicts that when matter was
  formed it would be made of 75 hydrogen and 25
  helium.
• We tend to find these percentages throughout the
  universe.

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Fourth Pillar of the Big Bang

• The theory predicts that as matter cooled it
  would be somewhat random.
• These random clumps attracted each other and
  formed the first stars and galaxies.

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Big Bang Unanswered

• There are a few things that scientists have not
  figured out
• Why is the density of matter so close to the
  critical value between constant expansion and
  re-collapse into the big crunch?
• Why does the universe look basically the same in
  all directions?
• Why was the density so uneven that galaxies could
  form?
• The dark matter/dark energy problem.
• Others that you will learn about when you are
  older!

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The Hubble Expansion

• Edwin Hubble discovered that the universe is
  expanding. He tried to measure this expansion to
  answer an important question
• Is the expansion slowing down or continuing?
• If the galaxies are not moving fast enough, their
  gravity will pull on each other, slow each other
  down, and eventually cause them to attract one
  another into a big crunch.
• If they are moving fast enough to escape from
  each other, the universe will continue to expand.

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The Hubble Constant

• Despite years of measurement, the question has
  still not been answered.
• The numbers seem to be so close to the critical
  number that scientists are still not at all sure
  if the universe will keep expanding or will
  collapse into the big crunch.

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New Dilemma

• The newest data seems to show that not only is
  the universe still expanding, but the rate of
  expansion is increasing!
• Scientists are really puzzled by this.

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Possibilities
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Birth of our Solar System

• Our solar system was formed from the dust and
  remains of an ancient supernova.
• The dust cloud that remained started collapsing
  about 5 billion years ago.

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Nebular Theory

• As the dust cloud gravitationally collapsed, it
  started to spin, just a a skater spins faster
  when she pulls her arms in.
• Most of the dust went to the center, where our
  sun was born when its internal temperature
  started the fusion of hydrogen.

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The Planets Form

• Clumps further out attached more dust and grew
  into the planets.
• When the sun ignited, it blew the lighter
  elements out into the 2nd zone, where the gas
  giants scooped all the lighter elements up.

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