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Stars, Galaxies, and the Universe


Stars, Galaxies, and the Universe Chapter 21 The First Pillar of the Big Bang The universe is expanding. Galaxies are moving away from us in all directions. – PowerPoint PPT presentation

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Title: Stars, Galaxies, and the Universe

Stars, Galaxies, and the Universe
  • Chapter 21

Tools of Modern Astronomy
  • 21-1

  • 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.

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
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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.

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

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

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

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

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.

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.

  • 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.

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

  • 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.

Characteristics of Stars
  • 21-2

  • A very large ball of mostly hydrogen and helium
    gas that shines extremely brightly by nuclear
  • 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!

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.

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
  • 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.

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.

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.

Color and Temperature
  • A stars color indicates its temperature.
  • Reddish color stars are fairly cool, about
  • 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.

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

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.

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
  • How bright it seems.

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
  • 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
  • 21-3

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.

  • 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.

Birth of a Star 2
Lifetimes of Stars
  • The mass of a star determines how long it will
  • 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!

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.

  • 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.

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.

Neutron Stars
  • A dying giant or supergiant star can suddenly
  • 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.

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
  • These are called pulsars.

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.

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.

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
  • These X-rays can show us where a black hole might
    be found.

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

  • 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.

Star Systems and Galaxies
  • 21-4

Star Systems
  • While our system has only one star, over half all
    stars are in groups of two or more called star
  • 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.

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.

  • 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

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

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

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.

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.

History of the Universe
  • 21-5

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
  • The universe expanded and continued to cool.
  • But there was no light, as the stars had not
    formed yet.

The Earliest Universe
  • As gravity started pulling matter into clumps,
    the clumps grew bigger and bigger.
  • Eventually their interiors got hot enough to fuse
  • 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.

The Universe Lives On!
  • These stars reached the end of their lives and
    exploded, scattering these new elements across
  • 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
  • So the universe existed 8-10 billion years before
    our solar system came along.

We are Stardust!
  • Nearby large stars went supernova over 5 billion
    years ago.
  • This created elements larger than Hydrogen
  • 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.

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!

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
  • In order to explain the expansion, this theory
    says that new matter is constantly being added.

The Big Bang
  • Most accepted model for the creation of
  • 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
  • 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.

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.

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

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

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
  • We tend to find these percentages throughout the

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.

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
  • The dark matter/dark energy problem.
  • Others that you will learn about when you are

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.

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.

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.

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.

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.

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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