AST 10011005

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AST 1001/1005 Exploring the Universe
Spring 2008 Instructor Geoff Lawrence
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Introductions

• Who am I?
• Geoff. Not Mr. , Not Dr.
• nth year physics graduate student at the U of M
• Who are you?
• Undergraduates with a BURNING desire to learn
  about astronomy

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• Extra Office Hours Kolthoff 155
• M 1115 a.m. - 100 p.m.
• Tu 1115 a.m. - 1215 p.m.
• W 1115 a.m. - 100 p.m.
• Th 1115 a.m. - 100 p.m.
• F 1115 a.m. - 1145 a.m.
• F 130 p.m. - 200 p.m.

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The Solar System
Solar System The Sun and all of its orbiting
bodies.
Stellar System A star and all of its orbiting
bodies
The Sun is a rather ordinary star
Is the Solar System a rather ordinary stellar
system?
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The Solar System
Sun 1 meter
Mercury 3.5 millimeter
Venus 8.7 millimeter
Earth 9.2 millimeter
Mars 4.9 millimeter
Jupiter 103 millimeter
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The Solar System
Sun 1 meter
Uranus 37 mm
Pluto 1.6 mm
Saturn 86 millimeters
Neptune 35 millimeters
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The Solar Neighborhood
Light Year The distance light travels in 1 year.
1 lyr 63,240 AU 5.9x1012 miles
Alpha Centauri 4.4 lyr
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The Milky Way Our Galaxy
Galaxy A huge assembly of stars, gas, and dust
held together by gravity
More than 100 billion stellar systems
100,000 lyr diameter
If Rsun 1 meter Rgalaxy 4.5 AU
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The Galactic Neighborhood
Andromeda 2.5 million lyr 25 Milky Way diameters
M33 3 million lyr 30 Milky Way diameters
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The Universe
The Universe The collection of EVERYTHING
The Observable Universe The collection of
everything that we can SEE
The edge of the observable universe is
approximately 14 billion light years away.
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The Scale of the Universe

• The Sun
• The Milky Way Galaxy
• The Earth
• The Solar System
• Northrop Mall
• The Moon
• The distance between galaxies
• Jupiter
• The Universe
• The Earth to Sun distance
• The distance between stars
• The Earth to Moon Distance

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The Answer is

• Northrop Mall
• The Moon
• The Earth
• Jupiter
• The Earth to Moon Distance
• The Sun
• The Earth to Sun distance
• The Solar System
• The distance between stars
• The Milky Way Galaxy
• The distance between galaxies
• The Universe

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Course Goals
What is Science?
What is modern astronomy?
What do we know about the Universe?
How do we know what we know?
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Science
Science The application of the Scientific Method
to the natural world.
Scientific Method Rigorous investigation
involving observation, formulation of hypotheses,
and experimentation to support or refute the
stated hypotheses.
Astronomy The scientific investigation of
astronomical phenomena.
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The Scientific Method

• Key Features
• Hypotheses are based on observations.
• Hypotheses must be testable.
• If the experimental results contradict the
  hypothesis, the hypothesis must be revised.

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Scientific Theory
Its just a theory.
Theory An explanation of a group of occurrences
in nature that has been confirmed by a
substantial number of experiments and
observations.
Scientific theory is not mere conjecture.
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Seasons
What causes the seasons?
The Earth is tilted with respect to the ecliptic
plane
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Seasons
Why does the Earths tilt matter?
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Moon The Dark Side of the
There is no dark side of the moon really
1 Lunar day about 29.5 Earth days
which is 1 Lunar month
The Moon is tidally locked to the Earth
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http//www.sumanasinc.com/webcontent/animations/co
ntent/sidereal.html
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Moon Phases
What causes moon phases?
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Moon Phases
What causes moon phases?
Only half of the moon is illuminated by the Sun.
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Eclipses
What causes a solar eclipse?
The Moon blocks the Sun.
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Eclipses
Partial Eclipse In the penumbra
Total Eclipse In the umbra
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What causes a lunar eclipse?
The Earth blocks the Sun.
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Eclipses
Shouldnt there be 2 every month?
The Moons orbital plane is tilted 5o with
respect to the ecliptic
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384-322 BCE
Aristotles geocentricearth-centeredcosmology.
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Claudius Ptolemy c. 90 168
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Nicolaus Copernicus 1473 1543

• Heliocentric solar system
• Period of planets
• Relative distances to planets

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Occam's razor

• Numquam ponenda est pluralitas sine necessitate
  
• Plurality ought never be posed without
  necessity.

1288 - c. 1347
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Tycho Brahe 1546 1601

• Collected a vast amount of naked eye
• data on Mars and other planets
• observations of the new star of 1572 and comet of
  1577

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• Johannes Kepler1571 - 1630

P
a
P2 a3
3 Laws of planetary motion
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Galileo Galilei 1564 1642

• His telescopes

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• Phases of Venus

Galileo Galilei 1564 1642
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Galileo Galilei 1564 1642

• Jupiters moons

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Galileo Galilei 1564 1642

• Lunar imperfections

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Isaac Newton 1642-1727
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http//physics.syr.edu/courses/java/demos/kennett/
Epicycle/Epicycle.html
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The orbits of the planets lie close to the
ecliptic plane.
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Conservation of angular momentum
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Chapter 6Telescopes Portals of Discovery
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Refraction

• Refraction is the bending of light when it passes
  from one substance into another
• Your eye uses refraction to focus light

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Example Refraction at Sunset

• Sun appears distorted at sunset because of how
  light bends in Earths atmosphere

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How does your eye form an image?
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Focusing Light

• Refraction can cause parallel light rays to
  converge to a focus

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Image Formation

• The focal plane is where light from different
  directions comes into focus
• The image behind a single (convex) lens is
  actually upside-down!

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How do we record images?
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Focusing Light
Digital cameras detect light with charge-coupled
devices (CCDs)

• A camera focuses light like an eye and captures
  the image with a detector
• The CCD detectors in digital cameras are similar
  to those used in modern telescopes

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What are the two most important properties of a
telescope?

• Light-collecting area Telescopes with a larger
  collecting area can gather a greater amount of
  light in a shorter time.
• Angular resolution Telescopes that are larger
  are capable of taking images with greater detail.

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Light Collecting Area

• A telescopes diameter tells us its
  light-collecting area Area p(radius)2
• The largest optical telescopes currently in use
  have a diameter of about 10 meters

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Bigger is better
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Thought QuestionHow does the collecting area of
a 10-meter telescope compare with that of a
2-meter telescope?

• Its 5 times greater.
• Its 10 times greater.
• Its 25 times greater.

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Thought QuestionHow does the collecting area of
a 10-meter telescope compare with that of a
2-meter telescope?

• Its 5 times greater.
• Its 10 times greater.
• Its 25 times greater.

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Angular Resolution

• The minimum angular separation that the telescope
  can distinguish.

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Angular Resolution

• Ultimate limit to resolution comes from
  interference of light waves within a telescope.
• Larger telescopes are capable of greater
  resolution because theres less interference

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Angular Resolution

• Ultimate limit to resolution comes from
  interference of light waves within a telescope.
• Larger telescopes are capable of greater
  resolution because theres less interference

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Angular Resolution

• The rings in this image of a star come from
  interference of light wave.
• This limit on angular resolution is known as the
  diffraction limit

Close-up of a star from the Hubble Space Telescope
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What are the two basic designs of telescopes?

• Refracting telescope Focuses light with lenses
• Reflecting telescope Focuses light with mirrors

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Refracting Telescope

• Refracting telescopes need to be very long, with
  large, heavy lenses

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Reflecting Telescope

• Reflecting telescopes can have much greater
  diameters
• Most modern telescopes are reflectors

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Designs for Reflecting Telescopes
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Mirrors in Reflecting Telescopes
Twin Keck telescopes on Mauna Kea in Hawaii
Segmented 10-meter mirror of a Keck telescope
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What do astronomers do with telescopes?

• Imaging Taking pictures of the sky
• Spectroscopy Breaking light into spectra
• Timing Measuring how light output varies with
  time

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Imaging

• Astronomical detectors generally record only one
  color of light at a time
• Several images must be combined to make
  full-color pictures

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Imaging

• Astronomical detectors can record forms of light
  our eyes cant see
• Color is sometimes used to represent different
  energies of nonvisible light

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Spectroscopy
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Spectroscopy

• A spectrograph separates the different
  wavelengths of light before they hit the detector

Diffraction grating breaks light into spectrum
Light from only one star enters
Detector records spectrum
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Spectroscopy

• Graphing relative brightness of light at each
  wavelength shows the details in a spectrum

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Timing

• A light curve represents a series of brightness
  measurements made over a period of time

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How does Earths atmosphere affect ground-based
observations?

• The best ground-based sites for astronomical
  observing are
• Calm (not too windy)
• High (less atmosphere to see through)
• Dark (far from city lights)
• Dry (few cloudy nights, less moisture on sight)

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Light Pollution

• Scattering of human-made light in the atmosphere
  is a growing problem for astronomy

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Twinkling and Turbulence
Star viewed with ground-based telescope
Same star viewed with Hubble Space Telescope

• Turbulent air flow in Earths atmosphere
  distorts our view, causing stars to appear to
  twinkle

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Calm, High, Dark, Dry

• The best observing sites are atop remote mountains

Summit of Mauna Kea, Hawaii
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Adaptive Optics
Without adaptive optics
With adaptive optics

• Rapidly changing the shape of a telescopes
  mirror compensates for some of the effects of
  turbulence

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Why do we put telescopes into space?
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Transmission in Atmosphere

• Only radio and visible light pass easily through
  Earths atmosphere
• We need telescopes in space to observe other
  types of light

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How can we observe nonvisible light?

• A standard satellite dish is essentially a
  telescope for observing radio waves

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Radio Telescopes

• A radio telescope is like a giant mirror that
  reflects radio waves to a focus

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IR UV Telescopes
SOFIA
Spitzer

• Infrared and ultraviolet-light telescopes operate
  like visible-light telescopes but need to be
  above atmosphere to see all IR and UV wavelengths

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X-Ray Telescopes

• X-ray telescopes also need to be above the
  atmosphere

Chandra
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X-Ray Telescopes

• Focusing of X-rays requires special mirrors
• Mirrors are arranged to focus X-ray photons
  through grazing bounces off the surface

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Gamma Ray Telescopes

• Gamma ray telescopes also need to be in space
• Focusing gamma rays is extremely difficult

Compton Observatory
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How can multiple telescopes work together?
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Interferometry

• Interferometery is a technique for linking two or
  more telescopes so that they have the angular
  resolution of a single large one

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Interferometry

• Easiest to do with radio telescopes
• Now becoming possible with infrared and
  visible-light telescopes

Very Large Array (VLA)
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Future of Astronomy in Space?

• The Moon would be an ideal observing site