Stars Galaxies and the Universe

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Stars Galaxies and the Universe

• Week 1
• Dr Bryce

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Class Policies

• This is a 3 or 4 semester hour class
• Meets 3 times a week MWF
• For 4 semester hour students there is an
  additional lab which meets once a week starting
  next week
• There will be 5 graded homework exercises, 3
  in-class exams and a final exam

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Contacting your Instructor

• Office is 402 Van Allen Hall
• Close to the western elevator (Dubuque Street
  side)
• Office hours are Mondays, Wednesdays, Fridays
  10am to 11am
• Email hmbryce at gmail dot com
• Please indicate which section you are enrolled in
  (i.e. B(without lab) or E (with lab)
• Drop/Add slips must go to the Physics general
  office 203 Van Allen Hall, the instructor cannot
  sign them for you

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Textbook

• Discovering the essential universe
• By Neil F Comins
• Published by W H Freeman
• We will cover chapters 1-3 and 7-12
• You do not need to purchase the textbook

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Homework

• Five graded homework assignments
• You will find the first assignment on the course
  website on Sept 11th
• First assignment is due on Sept 18
• Students are to submit a hard copy of their
  completed assignment directly to the instructor
• Each assignment is worth 20 marks
• Three of those marks are for attendance questions
• The attendance questions will be given in class
  the week the assignment is due. One question on
  each day.
• Two of those marks are to timely submission
• Students will receive both marks if the
  assignment is submitted on the due date and one
  mark if submitted on the Monday following the due
  date. Submissions will no longer be accepted one
  week from the due date

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Example

• Homework 1 due on Sept 18th
• Available on course website (www.physics.uiowa.edu
  /hbryce/stars) on Sep 11th
• Attendance questions given during class on Monday
  Sept 14th, Wednesday Sept 16th and Friday Sept
  18th
• Two submission marks for assignments received on
  Friday Sept 18th
• One submission mark for assignments received on
  Monday Sept 21st
• Last day for submissions is Friday Sept 25th.

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Exams

• 3 in-class 50 minute exam
• September 23
• October 21
• November 18
• The final will take place on Tuesday December 15
  at 945am
• These dates are subject to change

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Exams continued

• Will be multiple choice, with 4 possible
  responses
• Expect 25 questions in each class exam and 30 in
  the final exam
• The class exams will cover material in the
  previous 3 or 4 weeks of lectures, however the
  final will be 50 cumulative and 50 previous 2
  weeks
• Make up exams are available only in serious and
  documented cases, students are required to make a
  reasonable effort to contact the instructor prior
  to the exam that they will miss.

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Grading

• Plus and minus letter grading will be used
• Students with a final mark lower than 50 or who
  fail the lab section will fail the course overall
  
• Standard CLAS curve (15, 34, 40, 8, 3
  average 2.5) for an elementary class
• Students are reminded that the A grade will only
  be given in extraordinary situations

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3 Semester hour students

• Section A students
• 25 for each class exam
• 25 for homework total (5x20)
• Lowest in-class exam score will be dropped
• Plus 25 for final exam

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4 Semester hour (with lab) Students

• Section D students
• 18.75 for each class exam
• 18.75 for homework total
• Lowest in-class exam score will be dropped
• Plus 18.75 for final exam (which cannot be
  dropped)
• Plus 25 from Lab scores

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Tutorials

• Drop in tutorials are available
• http//www.physics.uiowa.edu/atutorial.html
• This is a great resource for your studies, you
  simply need to turn up and ask away!
• It is FREE!
• You are NOT being assessed on your knowledge in
  any way

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Calculators

• You will need a calculator for this course
• As we will be dealing some very large and some
  very small numbers, scientific function EE is a
  must.
• Also useful, square root, sin, cos and tan
• Cheap calculators are typically easier to operate
  than expensive overly complicated models.
• You will need to bring your calculator to exams,
  graphing calculators are forbidden in exams

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General Comments

• It is our aim to provide an atmosphere in which
  the majority of students can feel comfortable and
  can focus on learning
• With that in mind please switch off cell phones
  when entering the lecture theatre
• Please refrain from any behavior that will
  disrupt your classmates learning environment,
  i.e. eating, audible conversation, newspaper
  shuffling etc
• Students whose behavior is not deemed appropriate
  will be asked to leave the classroom

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Further General comments

• Students are reminded that CLAS guidelines
  suggest 2 hours of out of class study per week
  for every semester hour allotted to a class.
• If you may require a modification of seating,
  testing or any other aspect of the teaching and
  learning environment please contact the
  instructor.

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Science

• The intellectual and practical activity
  encompassing the systematic study of the
  structure and behavior of the physical and
  natural world through observation and
  experimentation.
• A systematically organized body of knowledge on
  any subject.

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Astronomy

• The branch of science which deals with celestial
  objects, space, and the physical universe as a
  whole
• Observation rather than experimentation
• Astrophysics concerned with the physical
  nature of stars and other celestial objects and
  the application of the laws and theories of
  physics to the interpretation of astronomical
  observations

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Scientific Theory

• Scientific theories are designed to be tested
• A theory that cannot be tested (even at a point
  in the future) is poorly thought out.
• Accepted theories have been subjected to many
  tests and we sometimes use the word law, eg
  The Universal Law of Gravitation.
• A theory can only be proved wrong, not right.
• Theories explain real life!
• Not to be confused with belief or faith.

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SI Units

• In 1960, the Eleventh General Conference on
  Weights and Measures was held in Paris and Le
  Systeme International d'Unites was adopted.
• This International System, or SI, as it is
  commonly referred to, is used for commerce and
  Science around the world. 
• There are seven SI base units.  
• Everything that is measurable, can be measured by
  these base units, or by units derived from these
  bases. 

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SI units

• For length we will use the meter (m)
• Not yards, feet, inches
• In astronomy we will use 3 other units for
  distance, depending on the scale we are
  observing
• For time we will use the second (s)
• For mass we will use the kilogram (kg)
• Not the pound

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The Kelvin temperature scale

• In everyday life you are probably pretty
  comfortable using the Fahrenheit temperature
  scale.
• The rest of the world used Celsius or Centigrade
  for everyday measurements. 100 degrees between
  freezing and boiling points of water

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Kelvin temperature scale

• Absolute zero is 0 Kelvin
• Absolute zero is the point at which atoms would
  have zero kinetic energy
• Only positive temperatures using Kelvins

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Powers of ten

• Astronomers use powers of ten to write numbers in
  more convenient forms.
• Just as we say million instead of 1 zero zero
  zero zero zero zero
• It is important to remember
• Negative powers mean small numbers
• Positive powers mean large numbers

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Example

• 3.0108
• 3.0 is the coefficient, which must be greater
  than or equal to 1 but less than 10
• 10 is the base
• 8 is the exponent, which shows the number of
  decimal places that the decimal point needs to be
  moved to change the number to standard notation. 
  A negative exponent means that the decimal point
  is moved to the left when changing to standard
  notation and a positive exponent means moved to
  the right.
• When you multiply numbers with scientific
  notation, multiply the coefficients together and
  add the exponents.  The base will remain 10.

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Frequently used Powers

• Nano 10-9
• Micro 10-6
• Kilo 103
• Mega 106
• 1 10-9 Instead of 0.000000001
• 210-6 0.000002
• 5103 5000
• 3.2106 3200000

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Some definitions

• Star Large balls of gas that produce heat and
  light by nuclear fusion.
• Planet Comes from the Greek word for Wanderer.
  An object that orbits a star. Shines by reflected
  light. Recently defined by IAU, so that the Solar
  System contains 8 planets.

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Continued

• Moon (or Satellite) an object that orbits a
  Planet.
• Galaxy A large grouping of stars in space, 108
  to 1012 members, held together by gravity and
  orbiting a common centre.
• Universe All matter and energy.

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Astronomical Unit

• The Earth-Sun distance
• 1.491011 m
• Used for describing the Solar System.
• Also used for describing stars with a large
  radius.
• And for describing other Solar Systems/Extra
  Solar Planets.

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Light Year

• The distance light will travel in one year.
• 9.461015m
• Light travels at a velocity of 3108 metres per
  second
• 3.15107 seconds in one year
• If an object is 2 million light years away then
  it has taken 2 million years for the light to
  reach us.
• We cannot see objects further than 14 billion
  light years away. Not enough time has passed for
  the light to reach us.

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• Light travels at a finite speed (300,000,000m/s).
• Thus, we see objects as they were in the past
• The farther away we look in distance, the
  further back we look in time.

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Rotation

• Spinning!
• Specifically rotating through an axis
• For the Earth the axis runs between the North and
  South poles.
• The Earth rotates once a day
• Stars (for example the Sun) also rotate.

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• We are moving with the Earth in several ways, and
  at surprisingly fast speeds

The Earth rotates around its axis once every day.
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Orbits

• The motion of one object around another.
• The Earth orbits around the Sun once each year.
• The Moon orbits around the Earth once every 27
  days.
• The Sun orbits the around the centre of the Milky
  Way once every 200 million years.

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Earth orbits the Sun once every year

• at an average distance of 1 AU 150 million km.
• with Earths axis tilted by 23.5º (pointing to
  Polaris)
• and rotating in the same direction it orbits,
  counter-clockwise as viewed from above the North
  Pole.

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We are constant motion
Earth rotates on axis gt 1,000 km/hr
Earth orbits Sun gt 100,000 km/hr
Solar system moves among stars 70,000 km/hr
Milky Way rotates 800,000 km/hr
Milky Way moves in Local Group
Universe expands
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To understand why our observations provide this
evidence.

• We need to know a little more.
• The most basic types of Observations.
• The History and development of Astronomy as a
  Science.
• Some basic physics, motion, gravity.
• As Astronomers can only observe light from our
  celestial objects we will be learning about the
  properties of light.

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Basic Observations

• Each Evening the Sun sets, the skies darken and
  on clear nights we are able to see stars/planets.
• We can also easily observe the westward motion of
  these heavenly/celestial bodies

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The paths of stars during the course of a night
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Stars appear to be grouped into Constellations

• A constellation is a region of the sky not a
  group of stars.
• 88 constellations fill the entire sky
• Although stars may be close together in a
  constellation they may actually be very distant
  from each other

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The Constellation Orion
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The Celestial Sphere
The 88 official constellations cover the
celestial sphere.
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More Celestial Sphere

• The North Celestial Pole is directly over the
  Earths North Pole
• The South Celestial Pole is directly over the
  Earths South Pole.
• The Celestial Equator is a projector of the
  Earths equator.
• The Ecliptic is the Suns path during the Year,
  more about this later

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Describing locations

• Our next problem is how do we describe or rather
  define locations in the sky?
• Consider how we describe locations on Earth
• Latitude
• Longitude
• North South East West

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Coordinates on Earth
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The Local Sky
An objects altitude (above horizon) and azimuth
(along horizon) specifies its location in your
local sky
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Special features
Zenith The point directly overhead Horizon
All points 90 away from zenith Meridian Line
passing through zenith and connecting N and S
points on horizon
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To describe Position

• Altitude will describe how far up in the sky an
  object is.
• With altitude 0o an object is on the horizon
  (setting or rising) and with altitude 90o an
  object is at the observers zenith
• Azimuth will describe how far round the sky an
  object is.
• With azimuth 0o an object is due North, 90o due
  East, 180o due South, 270o due West

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Altitude of the celestial pole your latitude
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Problems

• Stars rise and set during the course of the night
• This means that their altitudes and azimuths
  are constantly changing
• A stars altitude and azimuth will change with the
  observers location
• Not the most convenient system

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The Equatorial System

• Uses the Celestial Poles and the Celestial
  equator to define positions
• Declination (equivalent to latitude) gives the
  position angle north or south of the celestial
  equator
• Right ascension (equivalent to longitude) gives
  the position angle Eastwards around the Celestial
  equator from the Vernal Equinox
• These values remain constant as the Earth revolves

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We measure the sky using angles instead of
meters. The further away you are from an object
the smaller its angular size will be.
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Angular Measurements

• Full circle 360º
• 1º 60? (arcminutes)
• 1? 60? (arcseconds)

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Why do stars rise and set?

• Earth rotates east to west, so stars appear to
  circle from west to east.

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Our view from Earth

• The horizon (90 degrees from the zenith) splits
  the celestial sphere into two equal parts
• We can only see objects above the horizon
• An object moving below the horizon is setting
• Stars near the north celestial pole are
  circumpolar and never set.
• We cannot see stars near the south celestial
  pole.
• All other stars (and Sun, Moon, planets) rise in
  east and set in west.

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Seasons

• One of our most basic observations in Astronomy.
• In Summer the Sun is above the horizon for more
  time than it is in Winter.
• The days are longer and the nights are shorter.
• In Winter the Sun is above the horizon for less
  time and so it gets darker earlier.

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The stars we can see change throughout the year

• As the Earth orbits the Sun, the Sun appears to
  move eastward along the ecliptic.
• At midnight, the stars on our meridian are
  opposite the Sun in the sky.

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The Motion of the Sun

• Throughout the year the Sun appears to move
  relative to the background stars
• The constellations through which the Sun passes
  have well know names the Zodiac
• The Sun moves at a rate of about 1 degree a day
• No coincidence that there are 360 degrees in a
  circle

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The Suns path

• The Suns path across the Celestial Sphere is
  called the Ecliptic
• Tilted in relation to the Celestial Equator
• This means that the Suns declination changes
  throughout the year
• The ecliptic is the plane of the Earths orbit
• NOTE the vernal equinox

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Seasons depend on how Earths axis affects the
directness of sunlight
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The Real Reason for Seasons

• Earths axis points in the same direction (to
  Polaris) all year round, so its orientation
  relative to the Sun changes as Earth orbits the
  Sun.
• Summer occurs in your hemisphere when sunlight
  hits it more directly winter occurs when the
  sunlight is less direct.
• AXIS TILT is the key to the seasons without it,
  we would not have seasons on Earth.

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We can recognize solstices and equinoxes by Suns
path across sky
Summer solstice Highest path, rise and set at
most extreme north of due east. Winter solstice
Lowest path, rise and set at most extreme south
of due east. Equinoxes Sun rises precisely due
east and sets precisely due west.
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Dates to remember

• Sept 22, Autumnal Equinox, Sun will rise in the
  East and set in the West, the day and night will
  be of equal length.
• Dec 21, Winter Solstice, Sun rises in SE sets in
  SW, least amount of day
• March 21, Vernal Equinox
• June 21, Summer Solstice, Sun rises in NE, sets
  in NW, longest day

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The Moon

• Earths Satellite
• Takes about 27 days to
  orbit the Earth
• On average 380,000,000 m
• Or 3.8108 m from the Earth

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In another example

• Remember that Moonlight is reflected Sunlight.
• Please attempt the online tutorial for the phases
  of the Moon (good idea to do this before
  attempting the homework)

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Phases of the Moon 29.5-day cycle
new crescent first quarter gibbous full gibbous la
st quarter crescent

• waxing
• Moon visible in afternoon/evening.
• Gets fuller and rises later each day.

• waning
• Moon visible in late night/morning.
• Gets less and sets later each day.

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There is no dark side!
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Eclipses

• This photo shows an annular Solar Eclipse.
• The Moons angular diameter is too small to cover
  the Sun
• Just the Earth the Moons orbit is an ellipse so
  the distance to the Moon varies
• If the Moon is further away its angular diameter
  is smaller.

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Time lapse photography of a Total Solar Eclipse
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Some definitions

• Solar Eclipse the Moon is between the Earth and
  the Sun.
• Lunar Eclipse the Earth is between the Moon and
  the Sun
• As the Earth is much larger it has a bigger
  shadow so Lunar Eclipses are more common and
  visible from more of the Earth

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More definitions

• Totality The time period in which the Moon
  completely covers the Sun, normally just a couple
  of minutes. The sky becomes dark and you can see
  the stars.
• Partial The Moon doesnt completely cover the
  Sun and you see a crescent or a bite taken out of
  the Sun.
• Health warning Do not look at the Sun with your
  naked eye or with binoculars, even during an
  partial eclipse you can blinded.

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The Earths shadow

• The Earth and Moon cast shadows.
• When either passes through the others shadow, we
  have an eclipse.

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The Moons shadow

• Solar eclipses can occur only at new moon.
• Solar eclipses can be partial, total, or annular.
  
• Your view of the eclipse depends on your
  location, you have to be in the Moons shadow
• Annular eclipses occur when the Moons angular
  size is smaller than the Suns angular size

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The Moon isnt in the same plane as the Earth/Sun
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So we dont get eclipses every month
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Path of Totality
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Summary Two conditions must be met to have an
eclipse

• It must be full moon (for a lunar eclipse) or new
  moon (for a solar eclipse).
• AND
• The Moon must be at or near one of the two
  points in its orbit where it crosses the ecliptic
  plane (its nodes).

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Planets

• A view of the Solar System
• Notice Plutos orbit
• Notice that our part of
  the Solar System is
  close to the
  Sun.

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Our view of the planets
Mercury difficult to see always close to Sun in
sky Venus very bright when visible morning or
evening star Mars noticeably red Jupiter very
bright Saturn moderately bright