ASTRONOMY 1144 STARS, GALAXIES, UNIVERSE

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ASTRONOMY 1144STARS, GALAXIES, UNIVERSE

• Chapters ? Selected Topics. See daily outline and
  lecture notes on the website
• Emails ? Many per day! Please restrict to urgent
  ones. Preferably, see after class.
• Ask TAs first or visit during off. Hrs.
• Absence ? Follow lecture notes, and Contact
  someone in class for notes.
• Emergencies? Send email to TA and me.

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Science and Pseudo-Science

• Science Observable facts and verifiable
  theories
• Pseudo-science Belief and dogma based on
  faith, not subject to verification

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Astronomy vs. Astrology

• No proven connection NONE !
• As it has been practiced for thousands of years,
  astrology is at odds with astronomy
• Proof ? The positions of stars and
  constellations have changed since ancient times
  astrology does not account for that.
• Example Astrological birth signs ? Revised
  dates !!

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Sun and Constellations
Traditional dates are almost exactly (but
artificially) month apart true dates vary
See Uptodate Zodiac Signs and Dates on the
webpage
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Ancient Constellations Imaginary Figures
(Orion the hunter)
Three bright stars in the Orion belt
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Sun moves thru Leo and Virgo(exit Leo Sep 16,
exit Virgo Sep 30)
Leo
Virgo
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Sun thru Libra and Ophiuchus(enter Ophiuchus Nov
29, exit Dec 17)
Virgo
Ophiuchus
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Astrological Predictions Ophiucheans!

• Birth Sign ? Ophiuchus (Nov. 30 Dec. 17)
• You are a kind, gentle, and naïve person!
• Watch out for smooth operators out to get you
• Send 49.95 for a complete horoscope to
• Anil Pradhan (Stargazer for Suckers)
• Department of Astrology and Bogus Studies
• OSU

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History of Astronomy

• Fundamental human conflict
• Faith vs. Inquiry (Religion and Science)
• The Dark Ages, 1000 yrs before Copernicus
• Ancient Astronomy Before the Dark Ages
• China, India, Arabia
• Greeks systematized the study of science and
  math even as we study today

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Geocentric vs. Heliocentric

• The Greeks (Aristotle, Ptolemy) still held on to
  the Geocentric view. Why ?
• ? Could not observe stars to move !
• Copernicus, Galileo, Kepler
• ? Heliocentric planetary model

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Metric System Powers of Ten
Name Number Exponent Prefix
trillion 1,000,000,000,000 1012 tera
billion 1,000,000,000 109 giga
million 1,000,000 106 mega
thousand 1000 103 kilo
hundred 100 102 hecto
ten 10 101 deca
one 1 100 .
one-tenth 1/10 10-1 deci
one-hundredth 1/100 10-2 centi
one-thousandth 1/1000 10-3 milli
one-millionth 1/1,000,000 10-6 micro
one-billiionth 1/1,000,000,000 10-9 nano
one-trillionth 1/1,000,000,000,000 10-12 pico


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Stellar Properties and Evolution

• Stellar classification and their lives
• How are stars born?
• ? Gravity and matter
• Where are they likely to form and why?
• ? Giant molecular clouds
• What are they made of?
• ? Mostly Hydrogen and light elements
• How do you observe and study them?
• ? Telescopes and spectroscopy

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Stars and Nebulae
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Nebulae Ionized Gas
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Giant Molecular Clouds
Cold gas coalesces Gravitationally. Heats up to
high temperatures and densities
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Successive Stellar Formation
Young O,B stars with strong winds into the
molecular cloud may trigger more stellar formation
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The Orion Nebula Gas heated (ionized) by hot
stars
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X-rays from the SURFACE of the Sun
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What color is the Sun?
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Observational Astronomy
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Astronomy from spaceHubble Space Telescope
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Atomic Energy Theory of Relativity
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Nebulae stellar nurseries
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Spiral Galaxy and Nucleus
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Hubble Deep Field 10,000 Galaxies
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Universe Past and Present
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Cosmic Epochs and Evolution
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Relative Sizes in astronomyFrom very small to
very large(meters)
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Some Essential Numerical Figures

• Radius of the Earth 6500 Km
• Speed of light 300,000 Km/sec
• Astronomical distances are so large that we use
  the speed of light to measure them
• Mean Earth-Sun Distance 150 million Km
• 1 Astronomical Unit (AU) 8.3 Light
  Minutes
• 1 Light Year (Ly) 9.5 trillion Km 63,240 AU
• Parsec 3.26 Lys (parallax angle unit)

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The Milky Way100,000 Lys Across
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Stellar Constellations
Connect bright stars to discern some shape
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Ancient Figures and Constellations
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The Orion Constellation
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The North Star (Polaris)
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The Summer Triangle
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Winter Triangle of Bright Stars
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The Distance Scale

• http//htwins.net/scale2/

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LINEAR AND ANGULAR SIZE OF OBJECTS

• ? angle subtended by the object at the observer
• the farther the object, smaller the a

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Angular size of moon 30
1 Degree 60 (minutes) 60 x 60 3,600
(seconds)
What is the angular size of the Sun? How large
does the Sun appear ?
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Angular distance between stars
While angular distance can be measured by
observations, actual distances are difficult to
measure (What do we need?)
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Orbital and angular motion of the Earth
The Earth moves one degree in its orbit around
the Sun each day. Why?
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Parallax Measurement of Distance

• Apparent movement due to viewing position
• No actual motion of the viewed object
• Difference in angle between line of sight from
  the Earth and from the Sun (half the diameter or
  the radius of the Earths orbit, 1AU)
• Since the parallax of stars is very small, it is
  measured in parsec (parallax second)
• distance d 1/p OR p 1/d
• 1 parsec 3.26 LY 206,265 AU

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Distance Measure in AstronomyThe Parallax Method
Parallax is the change in angle due to motion
Circle 360o (degrees) 1 degree 60
(minutes) 1 minute 60 (arcseconds)
1 AU
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Measure of distances in angles The distance d of
an object that makes an angle of 1 as the
Earth moves to opposite sides of the Sun
d
d
a
a
d (pc) 1 / a
1 parsec (pc) 3.26 Light Years (Ly)
Object at a distance of 1 pc
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Stellar and Astronomical Distances

• 1 parsec (pc) 3.26 LY 205,000 AU
• The stars are very far away
• Nearest Star Alpha Centauri ? 4.3 LY, more than
  1 pc ! The parallax angle a is less than one
  arcsecond ()
• Thats why the Greeks could not see the stars
  move
• Galaxies have been seen up to more than 10
  billion Lys away

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Night Sky ExposureGeocentric or Heliocentric ?
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Earths rotation and the Sky
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Daily Rotation of the Earth and Stars
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Annual Revolution of the Earth around the Sun and
position of stars
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Location of Heavenly Objects

• How do you locate places on the Earth?
• Latitude and Longitude
• Latitude angle measured from the Equator (0o),
  up or down, N-S
• Longitude angle measured from the Prime
  Meridian, E-W, 0o 180o
• How would you find location in mid-ocean ?
• First rule of navigation Lookup angle of
  Polaris ? Latitude
• How do you find the longitude? Clock ?

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Celestial Map and Celestial Coordinates
Celestial Poles And Equator Extension of
the Earths poles And equator
Analogous to Latitude and Longitude on The
Earth Measured in Degrees
Celestial Equator is the extension of the
Earths Equator up to the CS
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Ecliptic and the Celestial Equator
Ecliptic is the apparent Path of the Sun on
the Celestial Sphere
Autumn Equinox
Summer Solstice
The Ecliptic and the Celestial Equator Intersect
at Vernal (Spring) and Autumn Equinoxes At an
angle of 23.5o to each other
Vernal Equinox
Winter Solstice
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THE CELESTIAL SPHERECoordinates and Map of
Objects in the Sky
Star
Declination d celestial latitude

Right Ascension a celestial longitude
Star at (a,d) celestial coordinates
Vernal Equinox Position of Sun In the Sky on
the first day of spring Day Night a 0

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Apparent Rotation of Celestial Sphere
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Models of the Solar System

• Positions of planets change, whereas stars
  appear relatively fixed
• Greeks held on to the Geocentric model because
  they could not observe stars to change their
  positions, and therefore thought that the earth
  must be stationary
• Ptolemy, Aristotle and others refined the
  geocentric model
• But there were problems.such as the path
  reversal by Mars ? Retrograde motion

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Retrograde motion of Mars(path reversal seen in
the Sky)
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Epicycles Ptolemic Geocentric Model
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How do we know the Earth is spherical ?

• Stars differ from place to place
• Northern and southern hemispheres
• What kind of an object always has a round shadow ?

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Earth Shadow during Lunar Eclipse
Multiple Exposure Photograph
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Cyrene
Syene
Tropic of Cancer
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The Spherical Earth

• The ancient Greeks had deduced not only that the
  Earth is spherical but also measured its
  circumference !

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Eratostheness method to measure the
circumference of the earth
7º
At noon on summer solstice day the Sun is
directly overhead at Syene, but at an angle of 7o
at Alexandria

• Distance (Alexandria - Syene)
• -- ---------------------------------------
• 360 Circumference of the Earth

Sunlight
Alexandria
Answer 40,000 stadia 25,000 mi !
Syene
Earth
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Earth-Moon-Sun GeometryAristarchuss
determination of distances(Closer the S-E-M
angle to 90, the farther the Sun)
If we replace the moon with a planet, then can
determine relative distances, as done by
Copernicus
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Copernicus
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Copernican ModelInferior and Superior
Planets(orbits inside or outside the Earths
orbit)

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Configurations of Inferior Planets, Earth, and
the Sun
Earth
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Configurations of Superior Planets, Earth, and
the Sun
Opposition
Conjunction
Earth
Synodic (apparent) period one conjunction to
next (or one opposition to next)
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Synodic and Sidereal Orbital Periods

• Inferior planets are never at opposition
  superior planets can not be at inferior
  conjunction
• Copernican model of orbital periods
• Synodic period is the apparent orbital period of
  a planet, viewed from the earth, when the
  earth-planet-sun are in successive conjunction or
  opposition
• Sidereal (with respect to stars) period is the
  real orbital period around the Sun
• Synodic periods of outer planets (except Mars)
  are just over one year

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Apparent (Synodic) and true (Sidereal with
respect to stars) orbital periods of planets
differ due to Earths relative motion
Synodic periods of all outer planets (except
mars) are just over 1 year because their Sidereal
periods are very long and they are in opposition
again soon after an earth-year
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Earth-Venus-Sun
Inferior planets appear farthest away from the
Sun at greatest elongation
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Measurements of Distances to Planets
Angle of max elongation
P-E-S
P
90 deg
Earth
P-E-S
E
S
Sin (P-E-S) PS / ES ES 1 AU
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Copernicus first determined the relative
distances of planets
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Copernican Heliocentric Model(Retrograde motion
of Mars seen when Earth overtakes Mars
periodically)
Earth is closer to the Sun, therefore moves
faster than Mars
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Tycho The most accurate pre-telescopic observer
Tycho charted very accurately the movement of
Mars in the Sky, but still believed In the
Geocentric Universe
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Kepler Tychos assistant(used Tychos data to
derive Keplers Laws)
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Planetary Orbits

• The Copernican heliocentric model is essentially
  correct
• But it consisted of circular orbits which did not
  exactly fit observations of planetary positions
• Kepler realized, based on Tychos data of the
  orbit of Mars, that orbits are elliptical ?
  Keplers First Law
• However, the difference for Mars is tiny, to
  within the accuracy of drawing a circle with a
  thick pen !

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Keplers First LawAll planetary orbits are
elliptical, with the Sun at one focus
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Eccentricity ee distance between foci/major
axis AB / ab
a
A
B
b
A circle has e 0, and a straight line has e
1.0
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Keplers Second LawPlanetary radius sweeps
equal area triangles in equal time
It follows that the velocity of the planet must
vary according to distance from the Sun --
fastest at Perihelion and slowest at Aphelion
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Keplers Third Law P2 a3P Orbital Period, a
semi-major axis
What is the size a of the orbit of a comet with
the period P of 8 years?
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Keplers Laws

• Empirically derived from observational data
  largely from Tycho (e.g. observations of the
  positions of Mars in its orbit around the Sun)
• Theoretical explanation had to await Newtons
  discovery of the Law of Gravitation
• Universally valid for all gravitationally
  orbiting objects (e.g. stars around black holes
  before falling in)

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Galileo
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Galileos Discoveries With Telescope

• Phases of Venus
• - Venus displays phases like the Moon as it
  revolves around the Sun
• Mountains and seas on the Moon
• - Other objects in the sky are like the
  Earth (not therefore special)
• Milky Way is made of stars like the Sun
• Sunspots
• - Imperfections or blemishes in
  otherwise perfect heavenly objects
• 4 Galilean satellites of Jupiter
• - Objects in the sky revolve around other
  objects, not the Earth (i.e. other moons)
• All of these supported the Copernican System
• Galileo also conducted experiments on
  gravity
• Regardless of mass or weight objects fall at
  the same rate

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Phases of Venus
Venus is never too far from the Sun, therefore
can not be in opposition like the Moon. Changing
phases of Venus demonstrate that it orbits the
Sun like the Earth.
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Orbits and Motions

• Orbits can not be circular since objects do NOT
  revolve around each other, but around their
  common center-of-mass
• The Earth and the Moon both revolve around each
  other
• This motion is in addition to Earths Rotation,
  Revolution, Precession

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The Earth-Moon Barycenter

• The earth and the moon both revolve around a
  common center of mass called the Barycenter
• The barycenter of Sun-planet systems lies inside
  the Sun
• As the earth is much more massive, the barycenter
  lies 1700 Km inside the earth
• Calculate its position O from
• M(E) x EO M (M) x MO

M
E
O