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Introductory Astronomy
ASTR 170  2010 S1  Daniel Zucker  E7A 317
[email protected]
I. Paradigm Shifts in the History of
Astronomy
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What is a Paradigm?
•  “A philosophical and theoretical framework of a
scientific school or discipline within which
theories, laws, and generalizations and the
experiments performed in support of them are
formulated.” (Merriam-Webster Dictionary)
•  In short: a way of understanding the world – a
world view
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History of Astronomy : Ancient Times
•  All cultures have myths and legends about the night
sky, the Sun, the Moon, the seasons, etc.
•  Science in Ancient Greece: –  Principle of cause and effect
–  Observe phenomena, seek explanations
–  World can be understood in precise geometrical
terms
–  Rational thought (“theory”) held above observation
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Science in Ancient Greece
•  Discoveries of Greek Science:
–  The Moon is a sphere
–  The Earth is a sphere
–  The Moon is between the Earth and the Sun
–  Measurement of the Earth’s circumference
–  (Aristarchus: the Sun is the centre of the Universe – not
widely accepted*)
•  But: bodies spherical, moving in circles (geometry!),
Earth at centre of Universe*
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Ptolemy to Copernicus
•  Ptolemy (~100 AD):
–  Almagest: Geocentric model with planets and
sun moving in epicycles on circular orbits,
Earth stationary at centre
–  Basis of European and Arabic world view for
next ~1500 years
•  Copernicus (1473-1543):
–  Heliocentric model with rotating Earth, still
with epicycles
–  Challenged the ancient world view of Earth at
the centre of the Universe
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Geocentric vs. Heliocentric
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Occam’s Razor
•  William of Ockham (14th century England): the
simplest explanation tends to be the best one
•  Not a scientific or logical result, but used as a
general rule of thumb in modern science
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Why Wasn’t the Heliocentric Model Accepted?
Problems with the model
•  Against Church teachings
•  New (vs. accepted wisdom)
•  Predicts parallaxes (≠
observations)
•  Rotating Earth (≠ Aristotle,
observations?)
•  Less accurate than Ptolemaic
model (required more
epicycles)
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A good idea isn’t always enough
•  Getting better data (Tycho
Brahe)
•  Fixing the model (Johannes
Kepler)
•  Promoting the new model
(Galileo Galilei)
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The Renaissance: Brahe and Kepler
•  Tycho Brahe (1546-1601): –  Last great “naked eye”
observer
–  very meticulous in collecting
data
•  Johannes Kepler
(1571-1630): –  None of the earlier models is
correct -- planets move in
ellipses, not circles
–  3 Laws: ellipses, planets move
faster near Sun, (period)2 is
proportional to (distance)3
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The Renaissance: Galileo
•  Galileo Galilei (1564- 1642):
–  First (?) to point a telescope at the night
sky
–  Tested and rejected Aristotle’s physics –
inertia and momentum, objects of
different composition fall at the same rate
–  Saw mountains on Moon, phases of
Venus, moons of Jupiter, Sunspots, stars
of the Milky Way
–  Tried for heresy!
•  Still missing: someone to put all the
pieces together into a coherent
physical theory...
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The Newtonian Revolution
•  Isaac Newton (1643-1727): invented
calculus*, reflector telescope, optical
spectrum, Theory of mechanics, Theory
of gravity, etc....
•  Mechanics: –  A body at rest/uniform motion will stay that
way without external force
–  Every action  equal and opposite reaction
GMm
F= 2
r
•  Equivalence: F = ma = GMm ⇒ a = GM
r2
r2
•  Gravity:
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The Relativistic Revolution
•  Albert Einstein (1879-1955): –  1905: Special Relativity, Photoelectric Effect,
Brownian Motion...
–  1907: Equivalence Principle: gravity = acceleration,
gravitational effects on light (bending/redshift)
–  1915: General Relativity (GR) – mass tells spacetime how to curve, the curvature of space-time
tells mass how to accelerate
•  GR predicts directly observable effects
–  Precession of Mercury’s orbit
–  Bending of starlight by Sun’s gravity (also
predicted by Newtonian gravity, but amount of
bending different)
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Observational Tests of General Relativity
•  Precession of Mercury’s orbit: –  Perihelion (closest approach to the Sun) advances 43
arcseconds/century
–  Other solutions: Venus more massive, another planet
closer to sun (“Vulcan”), Sun more flattened than
observed, Mercury had a moon, or... Newtonian
gravity incorrect
–  Precession directly explained by GR
•  Bending of starlight by Sun
–  Predicted to be 1.74 arcseconds at the Sun’s surface
–  1919 Solar Eclipse: Eddington  Africa, Crommelin
 Brazil, observational data not optimal for either
expedition
–  However, Eddington claimed results supported GR
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1922 Eclipse in
Western Australia:
much better data,
but credit still goes
to Eddington...
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The Next Great Paradigm Shift: The Size of the
Universe
•  What is the size of the
Milky Way?
•  What is the size of the
Universe?
•  What are the “nebulae”?
•  Definition – novae
(“new”): stars which
brighten dramatically
The Andromeda “Nebula”, M31
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The Size of the Universe: The Great Debate (1920)
Harlow Shapley: “Nebulae” part
of our Galaxy
Heber Curtis: “Nebulae” other
galaxies, like our Milky Way
•  The Galaxy is large (100 kpc),
kpc 30
and
kpc),
not at its center
we
areand
notwe
at are
its center
•  If “Nebulae” are other galaxies,
must be far away, but “novae” in
them are ~as bright as novae in
our Galaxy
•  van Maanen: claimed to measure
rotation of M31
•  Nebulae “avoid” the Milky Way
•  Nebulae all appear to be moving
away from us
•  The Galaxy is only 10 kpc across
and we’re at the centre – hard to
see what’s going on
•  Spectra of Nebulae look like
integrated light of many stars
•  Many Nebulae have rings of dust,
which could block the view of
Nebulae in the Galactic plane
•  Most “novae” in Andromeda are
much fainter than in the Milky Way
•  The Milky Way seems to have
spiral structure
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New Technology, New Techniques, and a Solution
•  Edwin Hubble (1889-1953):
–  1922 – 1923: used new 100”
telescope at Mt. Wilson,
California to discover Cepheid
variable stars in several
“Nebulae”, including Andromeda
–  1925: established that the
“Nebulae” were much too far
away to be part of the Milky Way,
and were in fact other galaxies
•  The Universe is a very big
place
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Another Paradigm Shift: The Expanding
Universe
•  When Einstein published GR (~1915),
the Universe was thought to be static
(not expanding or contracting), based on
observational data
•  But GR says a Universe with matter in it
must be expanding or contracting 
introduce a “fudge factor”, Λ: a
“cosmological constant” •  1929: Hubble paper reveals the
Universe is expanding  no need for
Λ(Einstein’s “greatest blunder”) ASTR170
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1998: The Year Λ Came Back
•  Type Ia Supernovae (SNe Ia):
explosion of a white dwarf star
•  Standard Candles: have ~same
luminosity (intrinsic brightness)
 can get distances
•  In 1998, two competing teams
observing SNe Ia to measure
the expansion of the Universe
•  Expectation: Universe’s
expansion slowing down
•  Discovery: Universe’s expansion
is speeding up!
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Two Competing Teams, Same Result
S. Perlmutter, G. Aldering, S. Deustua, S. Fabbro, G. Goldhaber, D. Groom,
A. Kim, M. Kim, R. Knop, P. Nugent, (LBL & CfPA)
N. Walton (Isaac Newton Group)
A. Fruchter, N. Panagia (STSci)
A. Goobar (Univ of Stockholm)
R. Pain (IN2P3, Paris)
I. Hook, C. Lidman (ESO)
M. DellaValle (Univ of Padova)
R. Ellis (CalTech)
R. McMahon (IofA, Cambridge)
B. Schaefer (Yale)
P. Ruiz-Lapuente (Univ of Barcelona)
H. Newberg (Fermilab)
C. Pennypacker
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Brian Schmidt (ANU)
Nick Suntzeff, Bob Schommer, Chris Smith (CTIO)
Mark Phillips (Carnegie)
Bruno Leibundgut and Jason Spyromilio (ESO)
Bob Kirshner, Peter Challis, Tom Matheson (Harvard)
Alex Filippenko, Weidong Li, Saurabh Jha (Berkeley)
Peter Garnavich, Stephen Holland (Notre Dame)
Chris Stubbs (UW)
John Tonry, Brian Barris (University of Hawaii)
Adam Reiss (Space Telescope)
Alejandro Clocchiatti (Catolica Chile)
Jesper Sollerman (Stockholm)
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Another Paradigm Shift: The
Accelerating Universe
•  Many different possible explanations were explored:
–  Dust affecting observations?
–  Are today’s SNe the same as those far in the past?
–  Other observational effects...?
•  Eleven years later, however, the general consensus
is that the data are best fit by an accelerating
expansion of the Universe, usually written as: Λ (!)
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Sneak Peek: What is the Universe
Made Of? The Current Paradigm
Only 0.6% in Stars!
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Some Lessons from the History of Astronomy
•  The evolution of our understanding of the Universe has
not always been moving smoothly towards the “truth”
•  The development of astronomy has been hindered by:
–  theoretical “wrong turns” and misinterpretation of data
–  bad observational data
–  missing / unavailable information
•  Progress has often happened through a combination
of new theories and new observations
•  Paradigm shifts happen!
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