Transcript document
Limitations of the Copernican Model
a) No better than Ptolemy’s model for
predictions
b) More complex use of circles than Ptolemy
c) Still no physical forces involved; used idea
of natural uniform motion
Tycho Brahe (1546 - 1601) &
Johannes Kepler (1571 - 1630)
Brahe made very accurate observations to
one minute of arc (1’ - there are 60 minutes
in a degree and 360 degrees in a full circle)
Kepler was a mathematician who used
Brahe’s data to develop a heliocentric model.
He used the concept of a force between the
Sun and planets but did not quantify it.
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Kepler’s Model
a) Heliocentric
b) Planets move in ELLIPSES not circles,
with the Sun at one focus Figs. Z3.15 & K2-20
c) The speed of a planet varies during its
orbit - Law of Equal Areas Figs. Z3.16 & K2-23
d) P2 is proportional to a3 - if we know a
planet’s orbital period (P) we can predict its
average distance from the Sun (a)
Proposed a “magnetic” force of attraction
between Sun and planets but did not investigate
its form. Now we know it as gravity.
Newton’s contribution:
F a constant
m M
r r
Universal Law of Gravitation
Gravitational force F acts on masses m and M
separated by a distance r.
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Giordano Bruno and Life
In 1584 Bruno proposed that the stars
were distant suns scattered throughout an
infinite Universe, and that around these
suns circled planets.
Not based on observations; this was a
philosophy.
He said that the Earth was not unique and
that the planets around other stars may
have intelligent life upon them.
Denounced by the Church as a heretic, he
was imprisoned by the Inquisition in
Venice for 8 years and burnt alive on
February 17th A.D. 1600.
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Astrophysics - an Observational Science
We can observe but not alter distant systems.
Stars, planets, gas clouds etc. emit
electromagnetic radiation:
long wavelength,
low frequency
radio waves, microwaves, infrared, visible
light, ultraviolet, X-rays, gamma rays.
Figs. Z5.7 & K4-3
short wavelength,
high frequency
Only visible light and some radio waves pass
easily through Earth’s atmosphere.
We must analyse all characteristics of the e.m.
radiation we receive to discover how the
Universe works.
To first find out how e.m. radiation is emitted,
we must look at the structure of atoms.
Zeilik Ch. 5, Kuhn & Koupelis Ch. 4
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Matter is made up of atoms
Atoms are made up of:
• protons (p+), positive electrostatic charge
• neutrons (n), no electrostatic charge
• electrons (e-), negative electrostatic charge
Atoms are electrically neutral i.e.
number of protons = number of electrons
p+ and n are about 2000 times heavier than e{ By the way…
Like charges repel, unlike attract
Force between charges q and Q is
F a constant
q Q
r r
Compare this with the expression
for gravitational attraction
F a constant
m M
r r
both forces depend on distance squared r r }
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Elements contain one type of atom
The atoms of different elements contain
different numbers of p+ and hence eThe atoms of different isotopes of an element
contain the same number of p+ but a different
number of neutrons (n).
e
n
p
e
p
• Hydrogen (H) 1p and 1e
• Deuterium 1p and 1e and 1n (isotope of H)
e
p p
n
e
p
e
n
p
n
e
• Helium 3 - 2p and 2e and 1n - unstable
• Helium 4 - 2p and 2e and 2n -stable
• Lithium 6 - 3p and 3e and 3n - stable
Hydrogen, helium and a trace of lithium made
in first 5 minutes of the hot Big Bang - most
of the other 89 stable elements must have been
produced later.
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Ions and Molecules
Ions are atoms that have had one or more
electrons ripped off by:
a) a light ray (photon, packet of e.m. radiation)
b) a collision with another atom or electron
The atom is then said to be ionised.
Some astronomical shorthand
Hydrogen H
H I Region
H+ H II Region
1e- lost
Molecules consist of two or more atoms bonded
together by sharing some of their electrons
e.g. molecular hydrogen H-H or H2
e
p
p
e
and water H2O
O
H
H
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Quantum Theory of the Atom
a) electrons orbit the nucleus only at fixed
distances
Figs. Z5.9 & K4-13
b) electrons can be “kicked out” of a nearer
orbit and forced into an outer orbit by a
light beam or by atoms colliding
c) e.m. radiation is emitted when an electron
moves from an outer orbit to a nearer orbit
d) e.m. radiation, including visible light, has a
characteristic frequency which depends
upon the energy lost by the electron
e) energy lost (E) = frequency (f ) times a
constant called Planck’s Constant (h)
E hf
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