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More about Electromagnetic Radiation
Speed of light = frequency  wavelength
= a constant c
Q.
How do we generate light?
A.
Heat things up
Heat up a solid, liquid or gas and the atoms
have more energy. They move about faster,
electrons get knocked out of normal orbits,
fall back and give out light.
Tungsten wire as a light bulb filament:
Red
2000 K
Yellow 6000 K
Blue
10,000 K
The colour tells us about the temperature
without contact.
Note the Kelvin temperature scale starts at the
lowest possible temperature “absolute zero” or
-273oC, so room temperature is about 290 K.
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Spectra
Kirchoff’s Rules (Zeilik p. 87, Kuhn p. 110)
1) Hot, opaque solids, liquids and gases emit a
continuous spectrum.
2) Hot, transparent gases produce a spectrum
of bright lines (emission lines). The number
of lines and their colour is unique to each
element.
3) If a continuous spectrum of light from a hot,
opaque body passes through a gas at a lower
temperature, the cooler gas will produce
dark lines (absorption lines). The pattern of
the lines depends on the elements in the
cooler gas.
Figs. Z5.4, Z5.5 & K4-10, K4-14
We can learn about the chemical composition
of stars solely from the light which they emit.
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Movement
From their changing surface features the
Sun, the planets and their moons can be
seen to be rotating about their axes.
Planets are seen to be orbiting the Sun.
Do other stars rotate or move? Can we see
this happening?
Resolving a star:
The images of stars in telescopic
photographs are different sizes, but this is
a limitation of the telescope and camera the stars are not truly resolved.
You could see a star as a disc if it made an
angle greater than the wavelength of the
observed light divided by the diameter (or
“aperture”) of your telescope.
Let’s put in some typical figures…
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Wavelength of yellow light = 0.00000056 m
Aperture diameter of telescope = 4 m
so
wavelength / diameter = 0.00000014
The angle made by the nearest star is its
diameter divided by its distance = 0.00000003
So we see a point of light, not a disc. We
certainly can’t see it rotating by watching for
surface features.
Can we see movement across the sky? Yes for
nearby stars, no for all others.
But the signature of movement is in the
spectra:
The Doppler Shift
• Object approaching along line of sight blueshift
• Object receding along line of sight redshift
• Object moving at right angles to line of sight no shift
Zeilik p.199, Kuhn & Koupelis p.120
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The Creation of the Elements
The Big Bang left the Universe with
hydrogen, helium and a little lithium plus a
few other light elements.
The only birthplaces for heavier elements are
the interiors of stars. How does this work?
Problem: can’t enter stars, only receive light.
Problem: stars are very stable. How do we
discover how they evolve?
Look for clues in our local star, the Sun.
What can we measure?
a) spectrum of light
b) size, mass, density
c) surface features, sunspots
d) solar atmosphere
e) energy output
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The Solar Spectrum
Continuous spectrum with absorption
lines.
Figs. Z13.8 & K4-10
Requires a very hot opaque gas below the
surface to give a continuous spectrum,
ending in a transparent atmospheric layer
a few 100 km thick at a temperature of
5800 K.
Figs. Z13.5 & K4-15
Absorption lines tell us the abundance of
gases in the photosphere:
74% hydrogen
25% helium
1% heavier elements
This does not tell us the composition of the
interior.
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Size
(a) Measure Earth to Venus distance using
radar echo.
(b) Use Kepler’s Third Law to calculate
Earth - Sun distance = 149,600,000 km
(see lecture 2)
(c) Measure the angular diameter of the Sun
as seen from Earth = 0.5o
Calculate the Sun’s actual diameter using (a),
(b) and (c) = 1,400,000 km
Using Newton’s Laws of Motion we can
calculate the Sun’s mass = 2 x 1030 kg
(the Sun constitutes 99.85% of the total mass
of the solar system)
Mass divided by volume gives us the Sun’s
average density = 1410 kg per cubic metre
The Sun is made up of gas - it is too hot to be
liquid.
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Surface Features
Sunspots have a temperature of about
4200 K, so they look darker than the
yellow photosphere.
As sunspots appear to cross the Sun’s
surface we can measure its rotation rate. It
is not a solid body and rotates fastest at its
equator - once in 25.4 days - and slowest at
its poles - once in nearly 40 days.
Sunspots have an 11 year activity cycle.
They occur in pairs and are connected with
magnetic effects.
Figs. Z13.21 & K11-27
They are probably linked to weather on
Earth; almost no sunspots were seen from
1645 to 1715 during an unusually cold spell.
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