Physics Observing The Universe
revision
Movement of celestial bodies
• The sun appears to travel east-west across the
sky once every 24hours.
Sidereal Day
• The star appear to move across the sky in a
slightly shorter time(23h 56min). This is called
a sidereal day.
Sidereal day is rotation to face
original direction.
A solar day the rotation
goes all the way back to
face the Sun.
The moon
• The moon appears to travel east to west across the sky
once every 24hrs 49mins. One complete cycle takes 28days.
• Why does the Moon take longer to cross the sky than the
Sun?
• Because it orbits the Earth in the same direction as the
Earth rotates. So by the time the Earth rotates enough for a
static object to have gotten all the way to the opposite
horizon, the Moon hasn't quite gotten there yet because it
was moving with the Earth's rotation a little.
Retrograde motion
Retrograde motion
• At certain points in the orbits of planets, other
planets (for example mars) appear to move
backwards or from west to east across the sky.
This is because earth is moving faster than
that planet and so overtakes it.
eclipses
Eclipses
• Eclipses do not happen very often as the Sun
and moon do not align very often.
• The moon’s orbit is tilted relative to the plane
of the Earth’s orbit.
• Usually Earth, Sun and the Moon are not in
line so no eclipse occurs.
S
Pegasus
Orion
E
Autumn equinox
Scorpius
W
W
Pegasus
Orion
S
Leo
E
Winter solstice
E
W
Scorpius
Orion
Spring equinox
Leo
S
E
Pegasus
Scorpius
S
Leo
W
Summer solstice
How does a
telescope
work?
Focusing parallel light
• Stars are
so far
away that
light
arriving
from
them is
parallel.
Power of a lens
• Calculate the power of a lens:
• Power (dioptre)= 1/focal length (m)
The more powerful a convex lens, the more
curved the surface.
Forming an image of an extended object
F
F
Real
image
Object
Ray 1.
Arrives parallel to the Principal Axis – then passes through F.
Ray 2.
Passes through the optical centre – undeviated.
Ray 3.
Passes through F first – then emerges parallel to the Principal Axis.
Any other rays will be refracted to pass through the same image point.
Note that the top of the image is now below the Principal Axis.
Magnification=
Focal length of the objective lens
Focal length of the eyepiece lens
Simple telescopes are made
of two converging lenses of Remember- the more powerful a lens
the shorter the focal length.
different powers. The more
powerful lens acts as the
eyepiece.
Concave mirrors
Most astronomical telescopes
have concave mirrors, not
convex lenses as their
objectives.
Modern telescopes have very
large mirrors to:
(a)Collect light/radiation
(b)Produce a more
defined/brighter/sharper
image
(c)See faint sources
(d)Reduce diffraction
What are the objects we see in the
night sky and how far away the are?
Parallax
α
• The parallax angle is half the angle moved in a
6 month period.
Parallax
• Parallax makes some stars seem to move
relative to others over the course of a year.
• The smaller the parallax angle is the further
away a star is.
Parsecs
• A parsec (pc) is the distance to a star with a
parallax angle of one arc second.
ArcSeconds
An hour can be broken
into 60 divisions called
minutes
Each minute can be
broken into 60 divisions
called seconds
So as a fraction of an
hour, 1 second is 1/3600
of an hour meaning there
are 3600 seconds in an
hour.
A degree can be broken
into 60 divisions called
minutes. They are written
as ‘ eg 20’
Each minute can be
broken into divisions
called seconds. They are
written ‘’ eg 20’’
So as a fraction of a
degree, 1 second is
1/3600 of a degree
meaning there are 3600
seconds in a degree.
Star distance
•
•
•
•
A parsec is similar in magnitude to a light-year
1 parsec = 3.1 x 1013 km
1 light-year = 9.5 x 1012 km
Typical interstellar distances are a few parsecs.
Luminosity
• Luminosity (intrinsic brightness) of a star depends
on its temperature and its size.
• Temperature: a hotter star radiates more energy
every second from each square metre of its
surface.
• Size: a bigger star has more surface that radiates
energy.
• Observed Brightness- depends on the stars
distance from the earth, as well as the stars
luminosity. Dust or gas between Earth and the
star may absorb some of its light.
Cepheid Variables
• These are stars that
pulse in brightness. They
have a period related to
their brightness.
Measuring the distance to a star
in a distant galaxy:
1. Look for a cepheid variable in
the galaxy of interest.
2. Measure its observed
brightness and its period of
variation.
3. From the period, determine
luminosity.
4. Knowing both the luminosity
and the intensity of its light at
the telescope, calculate the
distance of the star.
The great debate
Shapley
• Measured distance to nebulae.
Observed they form a
spherical cloud with a centre
far from the solar system.
• Guessed the nebula was a
cluster of stars and they
formed a sphere around the
Milky Way galaxy. (Globular
clusters)
• He claimed milky way was the
entire galaxy.
Curtis
• He challenged Shapley’s
claim about the universe.
• Curtis was studying ‘spiral
nebulae’ rather than
globular clusters.
• He felt that they were
distant objects- galaxies on
their own.
• Was proved correct by
Hubble’s discovery of the
Andromeda galaxy.
Hubble
• Hubble used the data from
cepheids to determine the
distances to galaxies.
• He discovered that all
galaxies appeared to be
moving away from us.
• There spectrum has been
redshifted.
• The more distant the galaxy
the faster the rate of
recession.
Hubble’s Constant
Speed of recession = Hubble Constant X distance
The first time Hubble estimated the constant he found it to be
500km/s. With more reliable data from the HST the current
excepted value is 72 ± 8 km/s-1Mpc-1
Moving Galaxies
• The fact that galaxies are moving lead to two
important ideas:
• The universe itself may be expanding, and
may have been much smaller in the past.
• The universe may have started by exploding
outwards from a single point- the big bang.
A closed Universe
An open Universe
flat Universe
closed Universe
open Universe
What are stars?
Alpha scattering-gold foil experiment
• Start with a metal foil. Use gold, because it can be
rolled out very thin- thickness of a few atoms.
• Direct the source of alpha radiation at the gold
foil. Do this in a vacuum as alpha is easily
absorbed.
• Watch for flashes of light as alpha particles strike
the detecting material around the outside of the
chamber.
• Count the flashes at different angles, to see how
much the alpha radiation is deflected.
Interpretation of results
• Most alpha particles passed straight through
the gold foil, deflected by no more than a few
degrees.
• A small fraction of the alpha particles were
actually reflected back towards the direction
from which they had come.
• Must be something positive repelling the
alpha particles.
What are stars?
All hot objects(including
stars) emit a continuous
range of electromagnetic
radiation, whose
luminosity and peak
frequency increases with
temperature.
Energy levels
A hydrogen atom has:
1 proton in the nucleus
1 electron (in the first shell)
The atom does have other shells too
…
but they are all empty …
most of the time.
+
Why does the electron normally occupy the innermost shell?
The innermost shell has the lowest energy. The electron drops down
through the shells, losing energy as it goes, until it has the lowest
possible energy.
Each shell represents a specific level of electron energy.
In Physics we refer to the shells as ENERGY LEVELS.
Excitation and de-excitation
I’m very
Taking a closerexcited!
look at the •first
An 4electron can absorb energy and jump to a
energy levels..
-0.9
higher energy level. This is called EXCITATION.
-1.5
The electron is excited!
I’m excited
I’m even more
now!
+
• But then it will fall back down
to a lower energy
excited!
level, giving out energy as it does so.
-3.4
• The electron can return to the Ground state in a
number of ways. How many?
-13.6
Ground state
-0.9
-1.5
4
3
-3.4
2
-13.6
1
Ground state
• Each jump between energy levels produces an
amount of energy determined by the
difference in energies between the 2 levels.
• Level 4 to 1
Energy change = -0.9 - (-13.6) = 12.7 units
• Level 4 to 3
Energy change = -0.9 - (-1.5) = 0.6 units
• Level 3 to 1
Energy change = -1.5 - (-13.6) = 12.1 units
In each case the energy is emitted as photons
of light.
Types of spectrum
Most very hot objects will emit a continuous spectrum.
Hot gases emit only those colours which correspond to the
energy released by de-excitation. A line EMISSION spectrum
But a cold gas would absorb exactly the same colours because
they have just the right energy to jump up to higher energy
levels (excitation).
A line ABSORPTION spectrum
Comparing the 3 types of spectrum
Comparing the 3 types of spectrum
Note that, for a given gas, the emission and absorption spectra are reverse
versions each other.
But something odd was noticed in the spectra of
distant stars
The whole spectrum seems to be shifted towards the
red end: a RED SHIFT. Why?
Boyles Law
Pressure
/ Pa x105
Volume
/cm3
0.96
33.5
1.59
2.00
2.41
2.62
2.97
21.0
16.5
13.5
12.5
11.0
Pressure vs Volume
3.5
Pressure /x10 5 Pa
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
30
35
Volume /cm 2
What is the equation relating pressure and volume?
40
Boyles Law
Pressure
/ Pa x105
0.96
1.59
2.00
2.41
2.62
2.97
Volume
/cm3
33.5
21.0
16.5
13.5
12.5
11.0
p x V 1/V
32.29
33.31
32.98
32.58
32.75
32.62
0.030
0.048
0.061
0.074
0.080
0.091
Boyles Law graph
Pressure vs 1/Volume
3.50
2.50
5
Pressure / x10 Pa
3.00
2.00
1.50
p a 1/V
1.00
0.50
0.00
0.000
0.020
0.040
0.060
1/Volume / cm-3
0.080
0.100
Charles Law
Temperature
/ oC
Volume (length of air
column) / mm
0
100
Room
3.8
5.2
4.1
L
Charles Law results
Charles Law
6
5
V /cm 3
4
3
2
1
0
0
10
20
30
40
50
temp / o C
60
70
80
90
100
Extrapolating to find temp at which volume is
zero
Charles Law
6
5
4
-273oC (Absolute zero)
V /cm 3
3
2
1
0
-300
-250
-200
-150
-100
-50
0
-1
temp / o C
50
100
Pressure Law
Temperature
/ oC
Pressure
/x105Pa
0
0.92
100
1.26
Room
Temp
0.99
Pressure Law results
Pressure Law
1.40
6
Pressure / x10 Pa
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0
20
40
60
80
o
Temperature / C
100
120
Extrapolating to find temp at which pressure is
zero
Pressure Law
1.4
Pressure / x 106 Pa
1.2
-273oC (Absolute zero)
1
0.8
0.6
0.4
0.2
0
-300
-250
-200
-150
-100
-50
Temperature / oC
0
50
100
Now logically,
• Volume of a gas cannot be less than 0.
• Pressure of a gas cannot be less than 0.
• Therefore both Charles Law and the Pressure
Law predict a minimum temperature of 273oC.
• Kelvin: If -273oC is the lowest possible
temperature then it should be the zero of an
Absolute Temperature scale.
Absolute Temperature Scale
• The unit is the kelvin, K.
• Absolute zero is assigned 0 K.
• Same size increment as oC, so
0 K = -273 oC
50 K = -223 oC (-273+50)
273 K = 0 oC
373 K = 100 oC
Conversion is easy.
Note: No degree
symbol
Absolute temperatures are represented by ‘T’.
So if we now plot the Charles Law and Pressure Law
results using Absolute Temperatures instead of oC ..
The Ideal Gas Equation (equation of state)
• Boyles Law
• Charles Law
• Pressure Law
p a 1/V (at constant temperature)
V a T (at constant pressure)
p a T (at constant volume)
where T is the Absolute temperature in kelvin, K.
Combining these we get:
pV a T
or
pV/T = constant,
Life of a star – Stage 1: NEBULA
This is a NEBULA, a
cloud of hydrogen and
dust.
Gravitational attraction
pulls the hydrogen and
dust together
compressing it.
Temperature and pressure
rise.
Life of a star – stage 2: PROTOSTAR
As the pressure and
temperature increases a
ball of hydrogen forms, so
hot that is glows.
This is a PROTOSTAR.
Nuclear fusion has not
really started to happen
yet.
Life of a star. Stage 3: Main Sequence
If there is enough mass,
gravity continues to
compress the hydrogen
until the temperature
reaches about 10 000 000
K.
Hydrogen nuclei now
collide at speeds where
nuclear fusion begins.
Life of a star - Stage 4: Red Giant
When the hydrogen starts to
run out the star fuses
helium and larger nuclei in
the core.
This generates less heat
than fusion of hydrogen.
The star cools down and
swells becoming a RED
GIANT
Life of a small/medium star –
Stage 5: White Dwarf
Eventually the cool outer layers drift
off into space forming a PLANETARY
NEBULA.
The remaining, collapsed inner core
is a WHITE DWARF.
It continues fusing larger nuclei until
it runs out of fuel.
As fusion stops it cools down to
become a BLACK DWARF
OR: Life of a large star –
Stage 5: Supernova
The star continues to collapse, fusing
increasingly larger nuclei.
Once fusion ceases the star
‘explodes’ ejecting the outer layers.
This is a SUPERNOVA.
Supernovae are very bright.
Life of a large star –
Stage 6: Neutron star
The remaining core is a neutron star.
Life of a large star –
Stage 6: Neutron star
Neutron stars have a very large mass in a
very small volume. They are very dense.
Life of a large star – Stage 6: Pulsars
Pulsars are highly magnetized neutron stars that emit a
beam of e/m radiation.
They rotate very rapidly. e,.g once every 1.4 milliseconds
to 8.5 seconds.
The radiation can only be observed when the beam of
emission is pointing towards Earth.
This is called the lighthouse effect and gives rise to the
pulsed nature that gives pulsars their name.
For some pulsars, the regularity of pulsation is as precise
as an atomic clock.
Life of a VERY large star –
Stage 7: Black hole
If there is enough mass the neutron star continues
to collapse to form a BLACK HOLE.
The gravitational force is so strong not even light
can escape.
Hertzsprung Russell
diagram
By convention, the temperature scale goes backwards.
Hertzsprung-Russell diagram
The majority of stars
(including the Sun) are in the
main sequence - a line which
runs from massive, luminous
hot stars at one end to low
mass, dim, cool stars at the
other end.
Hertzsprung-Russell diagram
Another group of stars,
the red giants, are
relatively cool - but
they are very luminous,
because their diameters
and surface areas are
very large compared
with main sequence
stars.
Hertzsprung-Russell diagram
Supergiants are very
large and luminous, and
their temperatures cover
the full range from very
hot to relatively cool.
Hertzsprung-Russell diagram
The white dwarfs are
hot but not very
luminous - because
their diameters are very
small.
Isotopes of hydrogen
The nucleus of a hydrogen atom can take 3 forms:
+
+
+
A single proton
H
1
A proton and 1 neutron
called deuterium.
A proton and 2 neutrons
called tritium.
2
H
1
H
3
1
Nuclear fusion reaction equations
The deuterium + tritium fusion reaction can be
written as:
2
1
H +
3
1
H
4
2
He +
1
0
n
2 helium nuclei can then go on to fuse:
Beryllium
4
2
He +
4
2
He
8
4
Be
Fusion of hydrogen nuclei
The simplest fusion reaction is between a
deuterium and a tritium nucleus.
1
H
+
n
+
2
+
+
4
2
3
1
He
H
Points to consider
Nuclei are positive. They repel each other.
To make nuclei collide with enough force to
fuse needs very high speeds only achieved at
temperatures of millions of kelvin.
1.Where does the energy come from to make
fusion happen?
2.What conditions are necessary for the
process to keep itself going?
collaboration
• More effort can be put in from international
collaboration than from a single experimental
group.
• It is often impossible for individuals to deal
with the huge amount of data accumulated
from surveys.
Why build a telescope on the top of a
mountain?
• For optical observatories a low turbulent
atmosphere(very good seeing)
• Dark skies- no light pollution
• High number of clear night skies
• For radio telescopes this doesn’t matter as
radiowaves pass through clouds
• Not windy due to refraction
Non astronomical reasons for choosing
a site.
• Must have reasonable logistical supply.
• Easy to travel to
• Accomodation, food, drink
HOW CLEARLY CAN WE SEE?
RESOLUTION
Which colour writing
can you read clearly first?
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middlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddlemiddle
rightrightrightrightrightrightrightrightrightrightrightrightrightrightrightrightright
easteasteasteasteasteasteasteasteasteasteasteasteasteasteast
middlemiddlemiddlemiddlemiddlemiddlemiddlemiddle
rightrightrightrightrightrightrightrightrightrightrightright
leftleftleftleftleftleftleftleftleftleftleftleftleftleft
middlemiddlemiddlemiddlemiddlemiddle
rightrightrightrightrightrightrightrightright
leftleftleftleftleftleftleftleftleftleft
middlemiddlemiddlemiddlemiddle
rightrightrightrightrightrightright
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middlemiddlemiddlemiddle
rightrightrightrightrightright
leftleftleftleftleftleft
middlemiddlemiddle
rightrightrightright
NOW TRY IT AGAIN LOOKING AT THE SCREEN THROUGH A
SMALL HOLE.
Is it easier or more difficult?
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southsouthsouthsouthsouthsouthsouthsouthsouthsouthsouthsouth
northnorthnorthnorthnorthnorthnorthnorthnorthnorthnorthnorthnorthnorthnorthnorth
easteasteasteasteasteasteasteasteasteasteasteasteasteasteasteasteast
southsouthsouthsouthsouthsouthsouthsouth
northnorthnorthnorthnorthnorthnorthnorthnorthnorth
easteasteasteasteasteasteasteasteasteasteasteast
southsouthsouthsouthsouthsouth
northnorthnorthnorthnorthnorthnorthnorth
easteasteasteasteasteasteasteasteastlef
southsouthsouthsouthsouth
northnorthnorthnorthnorthnorth
easteasteasteasteasteasteast
southsouthsouthsouth
northnorthnorthnorthnorth
easteasteasteast
southsouthsouth
northnorthnorthnorth
In general we can resolve blue better than
green or red.
This is because blue light has a shorter
wavelength than green or red light.
The shorter the wavelength the better the
resolution.
Which waves in the e/m spectrum would give
us the best resolution?
Looking at something through a small aperture
(hole) makes the resolution WORSE.
Therefore the bigger the aperture the better the
resolution
Which of these two
telescopes would give
the best resolution?
P