3. We know the stars by their light
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Radio and microwave
Visible, IR and UV
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The spectrum of a star tells us:
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It’s temperature
It’s luminosity
It’s distance
It’s size
And more.
The visible spectrum of the Sun
Stellar temperatures
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We have already seen that the colour of a star
enables us to find its temperature…
provided the colour is not altered as it passes
through interstellar dust!
Fortunately there are other ways to find the
temperature from the spectrum…
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By examining the temperature and spectra of nearby
stars, astrophysicists found that there were other
indicators of temperature
Certain spectral lines appeared consistently at certain
temperatures and disappeared at others
Different lines appear with different degrees of
ionisation – which results from different temperatures

It became possible to classify stars by the intensity of
certain lines in the spectrum

Stellar spectra are classified according to a
system which ranks them in order of surface
temperature (the letters were re-ordered from an
older system):
hot O B A F G K M cool (L T)

Oh Be A Fine Girl (Guy) Kiss Me . . .
The Hertzsprung-Russell diagram
brings order to this mass of information
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It plots stars
according to their
temperature and
luminosity
A definite pattern
emerges:
Bright stars are bluer
- that is, hotter.
90% of stars are on
the ‘Main Sequence’
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Another H-R
diagram
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If a main sequence star looks blue it must be a bright star
-- so if it looks dim it must be a very long way away
A red star is not as bright, so if it looks bright it must be
relatively close. For example:
Sirius and Alpha Centauri are similar in apparent
brightness but Sirius is bluish while A.Cent. is yellowish
So A.Cent must be
relatively close
Sirius is at 2.6 pc
A.Cent is at 1.3 pc.
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Binary stars give away their mass from their period and
distance apart.
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This enables us to study the relationship between
the mass and the other properties of stars
It is found that there is a simple massluminosity relationship for main sequence stars
The luminosity increases with the cube of the
mass (this is consistent with other clues about the
size, density and mass) – big is brighter (much!)
Big bright stars are burning ferociously and don’t
last long!
This tells us about the nuclear processes
occurring within stars and hints at their lifetimes.
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Because we know the relationship between the
energy output and the size (Wien’s law and the
Stefan-Boltzmann law etc.) we can determine
the area, and hence the radius of the star as
well.
This enables us to find the average density.
Because cooler stars will need to be bigger to
produce the same amount of luminosity, they
must be larger than equivalent hot stars.
The largest
stars are in
the brightcool corner
and the
smallest in
the dim-hot
corner
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Clusters of stars,
all born about the
same time, enable
us to study the life
cycle of stars.
For example, the
Pleiades…
The birth of a cluster
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All stars begin on the main sequence.
Brighter stars ‘die young’ and become giants
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All stars begin on the main sequence.
Brighter stars ‘die young’ and become giants
Dimmer stars are very long lived.
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Some stars end their
lives spectacularly!
They implode
producing such
enormous
temperatures that the
higher elements are
formed
Which is why we are
here talking about
them!
4. Whole new worlds
Andromeda Galaxy M31
The Milky Way is our galaxy
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It is about 50,000 pc (50 kpc) in diameter (160,000 ly)
but only 1000 pc thick (1 kpc)
- with a 2-3 kpc bulge in the centre
The Milky Way is our galaxy

We are about 8,000 pc (8 kpc) from the centre

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Henrietta Leavitt discovered that Cepheid variables had a
definite relationship between their period and luminosity
It turned out that
there were two types
of Cepheids, which
made the relationship
more accurate.
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Hubble was able to use this to determine the
distance to galaxies

His discovery revolutionised (that word again!) our
picture of the universe

The universe was not static and unchanging – as
even Einstein had believed

It was expanding!
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We can’t measure the speed of a distant galaxy
easily (radar guns don’t reach that far, besides
which... ?)
However we can use the same technique: Radar
guns measure the shift in frequency of the
microwaves bounced off a moving vehicle.
If the moving object is itself emitting waves we
also have a shift in frequency.
This is known as the ...
Doppler effect
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The truck emits waves which travel in the air at a
constant speed – whether or not the truck is moving.
If it is moving we hear a higher frequency in front of
the truck and a lower frequency once it passes us.
Truck not moving
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Doppler animation
Hubble found a definite relationship between the distance
of galaxies and their redshift
 This meant that the further away the galaxy, the greater
the rate at which it appeared to be moving away from us
 1990’s data
 v = Hod
 So Ho = ?
70 km/sec/Mpc

Hubble’s 1929 data

Galaxies are not speeding away from us through
space, it is space which is expanding – carrying
the galaxies with it!
A 2D analogy of a 4D universe

Astrophysics challenges many of our normal
assumptions – even the laws of physics
themselves

This is a wonderful opportunity for us to think
about the assumptions we (and others) make all
the time
 This
could even have political consequences!

For example, for the galaxies to move in the way they do
either Newton’s law (even when modified by Einstein) is
wrong or there is a lot of mass in the galaxies that we
can’t see – dark matter.
Is this dark matter?

Where did galaxies come from? Why did they form?
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Were Quasars present at the birth of a galaxy?
5. The expanding universe
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If the universe was expanding, what was it
expanding from? It seemed a very strange
idea to think it all came from nothing!

Fred Hoyle came up with a brilliant solution:
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It had always been, it was infinite, matter was
continually being created to keep the density
constant
... at the rate of a few atoms per day per
Cathedral

After all, how could it possibly have started
from nothing? - With a ‘big bang’?
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But how could we possibly tell the difference?

Astrophysicists love hard questions!

It must have been AWE FULL hot to start with!

That heat radiation should still be bouncing
around the universe

But would be MUCH colder by now.

Sure enough, astronomers were aware of radio waves
coming from the sky
COBE and WMAP have mapped this
radiation. It agrees very precisely with the
predictions of the big bang model
WMAP has also given us a very
accurate value for the Hubble
constant and therefore the age of
the universe

Age of universe – the time it has been expanding
at the observed rate – is equal to the reciprocal of
the Hubble constant (with a few adjustments)

T = 1/Ho = 1/71 km/sec/Mpc = 13.7 billion yrs.
How will it end?
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Is it closed, open or flat?
The expansion seems to be accelerating!
So why and what is the
universe?
Who knows?
But it’s sure fun trying to find out!