AstrophysParts3-4-5

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

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It became possible to classify stars by the intensity of certain lines in the spectrum

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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   

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

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His discovery revolutionised (that word again!) our picture of the universe

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The universe was not static and unchanging – as even Einstein had believed

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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

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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

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Astrophysics challenges many of our normal assumptions – even the laws of physics themselves

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This is a wonderful opportunity for us to think about the assumptions we (and others) make all the time

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could even have political consequences!

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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?

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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 

If the universe was expanding, what was it expanding from? It seemed a very strange idea to think it all came from nothing!

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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

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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?

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Astrophysicists love hard questions!

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It must have been AWE FULL hot to start with!

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That heat radiation should still be bouncing around the universe

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But would be MUCH colder by now.

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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)

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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!