3. We know the stars by 3. We know the stars by their lighttheir light

Radio and Radio and microwavemicrowave

Visible, IR and UVVisible, IR and UV

The spectrum of a star tells The spectrum of a star tells

us:us: It’s temperatureIt’s temperature It’s luminosityIt’s luminosity It’s distanceIt’s distance It’s sizeIt’s size And more.And more.

The visible spectrum of the Sun

We have already seen that the We have already seen that the colour of a star enables us to find its colour of a star enables us to find its temperature…temperature…

provided the colour is not altered as provided the colour is not altered as it passes through interstellar dust!it passes through interstellar dust!

Fortunately there are other ways to Fortunately there are other ways to find the temperature from the find the temperature from the spectrum…spectrum…

Stellar temperaturesStellar temperatures

By examining the temperature and spectra of By examining the temperature and spectra of nearby stars, astrophysicists found that there nearby stars, astrophysicists found that there were other indicators of temperaturewere other indicators of temperature

Certain spectral lines appeared consistently at Certain spectral lines appeared consistently at certain temperatures and disappeared at otherscertain temperatures and disappeared at others

Different lines appear with different degrees of Different lines appear with different degrees of ionisation – which results from different ionisation – which results from different temperaturestemperatures

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

Stellar spectra are classified according Stellar spectra are classified according to a system which ranks them in order of to a system which ranks them in order of surface temperature (the letters were re-surface temperature (the letters were re-ordered from an older system):ordered from an older system):

hothot OO BB AA FF GG KK MM cool cool (L(L T)T)

OOh h BBe e AA FFine ine GGirl (irl (GGuy) uy) KKiss iss MMee . . . . . .

The Hertzsprung-Russell The Hertzsprung-Russell diagram diagram brings order to this mass of brings order to this mass of informationinformation It plots stars It plots stars

according to their according to their temperature and temperature and luminosityluminosity

A definite pattern A definite pattern emerges:emerges:

Bright stars are Bright stars are bluer - that is, bluer - that is, hotter.hotter.

90% of stars are 90% of stars are on the ‘Main on the ‘Main Sequence’Sequence’

Another H-RAnother H-R

diagramdiagram

If a main sequence star looks blue it must be a bright 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 star -- so if it looks dim it must be a very long way awayaway

A red star is not as bright, so if it looks bright it must A red star is not as bright, so if it looks bright it must be relatively close. For example:be relatively close. For example:

Sirius and Alpha Centauri are similar in apparent Sirius and Alpha Centauri are similar in apparent brightness but Sirius is bluish while A.Cent. is brightness but Sirius is bluish while A.Cent. is yellowishyellowish

So A.Cent must So A.Cent must be relatively close be relatively close

Sirius is at 2.6 pcSirius is at 2.6 pc A.Cent is at 1.3 A.Cent is at 1.3

pc.pc.

Binary stars give away their mass from Binary stars give away their mass from their period and distance apart.their period and distance apart.

This enables us to study the relationship This enables us to study the relationship between the mass and the other properties of between the mass and the other properties of starsstars

It is found that there is a simple It is found that there is a simple mass-mass-luminosityluminosity relationship for main sequence relationship for main sequence starsstars

The luminosity increases with the cube of the The luminosity increases with the cube of the mass (this is consistent with other clues about mass (this is consistent with other clues about the size, density and mass) – the size, density and mass) – big is brighterbig is brighter (much!)(much!)

Big bright stars are burning ferociously and Big bright stars are burning ferociously and don’t last long!don’t last long!

This tells us about the nuclear processes This tells us about the nuclear processes occurring within stars and hints at their occurring within stars and hints at their lifetimes.lifetimes.

Because we know the relationship Because we know the relationship between the between the energy outputenergy output and the and the sizesize (Wien’s law and the Stefan- (Wien’s law and the Stefan-Boltzmann law etc.) we can determine Boltzmann law etc.) we can determine the area, and hence the the area, and hence the radiusradius of the of the star as well.star as well.

This enables us to find the This enables us to find the average average densitydensity..

Because Because coolercooler stars will need to be stars will need to be bigger to produce the same amount of bigger to produce the same amount of luminosity, they must be luminosity, they must be largerlarger than than equivalent hot stars.equivalent hot stars.

The The largestlargest stars are in stars are in the the bright-bright-coolcool corner corner

and the and the smallestsmallest in the in the dim-dim-hothot corner corner

Clusters of Clusters of stars, all born stars, all born about the about the same time, same time, enable us to enable us to study the life study the life cycle of stars.cycle of stars.

For example, For example, the Pleiades…the Pleiades…

The birth of a

cluster

All stars begin on the main sequence.All stars begin on the main sequence. Brighter stars ‘die young’ and become Brighter stars ‘die young’ and become

giantsgiants

All stars begin on the main sequence.All stars begin on the main sequence. Brighter stars ‘die young’ and become Brighter stars ‘die young’ and become

giantsgiants Dimmer stars are very long lived.Dimmer stars are very long lived.

Some stars end Some stars end their lives their lives spectacularly!spectacularly!

They implode They implode producing such producing such enormous enormous temperatures temperatures that the higher that the higher elements are elements are formedformed

Which is why we Which is why we are here talking are here talking about them!about them!

4. Whole new worlds4. Whole new worlds

Andromeda Galaxy M31

The Milky Way is our The Milky Way is our galaxygalaxy

It is about 50,000 pc (50 kpc) in diameter It is about 50,000 pc (50 kpc) in diameter (160,000 ly)(160,000 ly)

but only 1000 pc thick (1 kpc)but only 1000 pc thick (1 kpc) - with a 2-3 kpc bulge in the centre- with a 2-3 kpc bulge in the centre

The Milky Way is our The Milky Way is our galaxygalaxy

We are about 8,000 pc (8 kpc) from the We are about 8,000 pc (8 kpc) from the centrecentre

Henrietta Leavitt discovered that Cepheid Henrietta Leavitt discovered that Cepheid variables had a definite relationship variables had a definite relationship between their period and luminositybetween their period and luminosity

It turned out It turned out that there were that there were two types of two types of Cepheids, which Cepheids, which made the made the relationship relationship more accurate.more accurate.

Hubble was able to use this to Hubble was able to use this to determine the distance to galaxiesdetermine the distance to galaxies

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

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

It was expanding!It was expanding!

We can’t measure the speed of a We can’t measure the speed of a distant galaxy easily (radar guns don’t distant galaxy easily (radar guns don’t reach that far, besides which... ?)reach that far, besides which... ?)

However we can use the same However we can use the same technique: Radar guns measure the technique: Radar guns measure the shift in frequency of the microwaves shift in frequency of the microwaves bounced off a moving vehicle.bounced off a moving vehicle.

If the moving object is itself emitting If the moving object is itself emitting waves we also have a shift in waves we also have a shift in frequency.frequency.

This is known as the ...This is known as the ... Doppler effectDoppler effect

The truck emits waves which travel in The truck emits waves which travel in the air at a constant speed – whether or the air at a constant speed – whether or not the truck is moving.not the truck is moving.

If it is moving we hear a higher If it is moving we hear a higher frequency in front of the truck and a frequency in front of the truck and a lower frequency once it passes us.lower frequency once it passes us.

Truck not moving

Doppler animationDoppler animation

Hubble found a definite relationship Hubble found a definite relationship between the distance of galaxies and their between the distance of galaxies and their redshiftredshift

This meant that the further away the This meant that the further away the galaxy, the greater the rate at which it galaxy, the greater the rate at which it appeared to be moving away from usappeared to be moving away from us

1990’s data1990’s data v = Hv = Hoodd

So HSo Hoo = ? = ?

70 km/sec/Mpc70 km/sec/Mpc

Hubble’s 1929 data

Galaxies are not speeding away from Galaxies are not speeding away from us through space, it is space which us through space, it is space which is expanding – carrying the galaxies is expanding – carrying the galaxies with it!with it!

A 2D analogy of a 4D universe

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

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

This could even have political This could even have political consequences!consequences!

For example, for the galaxies to move in the For example, for the galaxies to move in the way they do either Newton’s law (even way they do either Newton’s law (even when modified by Einstein) is wrong or when modified by Einstein) is wrong or there is a lot of mass in the galaxies that we there is a lot of mass in the galaxies that we can’t see – dark matter.can’t see – dark matter.

Is this dark matter?

Where did galaxies come from? Why did Where did galaxies come from? Why did they form?they form?

Were Quasars present at the birth of a Were Quasars present at the birth of a galaxy?galaxy?

5. The expanding 5. The expanding universeuniverse

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

Fred Hoyle came up with a brilliant Fred Hoyle came up with a brilliant solution:solution: It had always been, it was infinite, matter It had always been, it was infinite, matter

was continually being created to keep was continually being created to keep the density constantthe density constant

... at the rate of a few atoms per day per ... at the rate of a few atoms per day per CathedralCathedral

After all, how could it possibly have After all, how could it possibly have started from nothing? - With a ‘big started from nothing? - With a ‘big bang’?bang’?

But how could we possibly tell the But how could we possibly tell the difference?difference?

Astrophysicists love hard questions!Astrophysicists love hard questions!

It must have been It must have been AWE FULL hotAWE FULL hot to to start with!start with!

That heat radiation should still be That heat radiation should still be bouncing around the universebouncing around the universe

But would be But would be MUCH colderMUCH colder by now. by now.

Sure enough, astronomers were aware of Sure enough, astronomers were aware of radio waves coming from the skyradio waves coming from the sky

COBE and WMAP have mapped COBE and WMAP have mapped this radiation. It agrees very this radiation. It agrees very

precisely with the predictions precisely with the predictions of the big bang modelof the big bang model

WMAP has also given us WMAP has also given us a very accurate value for a very accurate value for the Hubble constant and the Hubble constant and therefore the age of the therefore the age of the

universeuniverse Age of universe – the time it has been Age of universe – the time it has been

expanding at the observed rate – is equal to expanding at the observed rate – is equal to the reciprocal of the Hubble constant (with a the reciprocal of the Hubble constant (with a few adjustments)few adjustments)

T = 1/HT = 1/Hoo = 1/71 km/sec/Mpc = 13.7 billion = 1/71 km/sec/Mpc = 13.7 billion yrs.yrs.

How will it How will it end?end?

Is it closed, open or flat?Is it closed, open or flat? The expansion seems to be accelerating!The expansion seems to be accelerating!

So why and what is So why and what is the universe?the universe?

Who knows?Who knows?

But it’s sure fun trying to But it’s sure fun trying to find out!find out!