AstrophysiAstrophysicscs

for those who really want to for those who really want to know...know...

about life, the universe and about life, the universe and everythingeverything

The urge to make sense of The urge to make sense of their world has been a their world has been a

feature of every significant feature of every significant civilization in historycivilization in history

Stonehenge

Humans have always Humans have always wondered about the world wondered about the world

they live inthey live in

The night sky has The night sky has always fascinated always fascinated anyone who has some anyone who has some curiosity about their curiosity about their world.world.

It was one of the great It was one of the great topics of conversation topics of conversation among the early Greek among the early Greek philosophers.philosophers.

Natural philosophers, that is physicists, have Natural philosophers, that is physicists, have always wanted to understand how it works.always wanted to understand how it works.

It is the ultimate mystery story!It is the ultimate mystery story!

Our modern Our modern version of version of the ‘creation the ‘creation myth’ is myth’ is simply the simply the most most fascinating fascinating story ever story ever told!told!

‘Starbirth’

An antidote to An antidote to dogmatismdogmatism

It teaches us both that the universe is It teaches us both that the universe is actually a much more amazing place than actually a much more amazing place than we thought,we thought,

and that . . .and that . . . there is an enormous amount more to learn there is an enormous amount more to learn

about our place in the scheme of things.about our place in the scheme of things.

“The most beautiful thing we can experience is the mysterious. It is the source of all true art and science.”

Albert Einstein

Astrophysics is about…Astrophysics is about… howhow we know we know whatwhat

we know about the we know about the starsstars

particularly our particularly our own favourite star own favourite star – – the Sunthe Sun

whole new worlds – whole new worlds – the the galaxiesgalaxies

the life and death the life and death of the of the universe.universe.

Astrophysics is about… more Astrophysics is about… more than thisthan this

It is about who we are and even why It is about who we are and even why we are.we are.

It is about our place in the scheme of It is about our place in the scheme of things...things...

It may be that astrophysics can’t It may be that astrophysics can’t answer answer the ultimatethe ultimate questions, but questions, but anything that does claim to answer the anything that does claim to answer the ultimate questions must be consistent ultimate questions must be consistent with what we know about our universe.with what we know about our universe.

Astrophysics is about… usAstrophysics is about… us

Another quote from that great Another quote from that great astrophysicist:astrophysicist:

Science without religion is lame, religion without science is blind.

Albert Einstein

Astrophysics is about… our Astrophysics is about… our worldworld

Perhaps one of the greatest threats to Perhaps one of the greatest threats to our modern civilization is from those our modern civilization is from those who know who know THE ANSWERTHE ANSWER ... and pity ... and pity help the rest of us.help the rest of us.

But this is not just those who want to But this is not just those who want to destroy ‘decadent western society’...destroy ‘decadent western society’...

It is also the fundamentalists in our It is also the fundamentalists in our society who believe that they have the society who believe that they have the one true way – whether it is a religion, one true way – whether it is a religion, a political, or an economic ‘...ism’. a political, or an economic ‘...ism’.

Astrophysics is about… big Astrophysics is about… big ideasideas

All those ...isms seem somehow so small All those ...isms seem somehow so small minded and limited when we contrast their minded and limited when we contrast their view with the reality of the universe as we view with the reality of the universe as we can know it. There are several possible can know it. There are several possible reasons for this:reasons for this:

First, the imaginative thinking involved in First, the imaginative thinking involved in understanding the universe makes the sort understanding the universe makes the sort of thinking behind simplistic views look of thinking behind simplistic views look ridiculous.ridiculous.

...the idea of some ‘God’ sitting up there ...the idea of some ‘God’ sitting up there watching us to decide whether we go to watching us to decide whether we go to heaven or hell simply seems ‘childish’.heaven or hell simply seems ‘childish’.

Astrophysics is about… Astrophysics is about… responsibilityresponsibility

Second, it helps us to realise what a very Second, it helps us to realise what a very special place our Earth is, and how special place our Earth is, and how important it is for us to look after it.important it is for us to look after it.

Two things inspire me to awe - the starry heavens above and the moral universe within. Albert Einstein

Astrophysics is about… our Astrophysics is about… our humanityhumanity

Third, it helps us to realise that, despite Third, it helps us to realise that, despite all that we do know, there is a huge all that we do know, there is a huge amount that we don’t know.amount that we don’t know.

Is there a ‘purpose’ in the universe?Is there a ‘purpose’ in the universe? It could easily be said that the universe It could easily be said that the universe

seems ‘designed’ to bring about life. Does seems ‘designed’ to bring about life. Does this mean there is an ‘intelligent this mean there is an ‘intelligent designer’?designer’?

We don’t know the answer to this We don’t know the answer to this question... but surely it is one that is question... but surely it is one that is central to our sense of humanity – and one central to our sense of humanity – and one which can unite us rather than divide us.which can unite us rather than divide us.

Astrophysics is about…Astrophysics is about…

I maintain that cosmic religiousness is the strongest and most noble driving force of scientific research.

Albert Einstein

1. The stars – how far, how 1. The stars – how far, how bright?bright?

As the Earth revolves As the Earth revolves around its orbit, around its orbit, closer stars should closer stars should appear to move a appear to move a little against the little against the distant onesdistant ones

1. The stars – how far, how 1. The stars – how far, how bright?bright?

As the Earth As the Earth revolves around revolves around its orbit, closer its orbit, closer stars should stars should appear to move a appear to move a little against the little against the distant onesdistant ones

This is a This is a simulation of simulation of parallax – more parallax – more than most of the than most of the parallax that is parallax that is actually detected.actually detected.

1. The stars – how far, how 1. The stars – how far, how bright?bright?

The distance can be The distance can be found from this found from this ‘stellar parallax’‘stellar parallax’

1 1 parsec (pc) parsec (pc) is the is the distance to a star with distance to a star with a parallax angle of 1 a parallax angle of 1 arcsecondarcsecond

No stars are that No stars are that close!close!

Proxima Centauri is at Proxima Centauri is at 1.3 pc (4.2 ly).1.3 pc (4.2 ly).

HipparcosHipparcos has has measured the measured the stellar parallax of stellar parallax of over 100,000 stars over 100,000 stars

to better than 0.001 to better than 0.001 arcsec,arcsec,

that is out to 1000 that is out to 1000 parsecparsec

which might seem a which might seem a long way ….long way ….

High Precision Parallax Collecting

Satellite

... but it is only a tiny distance from our Sun ... but it is only a tiny distance from our Sun on the scale of a galaxy!on the scale of a galaxy!

However, it enables us to find the patterns However, it enables us to find the patterns which enable us to find the distances to which enable us to find the distances to virtually all the other stars.virtually all the other stars.

Stars come in a huge range of brightness Stars come in a huge range of brightness and coloursand colours

If we know the distance and the apparent If we know the distance and the apparent brightness we can work out the ‘intrinsic brightness we can work out the ‘intrinsic brightness’ of the star. (With the help of the brightness’ of the star. (With the help of the inverse square law.)inverse square law.)

The intrinsic brightness of a star is called it’s The intrinsic brightness of a star is called it’s ‘Luminosity’ and is measured in terms of the Sun’s ‘Luminosity’ and is measured in terms of the Sun’s luminosity, Lluminosity, L

Perhaps surprisingly, we can actually measure the Perhaps surprisingly, we can actually measure the total energy output of the Sun quite easily:total energy output of the Sun quite easily:

Every 1 mEvery 1 m22 at the Earth’s distance receives 1370 at the Earth’s distance receives 1370 W/mW/m22

Multiplying by the huge number of square metres Multiplying by the huge number of square metres in a sphere of that radius gives us… in a sphere of that radius gives us…

… … the Sun’s luminosity; the Sun’s luminosity; L = 3.9 x 1026 W.

The colour of stars tells us about their The colour of stars tells us about their temperaturetemperature

The Southern Cross with time defocusThe Southern Cross with time defocus

APOD 8 July 2004

Stars are (near Stars are (near enough) to ‘black enough) to ‘black bodies’, which bodies’, which radiate in a way radiate in a way dependent on dependent on their temperaturetheir temperature

The colour of a hot The colour of a hot object goes from object goes from red to white to red to white to blue as it gets blue as it gets hotter.hotter.

Wien’s law:Wien’s law:

T =T = k/ k/λλpp

(where k = 2.9 x 10(where k = 2.9 x 1066 nm.K)nm.K)

Good foundry and kiln operators can tell Good foundry and kiln operators can tell the temperature just by looking at the the temperature just by looking at the colourcolour

Physicists measure the colour with Physicists measure the colour with radiometers.radiometers.

Astronomers measure the colour of stars Astronomers measure the colour of stars by comparing the brightness of the star by comparing the brightness of the star in the light through three filters:in the light through three filters: U for ultravioletU for ultraviolet B for blueB for blue V for visibleV for visible

bbVV/b/bBB

is often used asis often used as

a colour indexa colour index

Orion includesOrion includes

Betelgeuse which is Betelgeuse which is one of the reddest one of the reddest stars with an index of stars with an index of 5.55 giving it a 5.55 giving it a temperature of ‘only’ temperature of ‘only’ 3,500 K.3,500 K.

Bellatrix which is one Bellatrix which is one of the bluest stars of the bluest stars with a colour index with a colour index (b(bVV/b/bBB) of 0.81 making ) of 0.81 making its surface its surface temperature 21,500 Ktemperature 21,500 K

If we know the If we know the luminosityluminosity

(total power output) (total power output) and the and the surface surface temperaturetemperature

we can find the we can find the sizesize of a of a star.star.

How?How?

The total energy radiated The total energy radiated from a hot object depends from a hot object depends

on two things:on two things:1. The surface area 1. The surface area

2. The temperature2. The temperature

The Stefan-Boltzmann law: The Stefan-Boltzmann law:

Energy radiated Energy radiated per square metreper square metre = = kkTT44

For an ideal “black body” k = 5.7 x 10For an ideal “black body” k = 5.7 x 10-8 -8 W/mW/m22.K.K44

At 5800 K

this means

every

square

metre of

the Sun is

giving out

65 MW!

So if we know the power of the star (its So if we know the power of the star (its luminosityluminosity) as well as its ) as well as its temperaturetemperature we can use the Stefan-Boltzmann law to we can use the Stefan-Boltzmann law to work out the work out the total areatotal area needed to needed to radiate that much powerradiate that much power

Once we know the area, the radius can be Once we know the area, the radius can be found (A = 4found (A = 4ππRR22 ) )

As we know the Sun is producing a total As we know the Sun is producing a total of 3.9 x 10of 3.9 x 1026 W, its area must be 3.9 x W, its area must be 3.9 x 101026/65 MW giving a radius of 690 million /65 MW giving a radius of 690 million metres metres

This checks well with the radius as This checks well with the radius as measured by triangulation techniques measured by triangulation techniques

All this depends on All this depends on knowing the luminosity – knowing the luminosity –

which depends on which depends on knowing the distanceknowing the distance ……and we can only measure the distance and we can only measure the distance

to a mere 100,000 stars.to a mere 100,000 stars. But if we could somehow find the But if we could somehow find the

(intrinsic) luminosity of the star we (intrinsic) luminosity of the star we could find its distance!could find its distance!

Knowing both would then enable us to Knowing both would then enable us to find the radius – and a whole lot more!find the radius – and a whole lot more!

So is there a way to find the luminosity So is there a way to find the luminosity of a star without knowing its distance?of a star without knowing its distance?

There is!There is!

But first we need to know more But first we need to know more about how stars work. So let’s look about how stars work. So let’s look more closely at …more closely at …

2. Our favourite star2. Our favourite star

The Sun photographed in soft X-rays

The Sun’s statsThe Sun’s stats

Distance:Distance: 1 AU=150 million km1 AU=150 million km Radius: Radius: 700,000 km (109 R700,000 km (109 Ree)) Mass: Mass: 2 x 102 x 103030 kg (330,000 kg (330,000

MMee)) Average density:Average density: 1.4 kg/litre1.4 kg/litre Surface temperature:Surface temperature: 5,800 K5,800 K Luminosity:Luminosity: 3.9 x 103.9 x 102626 watts watts

(joule/sec)(joule/sec)

The Sun’s stats – how do we The Sun’s stats – how do we know?know?

Distance:Distance: Radius: Radius: Mass: Mass:

Average density:Average density: Surface Surface

temperature:temperature: Luminosity:Luminosity:

TriangulationTriangulation

Triangulation and distanceTriangulation and distance

Newton’s theory of gravity Newton’s theory of gravity and the motion of the and the motion of the planetsplanets

Mass and volumeMass and volume

Colour & Wien’s law Colour & Wien’s law T =T = k/k/λλpp

Intensity and distance – with Intensity and distance – with the inverse square law I = the inverse square law I = k/dk/d22

It gives us life and It gives us life and energyenergy

Humans have always Humans have always revered the Sunrevered the Sun

But where does it But where does it get its energy from?get its energy from?

If it were coal it If it were coal it would only last would only last 10,000 years!10,000 years!

(Maybe the (Maybe the creationists were creationists were right!)right!)

The Sun’s energyThe Sun’s energy

Atkinson, 1920: Einstein’s equation, E = mcAtkinson, 1920: Einstein’s equation, E = mc22 means that somehow there is energy locked up means that somehow there is energy locked up in mass.in mass.

This energy is associated with the enormously This energy is associated with the enormously strong forces between the particles in the strong forces between the particles in the nucleusnucleus..

The forces between The forces between nucleonsnucleons are about are about 100,000,000 times as strong as those between 100,000,000 times as strong as those between the electrons around atoms.the electrons around atoms.

This is about the ratio of the force of gravity This is about the ratio of the force of gravity

on a mosquito compared to that on ...on a mosquito compared to that on ...

The Sun’s energyThe Sun’s energy

...on a Jumbo jet...on a Jumbo jet

This means that the This means that the energyenergy involved in involved in nuclear reactions will be nuclear reactions will be hundreds of hundreds of millions timesmillions times greater than that greater than that involved in chemical reactionsinvolved in chemical reactions

The Sun would last nearly 100 billion The Sun would last nearly 100 billion years on its hydrogen fuel – but other years on its hydrogen fuel – but other processes will cause problems before processes will cause problems before then!then!

(And bad luck for the creationists!)(And bad luck for the creationists!)

The Sun’s energyThe Sun’s energy

We now know a lot about how the Sun We now know a lot about how the Sun works from computer models based on works from computer models based on the normal laws of physics:the normal laws of physics:

Laws of pressure and volume of gases, Laws of pressure and volume of gases, fluid low, heat transmission and fluid low, heat transmission and radiation, buoyancy and pressure, and so radiation, buoyancy and pressure, and so on.on.

The corona, the source of the solar wind The corona, the source of the solar wind becomes visible in an eclipse.becomes visible in an eclipse.

The solar wind The solar wind brings us auroras.brings us auroras.

Sunspots are the result of ‘knots’ in the Sunspots are the result of ‘knots’ in the magnetic field of the Sun. They ‘untie’ every 11 magnetic field of the Sun. They ‘untie’ every 11 years.years.

Our insight into the Our insight into the workings of the Sun workings of the Sun

enables us to understand enables us to understand the stars better.the stars better.

The stars and nebulae of the constellation OrionThe stars and nebulae of the constellation Orion