1. Astrophysics for those who really want to know... about life, the universe and everything

2. The urge to make sense of their world has been a feature of every significant civilization in history Stonehenge

3. Humans have always wondered about the world they live in The night sky has always fascinated anyone who has some curiosity about their world. The night sky has always fascinated anyone who has some curiosity about their world. It was one of the great topics of conversation among the early Greek philosophers. It was one of the great topics of conversation among the early Greek philosophers.

4. Natural philosophers, that is physicists, have always wanted to understand how it works. Natural philosophers, that is physicists, have always wanted to understand how it works. It is the ultimate mystery story! It is the ultimate mystery story!

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

6. An antidote to dogmatism It teaches us both that the universe is actually a much more amazing place than we thought, It teaches us both that the universe is actually a much more amazing place than we thought, and that... and that... there is an enormous amount more to learn about our place in the scheme of things. there is an enormous amount more to learn 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

7. Astrophysics is about… how we know what we know about the stars how we know what we know about the stars particularly our own favourite star – the Sun particularly our own favourite star – the Sun whole new worlds – the galaxies whole new worlds – the galaxies the life and death of the universe. the life and death of the universe.

8. Astrophysics is about… more than this It is about who we are and even why we are. It is about who we are and even why we are. It is about our place in the scheme of things... It is about our place in the scheme of things... It may be that astrophysics can’t answer the ultimate questions, but anything that does claim to answer the ultimate questions must be consistent with what we know about our universe. It may be that astrophysics can’t answer the ultimate questions, but anything that does claim to answer the ultimate questions must be consistent with what we know about our universe.

9. Astrophysics is about… us Another quote from that great astrophysicist: Another quote from that great astrophysicist: Science without religion is lame, religion without science is blind. Albert Einstein

10. Astrophysics is about… our world Perhaps one of the greatest threats to our modern civilization is from those who know THE ANSWER... and pity help the rest of us. Perhaps one of the greatest threats to our modern civilization is from those who know THE ANSWER... and pity help the rest of us. But this is not just those who want to destroy ‘decadent western society’... But this is not just those who want to destroy ‘decadent western society’... It is also the fundamentalists in our society who believe that they have the one true way – whether it is a religion, a political, or an economic ‘...ism’. It is also the fundamentalists in our society who believe that they have the one true way – whether it is a religion, a political, or an economic ‘...ism’.

11. Astrophysics is about… big ideas All those...isms seem somehow so small minded and limited when we contrast their view with the reality of the universe as we can know it. There are several possible reasons for this: All those...isms seem somehow so small minded and limited when we contrast their view with the reality of the universe as we can know it. There are several possible reasons for this: First, the imaginative thinking involved in understanding the universe makes the sort of thinking behind simplistic views look ridiculous. First, the imaginative thinking involved in understanding the universe makes the sort of thinking behind simplistic views look ridiculous....the idea of some ‘God’ sitting up there watching us to decide whether we go to heaven or hell simply seems ‘childish’....the idea of some ‘God’ sitting up there watching us to decide whether we go to heaven or hell simply seems ‘childish’.

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

13. Astrophysics is about… our humanity Third, it helps us to realise that, despite all that we do know, there is a huge amount that we don’t know. Third, it helps us to realise that, despite all that we do know, there is a huge 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 seems ‘designed’ to bring about life. Does this mean there is an ‘intelligent designer’? It could easily be said that the universe seems ‘designed’ to bring about life. Does this mean there is an ‘intelligent designer’? We don’t know the answer to this question... but surely it is one that is central to our sense of humanity – and one which can unite us rather than divide us. We don’t know the answer to this question... but surely it is one that is central to our sense of humanity – and one which can unite us rather than divide us.

14. Astrophysics is about… I maintain that cosmic religiousness is the strongest and most noble driving force of scientific research. Albert Einstein

15. 1. The stars – how far, how bright? As the Earth revolves around its orbit, closer stars should appear to move a little against the distant ones As the Earth revolves around its orbit, closer stars should appear to move a little against the distant ones

16. 1. The stars – how far, how bright? As the Earth revolves around its orbit, closer stars should appear to move a little against the distant ones As the Earth revolves around its orbit, closer stars should appear to move a little against the distant ones This is a simulation of parallax – more than most of the parallax that is actually detected. This is a simulation of parallax – more than most of the parallax that is actually detected.

17. 1. The stars – how far, how bright? The distance can be found from this ‘stellar parallax’ The distance can be found from this ‘stellar parallax’ 1 parsec (pc) is the distance to a star with a parallax angle of 1 arcsecond 1 parsec (pc) is the distance to a star with a parallax angle of 1 arcsecond No stars are that close! No stars are that close! Proxima Centauri is at 1.3 pc (4.2 ly). Proxima Centauri is at 1.3 pc (4.2 ly).

18. Hipparcos has measured the stellar parallax of over 100,000 stars Hipparcos has measured the stellar parallax of over 100,000 stars to better than 0.001 arcsec, to better than 0.001 arcsec, that is out to 1000 parsec that is out to 1000 parsec which might seem a long way …. which might seem a long way …. High Precision Parallax Collecting Satellite

19. ... but it is only a tiny distance from our Sun on the scale of a galaxy!... but it is only a tiny distance from our Sun on the scale of a galaxy! However, it enables us to find the patterns which enable us to find the distances to virtually all the other stars. However, it enables us to find the patterns which enable us to find the distances to virtually all the other stars.

20. Stars come in a huge range of brightness and colours

21. If we know the distance and the apparent brightness we can work out the ‘intrinsic brightness’ of the star. (With the help of the inverse square law.) If we know the distance and the apparent brightness we can work out the ‘intrinsic brightness’ of the star. (With the help of the inverse square law.) The intrinsic brightness of a star is called it’s ‘Luminosity’ and is measured in terms of the Sun’s luminosity, L  The intrinsic brightness of a star is called it’s ‘Luminosity’ and is measured in terms of the Sun’s luminosity, L  Perhaps surprisingly, we can actually measure the total energy output of the Sun quite easily: Perhaps surprisingly, we can actually measure the total energy output of the Sun quite easily: Every 1 m 2 at the Earth’s distance receives 1370 W/m 2 Every 1 m 2 at the Earth’s distance receives 1370 W/m 2 Multiplying by the huge number of square metres in a sphere of that radius gives us… Multiplying by the huge number of square metres in a sphere of that radius gives us… … the Sun’s luminosity;  … the Sun’s luminosity; L  = 3.9 x 10 26 W.

22. The colour of stars tells us about their temperature The colour of stars tells us about their temperature

23. The Southern Cross with time defocus The Southern Cross with time defocus APOD 8 July 2004

24. Stars are (near enough) to ‘black bodies’, which radiate in a way dependent on their temperature Stars are (near enough) to ‘black bodies’, which radiate in a way dependent on their temperature The colour of a hot object goes from red to white to blue as it gets hotter. The colour of a hot object goes from red to white to blue as it gets hotter. Wien’s law: T = k/λ p (where k = 2.9 x 10 6 nm.K)

25. Good foundry and kiln operators can tell the temperature just by looking at the colour Good foundry and kiln operators can tell the temperature just by looking at the colour Physicists measure the colour with radiometers. Physicists measure the colour with radiometers.

26. Astronomers measure the colour of stars by comparing the brightness of the star in the light through three filters: Astronomers measure the colour of stars by comparing the brightness of the star in the light through three filters: U for ultraviolet U for ultraviolet B for blue B for blue V for visible V for visible b V /b B b V /b B is often used as a colour index

27. Orion includes Orion includes Betelgeuse which is one of the reddest stars with an index of 5.55 giving it a temperature of ‘only’ 3,500 K. Betelgeuse which is one of the reddest stars with an index of 5.55 giving it a temperature of ‘only’ 3,500 K. Bellatrix which is one of the bluest stars with a colour index (b V /b B ) of 0.81 making its surface temperature 21,500 K Bellatrix which is one of the bluest stars with a colour index (b V /b B ) of 0.81 making its surface temperature 21,500 K

29. If we know the luminosity (total power output) and the surface temperature we can find the size of a star. How?

30. The total energy radiated from a hot object depends on two things: 1. The surface area 2. The temperature

31. The Stefan-Boltzmann law: Energy radiated per square metre = kT 4 For an ideal “black body” k = 5.7 x 10 -8 W/m 2.K 4 At 5800 K this means every square metre of the Sun is giving out 65 MW!

32. So if we know the power of the star (its luminosity) as well as its temperature we can use the Stefan-Boltzmann law to work out the total area needed to radiate that much power So if we know the power of the star (its luminosity) as well as its temperature we can use the Stefan-Boltzmann law to work out the total area needed to radiate that much power Once we know the area, the radius can be found (A = 4πR 2 ) Once we know the area, the radius can be found (A = 4πR 2 ) As we know the Sun is producing a total of 3.9 x 10 W, its area must be 3.9 x 10/65 MW giving a radius of 690 million metres As we know the Sun is producing a total of 3.9 x 10 26 W, its area must be 3.9 x 10 26 /65 MW giving a radius of 690 million metres This checks well with the radius as measured by triangulation techniques  This checks well with the radius as measured by triangulation techniques 

33. All this depends on knowing the luminosity – which depends on knowing the distance …and we can only measure the distance to a mere 100,000 stars. …and we can only measure the distance to a mere 100,000 stars. But if we could somehow find the (intrinsic) luminosity of the star we could find its distance! But if we could somehow find the (intrinsic) luminosity of the star we could find its distance! Knowing both would then enable us to find the radius – and a whole lot more! Knowing both would then enable us to find the radius – and a whole lot more! So is there a way to find the luminosity of a star without knowing its distance? So is there a way to find the luminosity of a star without knowing its distance?

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

35. 2. Our favourite star The Sun photographed in soft X-rays

36. The Sun’s stats Distance:1 AU=150 million km Distance:1 AU=150 million km Radius: 700,000 km (109 R e ) Radius: 700,000 km (109 R e ) Mass: 2 x 10 30 kg (330,000 M e ) Mass: 2 x 10 30 kg (330,000 M e ) Average density:1.4 kg/litre Average density:1.4 kg/litre Surface temperature:5,800 K Surface temperature:5,800 K Luminosity:3.9 x 10 26 watts (joule/sec) Luminosity:3.9 x 10 26 watts (joule/sec)

37. The Sun’s stats – how do we know? Distance: Distance: Radius: Radius: Mass: Mass: Average density: Average density: Surface temperature: Surface temperature: Luminosity: Luminosity: Triangulation Triangulation and distance Newton’s theory of gravity and the motion of the planets Mass and volume Colour & Wien’s law T = k/λ p Intensity and distance – with the inverse square law I = k/d 2

38. It gives us life and energy Humans have always revered the Sun Humans have always revered the Sun But where does it get its energy from? But where does it get its energy from? If it were coal it would only last 10,000 years! If it were coal it would only last 10,000 years! (Maybe the creationists were right!) (Maybe the creationists were right!)

39. The Sun’s energy Atkinson, 1920: Einstein’s equation, E = mc 2 means that somehow there is energy locked up in mass. Atkinson, 1920: Einstein’s equation, E = mc 2 means that somehow there is energy locked up in mass. This energy is associated with the enormously strong forces between the particles in the nucleus. This energy is associated with the enormously strong forces between the particles in the nucleus. The forces between nucleons are about 100,000,000 times as strong as those between the electrons around atoms. The forces between nucleons are about 100,000,000 times as strong as those between 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...

40. The Sun’s energy...on a Jumbo jet...on a Jumbo jet

41. This means that the energy involved in nuclear reactions will be hundreds of millions times greater than that involved in chemical reactions This means that the energy involved in nuclear reactions will be hundreds of millions times greater than that involved in chemical reactions The Sun would last nearly 100 billion years on its hydrogen fuel – but other processes will cause problems before then! The Sun would last nearly 100 billion years on its hydrogen fuel – but other processes will cause problems before then! (And bad luck for the creationists!) (And bad luck for the creationists!) The Sun’s energy

42. We now know a lot about how the Sun works from computer models based on the normal laws of physics: We now know a lot about how the Sun works from computer models based on the normal laws of physics: Laws of pressure and volume of gases, fluid low, heat transmission and radiation, buoyancy and pressure, and so on. Laws of pressure and volume of gases, fluid low, heat transmission and radiation, buoyancy and pressure, and so on.

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

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

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

51. Our insight into the workings of the Sun enables us to understand the stars better.

52. The stars and nebulae of the constellation Orion