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 the most fascinating story ever told! Our modern version of the ‘creation myth’ is 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. 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 The distance can be found from this ‘stellar parallax’ The distance can be found from this ‘stellar parallax’ 1 parsec is the distance to a star with a parallax angle of 1 arcsecond 1 parsec 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).

9. 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

10. ... 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.

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

12. 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.

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

14. 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)

15. 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.

16. 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

17. 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

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

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

21. 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!

22. 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 

23. 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?

24. There is! But first we need to know more about how stars work. So let’s look more closely at our very own star, the Sun But first we need to know more about how stars work. So let’s look more closely at our very own star, the Sun