Module P7 L6

Which is Brightest? A glow worm 1 m away... They could both appear to be the same brightness.... or car headlights 1000 m away? The intrinsic brightness of the glow worm is 1/1000 th of the intrinsic brightness of the headlights but the glow worm is 1000 times closer so the observed brightness is the same.

Analysing the Results NOTE: The resistance of the LDR gets higher in the dark and lower in bright light 8. What can you say about the intrinsic brightness of the bulb during this experiment? 9. What is the relationship between distance and the resistance of the LDR? 10. What is the relationship between distance and observed brightness?

(4)A has a greater intrinsic brightness than B and is the same distance or closer but its light is dimmed by passing through dust clouds Two stars A and B could have the same observed brightness because: (3)B has a greater intrinsic brightness than A but is further away (2)A has a greater intrinsic brightness than B but is further away (1)A and B have the same intrinsic brightness and are the same distance away from us (5)A has a greater intrinsic brightness than B and is the same distance or closer but its light is dimmed by passing through dust clouds

Colour of a Star Stars can appear to be blue, yellow or red... Blue stars are the largest and hottest. Yellow stars are relatively small and cool (our Sun is classified as a ‘yellow dwarf star’). Red stars could be either: red giants (large but cool) red dwarfs (small and cool)

Star light, star bright Intensity of radiation at each frequency Smaller Frequency (Longer Wavelength) Stars emit light at all frequencies. However, some stars emit more (say) blue light and so they appear blue. The area under the graph represents the total energy emitted by the star. Blue stars give off more energy than red stars.

[3] Real exam question

86 light years from Earth 62 light years from Earth

Nearby stars also show an annual motion due to the movement of the Earth around the Sun. The effect is only measurable when nearby stars are viewed against a background of more distant stars.

Parallax There is no star (other than the Sun) which has an annual parallax of more than one second. The star with the largest parallax is Proxima Centauri: 0.77 seconds of arc. (Note: 1 second of arc = 1/60 th of 1 minute of arc = 1/3600 th of 1 degree of arc) Proxima Centauri is 1/0.77 = parsecs away 1 parsec (pc) is 3.26 light years

Cepheid Variables Some stars are variable stars – their brightness changes over time. Some stars are Cepheid Variables – their brightness changes in a regular pattern. The period of the pattern is fixed. The period of Eta Aquilae is 7.2 days. From the period or frequency, we can calculate their intrinsic brightness. If we know their intrinsic brightness, we can work out how far away they are.

Cepheid Variables 2 In the 1920s, Edwin Hubble used Cepheid variables to calculate the distances to a number of galaxies. He used the red shift to calculate the speed at which they were moving.

Hubble’s Discovery He found that the further a galaxy the faster it moved. He plotted a graph of: speed in kilometres per second ( km / s) on the y-axis against distance in megaparsecs (Mpc) on the x-axis The graph showed that the velocity of a galaxy is directly proportional to its distance from us. Hubble measured the gradient as 120 kilometres per second per Megaparsec. 120 km / s / Mpc

The Hubble Constant Astronomers have named this gradient the Hubble Constant in his honour. speed of recession = Hubble constant x distance The gradient of this graph was the very first indication that we live inside an expanding Universe. Modern measurements mean that the Hubble Constant is about 70 km / s / Mpc. Finding a more exact value for the Hubble Constant will allow us to find out whether we live in an open or closed Universe – we may be able to predict not just the future of the Universe, but the future of Time itself and then, just possibly (in the words of Professor Stephen Hawking) “we should know the mind of God.”

“If we find the answer to that, it would be the ultimate triumph of human reason - for then we should know the mind of God.” Stephen Hawking, Lucasian Professor of Mathematics, Cambridge University, writing in A Brief History of Time (p.193)