1. PHYS1142 17 More about Electromagnetic Radiation Speed of light = frequency  wavelength = a constant c Q. How do we generate light? A. Heat things up Heat up a solid, liquid or gas and the atoms have more energy. They move about faster, electrons get knocked out of normal orbits, fall back and give out light. Tungsten wire as a light bulb filament: The colour tells us about the temperature without contact. Red 2000 K Yellow 6000 K Blue 10,000 K Note the Kelvin temperature scale starts at the lowest possible temperature “absolute zero” or -273 o C, so room temperature is about 290 K.

2. PHYS1142 18 Spectra Kirchoff’s Rules (Zeilik p. 87, Kuhn p. 110) 1) Hot, opaque solids, liquids and gases emit a continuous spectrum. 2) Hot, transparent gases produce a spectrum of bright lines (emission lines). The number of lines and their colour is unique to each element. 3) If a continuous spectrum of light from a hot, opaque body passes through a gas at a lower temperature, the cooler gas will produce dark lines (absorption lines). The pattern of the lines depends on the elements in the cooler gas. Figs. Z5.4, Z5.5 & K4-10, K4-14 We can learn about the chemical composition of stars solely from the light which they emit.

3. PHYS1142 19 Movement From their changing surface features the Sun, the planets and their moons can be seen to be rotating about their axes. Planets are seen to be orbiting the Sun. Do other stars rotate or move? Can we see this happening? Resolving a star: The images of stars in telescopic photographs are different sizes, but this is a limitation of the telescope and camera - the stars are not truly resolved. You could see a star as a disc if it made an angle greater than the wavelength of the observed light divided by the diameter (or “aperture”) of your telescope. Let’s put in some typical figures…

4. PHYS1142 20 Wavelength of yellow light  = 0.00000056 m Aperture diameter of telescope = 4 m so wavelength / diameter = 0.00000014 The angle made by the nearest star is its diameter divided by its distance = 0.00000003 So we see a point of light, not a disc. We certainly can’t see it rotating by watching for surface features. Can we see movement across the sky? Yes for nearby stars, no for all others. But the signature of movement is in the spectra: The Doppler Shift Object approaching along line of sight - blueshift Object receding along line of sight - redshift Object moving at right angles to line of sight - no shift Zeilik p.199, Kuhn & Koupelis p.120

5. PHYS1142 21 The Creation of the Elements The Big Bang left the Universe with hydrogen, helium and a little lithium plus a few other light elements. The only birthplaces for heavier elements are the interiors of stars. How does this work? Problem: can’t enter stars, only receive light. Problem: stars are very stable. How do we discover how they evolve? Look for clues in our local star, the Sun. What can we measure? a) spectrum of light b) size, mass, density c) surface features, sunspots d) solar atmosphere e) energy output

6. PHYS1142 22 The Solar Spectrum Continuous spectrum with absorption lines. Figs. Z13.8 & K4-10 Requires a very hot opaque gas below the surface to give a continuous spectrum, ending in a transparent atmospheric layer a few 100 km thick at a temperature of 5800 K. Figs. Z13.5 & K4-15 Absorption lines tell us the abundance of gases in the photosphere: 74% hydrogen 25% helium 1% heavier elements This does not tell us the composition of the interior.

7. PHYS1142 23 Calculate the Sun’s actual diameter using (a), (b) and (c) = 1,400,000 km Using Newton’s Laws of Motion we can calculate the Sun’s mass = 2 x 10 30 kg (the Sun constitutes 99.85% of the total mass of the solar system) Mass divided by volume gives us the Sun’s average density = 1410 kg per cubic metre The Sun is made up of gas - it is too hot to be liquid. Size (a) Measure Earth to Venus distance using radar echo. (b) Use Kepler’s Third Law to calculate Earth - Sun distance = 149,600,000 km (see lecture 2) (c) Measure the angular diameter of the Sun as seen from Earth = 0.5 o

8. PHYS1142 24 Surface Features Sunspots have a temperature of about 4200 K, so they look darker than the yellow photosphere. As sunspots appear to cross the Sun’s surface we can measure its rotation rate. It is not a solid body and rotates fastest at its equator - once in 25.4 days - and slowest at its poles - once in nearly 40 days. Sunspots have an 11 year activity cycle. They occur in pairs and are connected with magnetic effects. Figs. Z13.21 & K11-27 They are probably linked to weather on Earth; almost no sunspots were seen from 1645 to 1715 during an unusually cold spell.