1. Dr Paul A DanielsGuildford Astronomical Society1

2. Dr Paul A DanielsGuildford Astronomical Society2 The motto of the Royal Astronomical Society: Quicquid nitet notandum “Whatever shines should be observed” Observing ‘whatever shines’ in the sky has been an interest of humanity for millennia! Sun & Moon Fixed stars Wandering stars ( asteres planetai ) – Planets Comets Meteor-ology Novae

3. Dr Paul A DanielsGuildford Astronomical Society3 The most numerous of these are the stars: What can be observed? Position  Fixed or moving? Brightness  Steady or varying? Colour  Steady

4. Dr Paul A DanielsGuildford Astronomical Society4 Hipparchus (190-120 BCE) classified stars into one of six brightness ‘magnitudes’ and may have proposed the early constellation patterns we use now. It’s likely that the 2 nd century Farnese Atlas (a Roman statue of the Titan Atlas holding up a celestial globe) is based indirectly on Hipparchus’ constellations.

5. Dr Paul A DanielsGuildford Astronomical Society5 Hipparchus’ magnitudes were in use for many years.

6. Dr Paul A DanielsGuildford Astronomical Society6 The basic Hipparchus’ magnitudes were in use for many years. Around 140 CE Claudius Ptolemy introduced ‘greater’ or ‘smaller’ sub-divisions of the magnitudes. Galileo, using a telescope, realised that stars fainter than sixth magnitude existed… InstrumentFaintest Magnitude Eye6 Binoculars9 6” Telescope13 Hubble Space Telescope31

7. Stars within each constellation are usually assigned successive lowercase greek letters according to their brightness: α (alpha), β (beta), γ (gamma), δ, ε, etc. Dr Paul A DanielsGuildford Astronomical Society7

8. Dr Paul A DanielsGuildford Astronomical Society8 In 1856 Norman Pogson at the Radcliffe Observatory in Oxford noted that stars classified by Hipparchus as 1 st magnitude were about 100 times brighter than 6 th magnitude stars. Pogson standardised the idea of magnitude by using the brightness ratio of 100 ⅕ (fifth root of 100) between adjacent magnitudes. This Pogson Ratio is about 2.512.  Retained ‘bigger’ magnitudes for fainter stars.  -3-2-10123456789 Magnitude x 2.512 x 100 x 2.512

9. Dr Paul A DanielsGuildford Astronomical Society9 There are two broad types of stellar magnitude: Apparent Magnitude  The brightness of a star as seen from just above the Earth’s atmosphere Absolute Magnitude  The brightness of a star as seen from a standard distance of 10 parsecs.  Used for comparing the intrinsic brightness of stars.

10. Dr Paul A DanielsGuildford Astronomical Society10 Typical magnitudes: Body Apparent Magnitude Distance (pc) Absolute Magnitude Sun-26.744.84 x 10 -6 4.83 Full Moon-12.74 Venus-3.8 to -4.9 Mars+1.6 to -3.0 Jupiter-1.6 to -2.94 Pluto+16.3 to +13.65 Sirius-1.442.641.45 Arcturus-0.0511.25-0.31 Vega0.037.760.58 Spica0.9880.39-3.55 Barnard’s Star9.541.8213.24 Proxima Centauri11.011.2915.45

11. Dr Paul A DanielsGuildford Astronomical Society11 Next time: Colour Temperature Hertzsprung-Russell Diagram