Transcript Astronomy Basics 1

Dr Paul A Daniels
Guildford Astronomical Society
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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
Dr Paul A Daniels
Guildford Astronomical Society
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The most numerous of these are the stars:
What can be observed?
• Position

Fixed or moving?
• Brightness

Steady or varying?
• Colour

Dr Paul A Daniels
Steady
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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 2nd century Farnese Atlas (a Roman statue of the Titan Atlas holding up a
celestial globe) is based indirectly on Hipparchus’ constellations.
Dr Paul A Daniels
Guildford Astronomical Society
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Hipparchus’ magnitudes were in use for many years.
Dr Paul A Daniels
Guildford Astronomical Society
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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…
Instrument
Eye
Binoculars
6” Telescope
Faintest Magnitude
6
9
13
Hubble Space Telescope
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Guildford Astronomical Society
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Stars within each
constellation are
usually assigned
successive
lowercase greek
letters according to
their brightness:
α (alpha), β (beta),
γ (gamma), δ, ε,
etc.
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Guildford Astronomical Society
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In 1856 Norman Pogson at the Radcliffe Observatory
in Oxford noted that stars classified by Hipparchus as
1st magnitude were about 100 times brighter than
6th magnitude stars.
Pogson standardised the idea of magnitude by using
the brightness ratio of 100⅕ (fifth root of 100)
between adjacent magnitudes. This Pogson Ratio is
about 2.512.
x 100
x 2.512
-3
x 2.512
-2
x 2.512
-1
x 2.512
0
x 2.512
1
x 2.512
2
x 2.512
x 2.512
3
4
Magnitude
5
x 2.512
6
x 2.512
7
x 2.512
8
x 2.512
9
 Retained ‘bigger’ magnitudes for fainter stars. 
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Guildford Astronomical Society
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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
Dr Paul A Daniels

The brightness of a star as
seen from a standard
distance of 10 parsecs.

Used for comparing the
intrinsic brightness of
stars.
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Typical magnitudes:
Body
Distance
(pc)
Sun
-26.74 4.84 x 10-6
Full Moon
-12.74
Venus
-3.8 to -4.9
Mars
+1.6 to -3.0
Jupiter
Absolute
Magnitude
4.83
-1.6 to -2.94
Pluto
+16.3 to +13.65
Sirius
-1.44
2.64
1.45
Arcturus
-0.05
11.25
-0.31
Vega
0.03
7.76
0.58
Spica
0.98
80.39
-3.55
Barnard’s Star
9.54
1.82
13.24
11.01
1.29
15.45
Proxima Centauri
Dr Paul A Daniels
Apparent
Magnitude
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Next time:
• Colour
• Temperature
• Hertzsprung-Russell Diagram
Dr Paul A Daniels
Guildford Astronomical Society
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