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Magnitude
Absolute vs. Apparent
• 2
The analogy
• Imagine a birthday candle
and a spotlight are both 50
feet in front of you
– Which would be brighter?
• Now move the candle right
in front of your face.
– Which would be brighter?
• Put the candle back and
move the spotlight 1000
feet away.
– Which would be brighter?
Magnitude = Brightness
• Brightness is measured in two ways:
• Absolute Visual Magnitude (M) - the
actual luminosity of the star based
on energy output.
– Candle and flashlight at same distance
• Apparent Visual Magnitude (m) –
how bright the star appears
(depends on distance)
– Candle/flashlight at different distances
• abs
Apparent Magnitude
• cas
• Aparrent
Using Abs. and App. Magnitudes
• Absolute mag. (M) = apparent mag (m) at a
distance of 10 parsecs (Baseline Distance)
• Star is closer than 10 parsecs then Apparent is
brighter
– (If m is higher than M, star is closer than 10 parsecs)
• Star is farther than 10 parsecs, Absolute is
brighter
– (If m is lower than M, star is further than 10 parsecs
Distance Modulus
• A value that helps us get an estimate of
distance.
• DM = M-m
– (Distance Modulus = Absolute Mag – apparent mag)
– If positive, apparent is smaller (brighter), star is
closer than 10 parsecs away
• More positive means more closer
– (way more positive means way more closer)
– If negative, apparent is larger (dimmer), and the
star is farther than 10 parsecs away
• More negative means more fartherer
History of Magnitudes
• Hipparchus (maybe Ptolemy?) – 2000 BCE
– A ranking system
– Brightest stars = 1st class, then 2nd, 3rd…
– 6th magnitude are faintest stars seen at
night
– Result: lower number = brighter
“There is no other rule for classing the stars but the
estimation of the observer; and hence it is that
some astronomers reckon those stars of the first
magnitude which others esteem to be of the
second.” (Ewing 1812)
Why Negative?
• 1856 proposal to standardize
• standard ratio of 2.512 between magnitudes.
• 5 magnitude steps correspond to 100x factor
brightness change
• The Star Vega selected as baseline (magnitude 0.0)
– 5th brightest star, 2nd in North hemisphere
– Is now magnitude 0.03, by redefined baseline scale
• Made it possible to go brighter than first class
• 0th class? -1st class? And so on.
Essential skills
• Know the difference between Absolute and
apparent magnitudes
• Know how to compare Absolute and Apparent
magnitudes to estimate stellar distances (the
“distance modulus” or M-m)
• Know when Abs. and App. are the same
– (what distance for M=m)
Animation
• Hypothesize.