cascas

Magnitude

Absolute vs. Apparent

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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.