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Brightness + Magnitude
of Stars
- There are approximately 5000 stars viewable with the unaided
eye from the Earth’s surface
- At any one position you could view approximately 300 stars
- HOWEVER – Light and air pollution reduces this number
- A system of stellar labeling was developed by assigning
numbers based on the relative brightness of stars =>
Magnitude system
1. Magnitude System
- original idea came during 2nd century B.C. from a Greek Scholar named
- He organized all the visible stars according to their apparent brightness. The
brightest stars were given a magnitude of 1. The Next brightest a magnitude of 2
and so on till the faintest stars were given a magnitude of 6.
A. Apparent or Relative BrightnessAmount of light energy striking a surface.
(Observer’s eye or telescope). The brightness is a
factor of two things:
1. Luminosity of StarThe amount of Light Energy being given off by
2. Distance to Star
A. Apparent or Relative Brightness-(cont.)
*** As distance to Star Decreases
brightness Increases (Inverse Relationship)
*** As Luminosity of Star increases
brightness Increases (Direct Relationship)
B. Apparent Magnitude
A number assigned to a celestial object that is a
measure of its relative brightness. (Based on
Distance and Luminosity)
1. The more positive the number the dimmer
the star
2. The more negative the number the
brighter the star
B. Apparent Magnitude (cont.)
It turns out that a an average 1st magnitude star appears 100 times brighter
than a 6th magnitude star so therefore a change in magnitude of 5
corresponds exactly to a factor of 100 in brightness.
*** One change in magnitude corresponds to a fifth
root of 100 or 2.5 times in brightness
Ex. Magnitude 3 star is 2.5 times brighter than a
magnitude 4 star
Magnitude 4 star is 2.5 times brighter than a
magnitude 5 star
1. How much brighter is a magnitude 2 star than a 4 star?
2. How much dimmer is a magnitude 2 star than a –1 star?
C. Absolute Magnitude- A star’s magnitude when viewed from a fixed point of
10 parsecs Away (32.6 light years from earth)
- A true measure of a star’s luminosity (Since distance is not a
- Basically takes all the stars and place them at the same distance from
- Stars closer than 10 parsecs will have a much more
positive absolute magnitude (dimmer) than apparent
Ex. Sun’s Apparent Magnitude -26.8
Sun’s Absolute Magnitude +4.58 (barely visible)
Star B is
much more
than Star A.
Both stars
have same
but NOT the