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 significantly - 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 2 nd century B.C. from a Greek Scholar named Hipparchus - 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 Brightness- Amount of light energy striking a surface. (Observer’s eye or telescope). The brightness is a factor of two things: 1. Luminosity of Star- The amount of Light Energy being given off by a star. IT DOES NOT DEPEND ON THE LOCATION OF THE OBSERVER 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 1 st magnitude star appears 100 times brighter than a 6 th 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 Question: 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 factor) - Basically takes all the stars and place them at the same distance from Earth. - Stars closer than 10 parsecs will have a much more positive absolute magnitude (dimmer) than apparent magnitude. Ex. Sun’s Apparent Magnitude Sun’s Absolute Magnitude (barely visible)

Star B is much more luminous than Star A. Both stars have same Apparent magnitude but NOT the same Absolute magnitude