Luminosity (A.K.A.: Brightness)

Luminosity vs Brightness These two terms are related, but are NOT the same. Luminosity is the total light energy output per second of a luminous object (produces its own light) Luminosity is a function of a star’s… Temperature: the hotter the star, the more light energy it produces Radius: the larger the star, the more surface area there is to emit its light Brightness is related to how much light actually reaches a point per second As you go further from a light source, the brightness decreases as the square of the distance Eg. If the distance doubles, the brightness is ¼ (÷22)

Measuring Brightness The magnitude system was invented by a Greek astronomer around 150 B.C.E. The brightest stars were classified as ‘1’s, the next brightest as ‘2’s… the faintest visible stars were ‘6’s We have kept the same idea today, even though we now know of several brighter objects than magnitude 1, and countless dimmer objects than magnitude 6 Notice that this is an inverse scale: as brightness goes up, magnitude goes down (even less than 0)

Measuring Brightness This scale is also a logarithmic scale, not a linear scale. That is, a magnitude 1 star is not twice as magnitude 2! As it turns out, for every 1 the magnitude is increased, the brightness goes down by a factor of about 2.512 This affect is cumulative: the difference in brightness between a magnitude 2 and a magnitude 6 is: 2.512 x 2.512 x 2.512 x 2.512, or 2.5124 = 39.8 (Mag. 2 is almost 40 times brighter than mag. 6.) (By definition, a difference of 5 in magnitude, is a 100x difference in brightness.)

Some magnitudes… Sun -26.74 100 W Lightbulb at 30 m -13.5 Full moon -12.7 Venus (at its brightest) -4.22 Jupiter (at its brightest) -2.6 Mars (at its brightest) -2.02 Sirius (brightest star) -1.45 Alpha Centauri (closest star) -0.1 Andromeda galaxy +3.5 Faintest object visible to humans +6.0 Neptune (at its brightest) +7.9 Brightest quasar +12.8

Absolute vs Apparent Brightness We know that a luminous object will appear dimmer when it is further away So how do we take into account how far away the object is? Answer: We make a distinction between apparent magnitude (how bright it looks) and absolute magnitude Absolute magnitude is defined as how bright the object would be if it was 32.6156 light-years away (10 parsecs). Examples: -The sun has an apparent magnitude of -26.74. If it we were 10 parsecs away, it would have a magnitude of 4.8 (a fairly dim star) -The Andromeda galaxy has an apparent magnitude of 3.5, but is 2.5 million light-years away! If it were only 32.6 light-years away, it would have a brightness of -21.1!

Magnitude Revisited… Absolute Magnitudes Object Apparent Absolute Sun -26.74 +4.83 100 W Light bulb at 30 m -13.5 +66.3 Full moon -12.7 +31.8 Venus (at its brightest) -4.22 +28.2 Jupiter (at its brightest) -2.6 Mars (at its brightest) -2.02 Sirius (brightest star) -1.45 +1.41 Alpha Centauri (closest star) -0.1 +4.35 Andromeda galaxy +3.5 -21.1 Faintest object visible to humans +6.0 N/A Neptune (at its brightest) +7.9 Brightest quasar +12.8 less than -28