Lecture 12
March 13, 2008
Luminosity and Distances to Stars

Key Concepts:

Reminder: extracredit report 1 & 2 due on April 1.

luminosity = total amount of power radiated into space per unit time
inverse square law = a relationship between luminosity, distance, and apparent brightness
"a light appears brighter as you approach the light"

the same number of photons leave the star
a spherical shell surrounding the star increases its area as distance squared
apparent brightness (number of photons per unit area) decreases as distance squared

seeing how bright the light is, you can figure out how far the house is
if you know L, you can measure B and determine distance, d
"standard candle" = any object of known luminosity that can be used to determine distances (e.g. stars of known mass and luminosity)

a logarithmic system of describing stellar brightness
1 magnitude = (100)^1/5 = a factor of 2.512
the brightest stars are 1st magnitude, the next brightest stars are 2nd magnitude, and so on. A larger magnitude means fainter -- see an example below of Ursa Major
apparent magnitude: how bright different stars appear in the sky
absolute magnitude: a measure of stellar luminosity (a star's apparent magnitude if it were at a distance of 10 parsecs away)

a method of triangulation for measuring distance to nearby stars
arcsec = 1/60 of arcmin, which is 1/60 of a degree of arc
if A = 1 AU, B (distance) is inversely proportional to angle p if the angle in radian is much smaller than 1.

parsec (pc) = distance corresponding to a parallax of 1 arcsec. 1 pc is about 3.3 light year or 3 x 10¹³ km.
Quiz 12C
Measuring the Distance to the Orion Molecular Cloud using parallax