Measuring 3 Main Properties of Stars
● Luminosity-Amount of power that a star radiates (energy per second = watts)
● The apparent brightness of a star depends on both distance and luminosity
● Apparent Brightness-Amount of starlight that reaches Earth (energy per second per
● The relationship between apparent brightness and luminosity depends on distance
● Brightness = luminosity/4pi(distance^2)
● We can determine a star’s luminosity if we can measure its distance and apparent
● Luminosity = 4pi (distance)^2 x brightness
● The amount of luminosity passing through each sphere is the same
● Surface area of sphere= 4 pi (radius)^2
● Divide luminosity by area to get brightness
Distance of Stars
● Parallax us the apparent shift in position of a nearby object against a background of more
● Parallax angle depends on distance.
● Apparent positions of nearby stars by about an arcsecond as Earth orbits the Sun.
● Parallax is measured by comparing snapshots taken at different times and measuring the
shift in angle to star.
d (in parsecs) = 1 / p (in arcseconds)
d (in light years) = 3.26 x 1 / p (in arcseconds)
● Most luminous stars : 10 ^6 Lsun
● Least luminous stars: 10 ^-4 Lsun
● Lsun is luminosity of the Sun
The Magnitude Scale
● m = apparent magnitude , M = absolute magnitude
● apparent brightness of star 1 / apparent brightness of star 2 = (100^⅕)^m1-m2
● luminosity of star 1 / luminosity of star 2 = (100^⅕)^m1-m2
● Every object emits thermal radiation with a spectrum that depends on its temperature.
● An object of fixed size grows more luminous as its temperature rises.
Properties of Thermal Radiation
1. hotter objects emit more light per unit area at all frequencies
2. hotter objects emit photons with a higher average energy
● Hottest Stars: 50000 K
● Coolest star: 3000 K ● Suns surface: 5800 K
● Level of ionization also reveals a star’s temperature