Astrophysics 06 - Brightness of Stars

Define luminosity, giving its unit.

The power output of a star in Watts.

Define light intensity.

The effective brightness of an object. The power output per unit area at the point of observation.

Define apparent magnitude.

The apparent brightness of stars when observed from Earth.

Dependent on the power output of a star and its distance from Earth.

Is magnitude linear or logarithmic?

Logarithmic

Give the definition for magnitude.

A magnitude 1 star has an intensity 100x greater than a magnutide 6 star.

Give the equation relating the intensities of two stars to their apparent magnitudes.

(Intensity of star 1 / intensity of star 2) ≈ 2.51^{(apparent magnitude of star 1 - apparent magnitude of star 2)}

I₂ / I₁ ≈ 2.51^{m₂ - m₁}

Define Absolute Magnitude.

The apparent magnitude of a star if it were 10 parsecs away.

Dependent only on power output.

Give the equation relating absolute and apparent magnitude to the distance of the star away from earth.

apparent magnitude - absolute magnitude = 5 x log (distance from earth in pc / 10)

m - M = 5 x log(d / 10)