**Distance Modulus**

For this purpose astronomers use the Distance Modulus which is derived from the inverse square law and the definition of the magnitude scale.

This is described in more detail, together with numerical examples in our EBook **Magnitude & Distance**.

The expression that relates distance * D* with apparent magnitude (

*) and absolute magnitude (*

**m***) is a logarithmic one:*

**M**This formula gives the distance to the celestial object in pc (parsec).

Because absolute magnitude is defined as the magnitude at a distance of 10 pc, the difference (m - M) indicates how far a celestial object is inside or outside a circle with radius of 10 pc.

For a calculated example see our EBook **Magnitude & Distance**.

The reason that we discussed luminosity, flux, the inverse square law, magnitude and the distance modulus in this chapter is that

**IF**we have a way to know the luminosity or absolute magnitude of a star,**AND**we measure its apparent magnitude,

we have found a way to calculate the distance.

Celestial objects of which we (think we) know the luminosity or absolute magnitude are termed **Standard Candles**.