**Luminosity**

A star is very nearly a blackbody, therefore its brightness is directly related to the total amount of energy radiated from its surface per unit time.

In astronomy this is called **Luminosity** which is the amount of energy emitted per second.

This is exactly the same as **Power** which is energy per unit time (Joule per second).

We all know this when we buy a light bulb that has a certain number of Watt.

The unit for power is Watt (W) or Joule (energy) per second or W = J s^{-1}.

In astronomy luminosity is not expressed in Watt but in multiples of the Sun’s power (i.e. the Sun’s luminosity),

denoted** L _{Sun}**.

**L _{Sun}** = 3.83x10

^{26}W.

We can measure the power received on Earth on every square metre. This is called the Solar Constant expressed in W/m^{2}.

Multiply this with the surface area of a sphere of radius 1 AU (distance Sun - Earth) and you get **L _{Sun}. **