Star cluster NGC6496
The star cluster NGC6496
Credit:ESA/Hubble & NASA

Light is the main signal we receive from space. Almost everything we know about space has been found by measuring light. One of the main ways to measure light is to look at the brightness. Measuring the brightness of something in space isn't as easy as you'd think though. It can be done in a few different ways.

Often magnitudes are used to describe the brightness of an object. They come in 2 forms: absolute and apparent. This is because how bright an object in space looks depends on how far away from Earth it is. 

The intrinsic brightness of an object is how bright is really is. This is how much light the object is actually giving off. We use the absolute magnitude to estimate this brightness. The absolute magnitude is how bright the object would be if it were placed at a set distance from the Earth. This lets us compare the brightness of different objects, like stars. A distance of 10 parsecs is used. A parsec is a distance unit used in astronomy. It is the same as 3.26 light years, or 31,000 billion kilometres (19,000 billion miles)! 

The apparent magnitude is how bright we actually see the object from Earth. When objects get further away from Earth they appear fainter. A star more than 10 parsecs away from Earth will look fainter than its absolute magnitude. A star which is closer will be brighter than its absolute magnitude.

Magnitudes use an unusual scale. They come from Ancient Greece when they were invented to compare the brightness of stars. This means that sometimes magnitudes can have negative values. The lower (or more negative) the value the brighter the object is. 

Brightness in astronomy can also be measured using luminosity. This is a more physical measurement. It measures the energy output of the object. This is done by looking at how much light is given out by the object over time. The units of luminosity are more standard, given in Watts - much like a light bulb! A typical LED light bulb in a house might be 20 Watts - the Sun is 20 million billion billion (2 x 1025) times brighter!