In the study of cosmolgy, one encounters distances and masses which are extremely large compared with what we can easily comrehend.
It would be fair to say we can judge numbers that are a few hundred times a billion. We hear of companies worth 100 billion pounds etc. and appear to be comfortable with such statements, but a million billion will be difficult to understand.
Average distance of the Sun from the Earth is 150 million km and the diameter of the Milky Way galaxy is about a million million million km.
Mass of the Sun is 2000 billion billion billion kg and the numbers get bigger as we study the galaxies.
Therefore in astronomy we talk about a different set of unit.
Unit of mass is one solar mass = 2000 billion billion billion kg
Rest of the heavenly bodies are weighed relative to the Sun.
The situation about length is not so simple. There are three different units used depending on the context. These are:
1. The astronomical unit or AU:
1 AU ~ 150 million km = 149,597,871 km
An Astronomical Unit is approximately the mean distance between the Earth & the Sun.
AU is used when discussing distances of planets and other objects in the Solar System.
2. The light Year or Ly:
A Ly = about 10 million million km
A light year is the distance traveled by light in vacuum in one year.
It takes light 8.32 minutes to travel from the Sun to the Earth.
Ly is a big unit, it is about 63,000 astronomical units.
Diameter of the Milky Way galaxy is about 100,000 Ly
and the size of the observable Universe is measured in billions of Lys!
Light Year is the most commonly used unit when one is talking about galaxies and the Universe. It is also starightforward to understand.
3. Parsec or pc:
One parsec = 31 million million km or 210260 AU or 3.26 Ly
Parsec is based on the change of angle in a star's position when viewed from the Earth as it revolves round the Sun. We shall not use this unit in our discussions.