All our lives we have grown up with the familiar units of distance used on Earth, namely centimetres, metres, and kilometres. We tend to think perhaps of a centimetre as a small unit and a kilometre as a long way.
We find, however, that when we venture to think of the distances in space that earthly units are completely inadequate to express most distances. To use the same units as employed on Earth we would have to apply very large numbers which would be like expressing the distance between Adelaide and St Petersburg in centimetres (2,168,225,200) instead of the more usual units of kilometres (21,682.)
Astronomers therefore use quite different measurement units. The best known is the light year, which is the distance light travels in one year. Light travels at a finite speed of 299,792,500 metres per second which is about 9,460,000,000,000 kilometres per year.
This means that light travels the distance from the Moon to the Earth, 384,400km in 1.28 seconds, and it travels from the Sun to the Earth, 149,597,870km in 8.3 minutes. So instead of saying that the sun is nearly 150 million km away we say that the Sun is 8.3 minutes light time away. Now when we come to the nearest star, Alpha Centauri (Rigil Kentaurus), we say it takes light 4 years 4 months to reach us. So Alpha Centauri is 4.3 light years away from Earth. In kilometres this is the unwieldy number of about 40,681,156,000,000 km. The brightest star in the night sky is Sirius which is twice as far away at 8.7 light years.
If we were able to fly a commercial airplane (at a constant speed of 800km/h) to Alpha Centauri (4.3 light years distant) it would take about 5.8 million years to reach Alpha Centauri from Earth. Consider that our galaxy, the Milky Way is about 100,000 light years in diameter then this gives us some idea of the vastness of space.