How far? How big ? How many ?

Light - our measuring stick in space

How far does light travel in one second ? 300 000 kilometers, which equals seven times around the Earth.

Let's call this distance one light second. We can use this as a ruler to measure distances in space. Beyond the Sun and its planets we need a longer ruler, which is the distance light travels in one year. This is called a light year - it is 32 million times longer than a light second. Multiplying a light second by the number of seconds in a year, we can answer the question:
How far is a light year? 10 trillion kilometers (or more exactly, 9.5 x 1012 km).
The Earth, which is about 12000 kilometers in diameter (NASA).

The Sun and its planets

The Stars

Our Galaxy, the Milky Way

milky way
Stars and dust clouds towards the center of the Milky Way. Stars form half the detectable mass of the Milky Way. The other half is in the form of gas and dust.

The Universe

distant galaxies
Distant galaxies seen by the Hubble Space Telescope (STSCI/NASA).

More about Space and Time, Stars and Planets