The Astronomical Unit (AU) is the distance from the Sun to the Earth; this is 93 million miles (150 million km)
The speed of light is 299,792,458 metres per second (approximately 300,000 km/sec or 186,000 miles/sec).
The light year is a unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.4607 × 1012 km (nearly 6 million million miles).
The parsec is a unit of length used to measure large distances to astronomical objects outside the Solar System. A parsec is defined as the distance at which one Astronomical Unit (AU) subtends an angle of one arcsecond, which corresponds to 648000/π AUs. One parsec is equal to about 3.26 light-years (30 trillion km or 19 trillion miles) in length.
The nearest star to the Sun, Proxima Centuri, is about 1.3 parsecs (4.2 light-years) distant. Most of the stars visible to the naked eye in the night sky are within 500 parsecs of the Sun.
The kiloparsec (kpc, one thousand parsecs) is also used as a measure of distance. Similarly, a distance of one million parsecs is commonly denoted by the megaparsec (Mpc). Astronomers typically express the distances between neighbouring galaxies and galaxy clusters in megaparsecs.
One gigaparsec (Gpc) is one billion parsecs — one of the largest units of distance commonly used. One gigaparsec is about 3.26 billion light-years, or roughly 1/14of the distance to the horizon of the observable universe (dictated by the cosmic background radiation).
The Milky Way is about 1,000,000,000,000,000,000 km (about 100,000 light years or about 30 kpc) across. The Sun does not lie near the centre of our Galaxy. It lies about 8 kpc from the centre on what is known as the Orion Arm of the Milky Way.
Other distance examples:
There are about 10 billion galaxies in the observable universe. The number of stars in a galaxy varies, but assuming an average of100 billion stars per galaxy means that there are about 1,000,000,000,000,000,000,000 (that's 1 billion trillion) stars in the observable universe