SCALE OF THE UNIVERSE

The Lightyear and Astronomical Unit

A lightyear is the distance light travels in one year. The speed of light is so fast that it can zip around the Earth 7 times on 1 second.

The distance from the Sun to the Earth is called an Astronomical Unit (A.U.). This distance is about 93 million miles or 152 million kilometers. It also equals 8.3 light minutes (the distance light travels in 8 min 20 seconds). That means we see the sun as it was 8.3 minutes ago.

The Solar system is about 22 light hours across.

We Live in a Galaxy

The Sun and our solar system is in a galaxy, a system of about 400 billion stars. The stars orbit center of mass of the galaxy. The time of the sun's orbit is 250 million years. The sun has orbited the Milky Way about 20 times since it was created. The galaxy is about 100,000 lightyears across.

Galaxies and Where They Live

Galaxies come in groups and clusters, and these clusters form a cosmic-web of galaxies. All in all, there are about 100 billion galaxies in the universe.

The Milky Way belongs to a small group of about 20 galaxies called The Local Group. The biggest neighbor is the Andromeda Galaxy. It is about 2.2 million lightyears from the Milky Way.

Large clusters of galaxies are about 1 million lightyears across in size.

Lookback Time

When we look at very distant objects, we see the light from them that was emitted long ago. If a galaxy is 1 billion lightyears away, then what we see "now" is what the galaxy emitted 1 billion years ago- we see it as it was 1 billion years ago. This is called look back time.