The Scale of the Universe

The Universe is extremely large. To make sense of it, astronomers use special distance units and organise space from small, nearby objects to vast, faraway structures.

Measuring distance: the light-year

A light-year (ly) is the distance light travels in one year. Light moves very fast, at about $c = 3.00 \times 10^8\,\text{m s}^{-1}$. There are about $3.16 \times 10^7$ seconds in a year.

$$1\ \text{light-year} = c \times 1\ \text{year} \approx 9.46 \times 10^{15}\ \text{m}$$

Key idea: a light-year is a distance, not a time.

Our cosmic address (from small to huge)

• Earth–Moon: about 384 400 km.
• Earth–Sun (1 AU): about 150 million km, which is about 8 light-minutes.
• Nearest star (Proxima Centauri): about 4.24 light-years.
• The Milky Way (our galaxy): a disc about 100 000 light-years across, containing many billions of stars. The Sun is one of these stars.
• Local Group: our Milky Way plus nearby galaxies, including Andromeda, about 2.5 million light-years away.
• Clusters and superclusters: groups of many galaxies bound by gravity.
• Observable Universe: contains many billions of galaxies and stretches for tens of billions of light-years across.

Looking far means looking back in time

Light takes time to travel. Seeing a galaxy 2.5 million light-years away means seeing it as it was 2.5 million years ago. Space is like a history book: the farther we look, the older the light.

Expansion and redshift (link to scale)

Most distant galaxies are moving away from us, so their light is stretched to longer (redder) wavelengths. This redshift shows the Universe is expanding. The speed $v$ a galaxy recedes increases with distance $d$ (Hubble’s law): $v = H_0 d$. This relationship helps estimate huge distances and the overall scale of the Universe.