Because distances in deep space are so vast, astronomers have adopted a number of stellar distance units to make them more manageable. Three of the most important of these are the astronomical unit (AU), the parsec, and the light-year.
Without such units, it would quickly become very difficult to talk or write about stellar distances. Beginning from Earth, Mars is a relatively manageable 50 million miles away. However, the closest star (apart from the Sun) is Alpha Centauri, which is 26 trillion miles away. The Milky Way Galaxy is hundreds of quadrillions of miles in diameter. It is virtually impossible to talk about distances to other galaxies in terms of small units like miles or kilometres. For this reason, astronomers have developed several shorthand units for referring in general terms to the distances between celestial objects.
The smallest of these is the Astronomical Unit (AU). The AU is equal to the average distance between the Earth and the Sun, or about 93 million miles (150 million kilometres). Astronomers often refer to other distances within the solar system in terms of AU. For instance, Jupiter is about five AU away from the Sun, Saturn orbits at a distance of 9.5 AU, and Neptune at 30 AU. One of the most distant known dwarf planets, Eris, reaches a maximum of almost 100 AU from the Sun.
Beyond the solar system, even the AU begins to get awkward, however. For instance, Alpha Centauri (the closest star to the Sun) is about 276,000 AU from us. The nearest galaxy to the Milky Way, the Andromeda Galaxy, is 159 billion AU away. These numbers are somewhat more manageable than miles, but still get awkward quite quickly.
Instead, astronomers use two additional units: the parsec and the light-year. The parsec was used first and is an angular measurement based on trigonometry. One parsec is equivalent to 19 trillion miles.
Today, astronomers usually speak in terms of light-years rather than parsecs. The light-year is equivalent to the distance that a beam of light travels through deep space in one Earth year. This is equivalent to about 5.88 trillion miles or 63,240 AU. One parsec is equivalent to about 3.26 light-years.
Light-years are so large that they are really only useful for discussing interstellar distances, although at much shorter distances it is also possible to speak in terms of light-minutes (the distance light travels in a minute) or even light-seconds. Light-years make interstellar and intergalactic distances easier to discuss, but still no less breathtaking. Alpha Centauri lies about 4.3 light-years from the Earth, while the Andromeda Galaxy is about 2.5 million light years away. However, the observable universe stretches about 13.7 billion light-years in every direction from the Earth. This means that light reaching telescopes from the most distant objects in the night sky began its journey 13.7 billion years ago – long before Earth itself even existed. Finding the edge of the observable universe in light-years is one means of estimating the age of the universe.