news sciences: How far is a light-year?

Schematic portraying a light-year: A large yellow shell outside smaller shell with arrows point from center to each. — The large yellow shell depicts a light-year; the smaller yellow shell depicts a light-month. Read more about this image at Wikimedia Commons.

Objects in our universe are extremely far away. They’re so far away that kilometers or miles aren’t a useful measure of their distance. So we speak of space objects in terms of light-years, the distance light travels in a year. Light is the fastest-moving stuff in our universe. It travels at 186,000 miles per second (300,000 km/sec). And thus a light-year is 5.88 trillion miles (9.46 trillion kilometers).

But stars and nebulae – not to mention distant galaxies – are vastly farther than one light-year away. And, if we try to express a star’s distance in miles or kilometers, we soon end up with impossibly huge numbers. Yet miles and kilometers are what most of us use to comprehend the distance from one place on Earth to another. In the late 20th century astronomer Robert Burnham, Jr. – author of Burnham’s Celestial Handbook – devised an ingenious way to portray the distance of light-years in terms of miles and kilometers.

Keep reading, for a way to comprehend the vastness of the universe, using units of distance we know and use every day.

Burnham started by relating the light-year to the astronomical unit – the Earth-sun distance.

One astronomical unit, or AU, equals about 93 million miles (150 million km).

Another way of looking at it: the astronomical unit is a bit more than 8 light-minutes in distance.

Diagram of sun, Earth, moon, labeled, with line between labeled 150,000,000 kilometers. — A light beam takes 8 minutes to travel the 93 million miles (150 million km) from the sun to the Earth. Image via Brews Ohare/ Wikimedia Commons.

A light-year, pictured as a mile

Robert Burnham noticed that, quite by coincidence, the number of astronomical units in one light-year and the number of inches in one mile are virtually the same.

For general reference, there are 63,000 astronomical units in one light-year, and 63,000 inches (160,000 cm) in one mile (1.6 km).

This wonderful coincidence enables us to bring the light-year down to Earth. If we scale the astronomical unit – the Earth-sun distance – at one inch, then the light-year on this scale represents one mile (1.6 km).

The closest star to Earth, other than the sun, is Alpha Centauri at some 4.4 light-years away. Scaling the Earth-sun distance at one inch places this star at 4.4 miles (7 km) distant.

See?

Star field with a red star at center. — The red star in the center of this picture is Proxima Centauri, our sun’s nearest neighbor among the stars. A beam of light from this star takes about 4 years to travel to Earth. Image via Max Planck Institute for Extraterrestrial Physics.

Familiar space objects, conceptualized

Scaling the astronomical unit at one inch (2.5 cm), here are distances to various bright stars, star clusters and galaxies:

Alpha Centauri: 4.4 miles (6.4 km)

Sirius: 8.6 miles (14 km)

Vega: 25 miles (40 km)

Pleiades open star cluster: 444 miles (715 km)

Antares: 555 miles (893 km)

Hercules globular star cluster (aka M13): 22,200 miles (35,700 km)

Center of our Milky Way galaxy: 26,100 miles (42,000 km)

Great Andromeda galaxy (M31): 2,540,000 miles (4,100,000 km)

Sombrero galaxy (M104): 28,000,000 miles (45,000,000 km)

Whirlpool galaxy (M51): 31,000,000 miles (50,000,000 km)

And so on, back to approximately 13 billion+ light-years to the farthest galaxies: 13,000,000,000 miles (21,000,000,000 km)

Okay, the numbers are still pretty big! But hopefully they can help you see that our universe is very vast.

Schematic of the stellar neighborhood of the sun, with sun in center and labeled stars, scale and arrow towards galactic center. — There are 33 stars within 12.5 light years of our sun. Click into Atlas of the Universe to find an interactive page that’ll let you keep zooming farther and farther out. Image via Atlasoftheuniverse.com.

The fastest-moving stuff in the universe

As mentioned above, light travels at an incredible 186,000 miles per second (300,000 km/sec). That’s very fast. If you could travel at the speed of light, you would be able to circle the Earth’s equator about 7.5 times in just one second!

In other words, a light-second is the distance light travels in one second, or 7.5 times the distance around Earth’s equator. A light-year is the distance light travels in one year.

How far is that? Multiply the number of seconds in one year by the number of miles or kilometers that light travels in one second, and there you have it: one light-year. It’s about 5.9 trillion miles (9.5 trillion km).

Diagram of earth with arrows to moon, sun, star, cluster and galaxy, depicting distances. — This scale starts close to home but takes us all the way out to the Andromeda Galaxy, the most distant object most people can see with the unaided eye. Image via Bob King/ Skyandtelescope.com.

Bottom line: Here’s a way to understand the scale of light-years in miles and kilometers.

Read another article answering the same question: What is a light-year?

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