What Are Light Years, And How Do They Let Us Look Into The Past?

An image showing the different units of measure for space.

When going over the concept at first, light years can be confusing, but they're just measuring distance in space. A light year is the distance light has traveled in one year. Light moves fast, at 300,000 kilometers per second in space. That's 186,000 miles per second if miles are your preference. In a year, which is 31,536,000 seconds, light moves approximately 9.46 trillion kilometers. Scientists call this distance one light year. It's a gigantic figure because space is huge and light travels fast, and kilometers or miles are not sufficient when it comes to measuring the distance between stars and galaxies.

The term “light year” was derived from the need for a convenient unit for astronomy. Space, as mentioned before, is so big that regular measures become irrelevant. Consider the closest star system to our planet, Alpha Centauri, at 41 trillion kilometers away. To spell it out every time is exhausting, so instead, astronomers say it’s 4.37 light years away. It's simpler to comprehend, and it is directly related to the nature of light, which is relevant to this field. The speed of light is constant when moving in a vacuum, so it's a convenient standard.

And light doesn't just help us figure out distance because it also helps us see the past. When you look at the sun, you're not seeing it right now. The sun is 150 million kilometers away, and it takes light about 8 minutes and 20 seconds to travel that distance. So, you're seeing the sun as it was over 8 minutes ago! If the sun blinked out of being right now, you would never even know until 8 minutes from now.

This time lag gets larger as things get farther away. Consider Alpha Centauri, for instance. The light travels 4.37 years to get to us, so what you are seeing there in the evening sky is 4.37 years old. If something happened to Alpha Centauri last week, we’d still see it shining for over four years before the change hit our view.

The same is true for galaxies. The Andromeda Galaxy, our closest large neighbor, is 2.5 million light years away. In other words, the light that's hitting telescopes left Andromeda 2.5 million years ago. Earth didn't yet have humans then. Dinosaurs had been wiped out, and mammals were only just taking over. When astronomers look at Andromeda, they're looking at a snapshot taken of the galaxy all that time ago.

Light years turn telescopes into time machines, literally. The farther away an object is, the further back the light we're capturing from it. Scientists have found galaxies that are over 13 billion light years away. That's almost as old as the universe, which is estimated to be 13.8 billion years old. When they look at those distant galaxies, they're looking at light that's been traveling since the universe was still young. It's not just distance they're measuring—it's time too. Those galaxies might be extinct by now, or altered beyond recognition. However, we wouldn't know until newer light gets here, billions of years later.

How does it work in the real world? Light leaves a star or galaxy and travels in straight lines through space. It keeps going unless something, like a planet or a black hole, catches it or bends it. Space isn't totally empty, but it’s empty enough that light will often go through unscathed for billions or millions of years. When it finally reaches a telescope, however, that light carries information with it. It tells us about how the object looked like when the light started out. The color and brightness of the light also relays information about their temperature, size, and composition.

Astronomers break light down into its elements using instruments like spectroscopes. A spectroscope disperses light into a spectrum, just like a prism disperses sunlight. Each element (hydrogen, helium, etc.) in a star absorbs light at specific wavelengths. Those missing ones show up as dark lines in the spectrum. By comparing those lines to known elements, scientists figure out what the star or galaxy is made of. The light from 13 billion years ago reveals a universe with more hydrogen and fewer heavy elements than now.

This time-travel “magic” isn't flawless. Space dust or gas occasionally dulls or scatters light, obscuring dim objects. Telescopes also have limitations because they can only detect so much light. The James Webb Space Telescope, launched a few years back, pushes those limits even more. It records infrared light, which can travel more easily through dust and detect the redshifted glow of very ancient galaxies. With it, astronomers see further, and therefore farther back, than ever. They've already spotted galaxies when the universe was less than 300 million years old.

This alignment sets up how we view space. Every nighttime sky is a mix of multiple times. Stars that twinkle nearby show light from a few years ago, while it’s light from centuries or millennia ago for those further away. Light years don’t just measure space—they measure history. Every kilometer light travels is a fraction of a second ticked off the clock. Stack enough kilometers, and you’re counting years, centuries, or eons. That’s why astronomers chase distant objects. They’re usually not after what’s out there today. Rather, they’re digging into what used to be there. The deeper they look, the closer they get to the universe’s start.