A light-year is a concept frequently used in astronomy. Indeed, I’ve used it dozens, if not hundreds of times on Love the Night Sky and the Virtual Astronomy Club to describe the distances to objects like star clusters and galaxies.
Yet many people don’t fully grasp its significance.
For instance, the Andromeda Galaxy is 2.5 million light-years away. We intuitively know that this is a staggeringly large distance, don’t we?
Stars within our own galaxy, however, are actually quite close. Aren’t they?
Well, yes…
And no!
The closest star to Earth, after our sun, is Proxima Proxima, which is about four light-years away, but even that ‘short’ distance is not as cozy as it sounds!
What Exactly is a Light-Year?
Despite its name, a light-year is a measure of distance, not time.
It defines the distance that light travels in one year.
Light moves at an astonishing speed of nearly 300,000 kilometers per second (or about 186,000 miles per second). Given that there are over 31.5 million seconds in a year, light travels an incredible 5.88 trillion miles or 9.46 trillion kilometers in a year.
A trillion is a difficult number for us mere humans to fathom, so let’s try and put it into perspective on a scale we can grasp.
Traveling a Light Year in an Airplane
Think about the last time you flew on a commercial jet. We use them to cover big distances in short times, often flying at 600 miles per hour or more. To get from where I live in the UK to the USA takes just a few short hours.
But this is where we get our first proof that our intuition about light years is wrong because to travel one in that same airplane would take us around 1.1 million years!
How Big is a Light Year in a Shrunken Universe?
Alternatively, instead of thinking about human-scale travel speeds, we could shrink the universe down to a scale we’re familiar with.
Imagine if the Sun were the size of a golf ball, with Earth, a tiny grain of sand, just 0.4 mm across and about 15 feet away.
At this scale, a light-year would be equivalent to 180 miles, and the next nearest star, Alpha Proxima, would be over 750 miles away.
This helps illustrate just how vast the empty space between stars is. Our planet is a spec of sand, the sun is 15 feet (five meters) away, and the next star after that needs a journey of over 750 miles.
How many stars are within 100 light-years of Earth?
Man-Made Objects and the Vastness of Space
We humans dream about traveling to other stars and exoplanets. We’re a smart bunch and we’ve created some super fast space vehicles, so how would it be to do that?
Voyager 1: Our Furthest-Traveled Man-Made Object
Launched in September 1977, Voyager 1 has been traveling through space for nearly 47 years at the time of writing. It holds the record for the furthest-traveled man-made object, sitting just over 15.2 billion miles away.
We saw earlier that a light year is the same as 5.88 trillion miles. What’s hard for us to appreciate is the difference in scale between billions and trillions.
Even at the almost inconceivable distance Voyager has journeyed away from Earth, this is equivalent to only 22.5 light hours. Despite its lengthy, half-century journey, Voyager 1 hasn’t even reached a single light day away from Earth.
And this probe is not moving slowly. Voyager 1 travels about 900,000 extra miles every day at a speed of 38,000 mph. Yet, it’s still roughly 1,000 days (just under three years) away from crossing the light-day boundary.
Use this website to track how far away it is right now – look at the “One-Way Light Time” row to see how many light hours or light days away it it.
It’s at this point we can grasp why journeys to other stars are not yet within our reach.
If Voyager 1 were heading towards Proxima Centauri, the nearest star after the Sun, it would take approximately 75,000 years to cover the distance. Learn more about how we measure the distance to stars (opens in a new tab).
The Fastest Man-Made Object: Parker Solar Probe
While Voyager 1’s speed is impressive, it isn’t the fastest object humans have created.
That title belongs to the Parker Solar Probe, which studies the Sun and can reach speeds over 430,000 mph at its closest approach to our star. It sounds fast, but this is a distance light can travel in 2.4 seconds, or 1,500 times faster than Parker.
Even though Parker’s speed equates to an otherworldly 119 miles per second, it would still take over 1,500 years for it to travel a single light-year.
My YouTube Light Year YouTube Video
If you prefer watching over reading, then check out my YouTube video covering all you need to know about the vast distance of a light year.
The Immense Scale of the Universe
All this mind-blowing information underscores the immense scale of the universe, even at the local level of nearby stars. Next time you hear that the nearest star is only four light-years away, remember that this means it’s at least 1,500 years of travel time away from us!
That means when astronomers talk about Andromeda Galaxy being two million light-years away, they’re really saying we have no hope of ever getting there without learning to travel at speeds significantly closer to that of light.
Understanding the concept of a light-year helps us appreciate the vastness of our universe and the incredible distances that separate us from even our closest stellar neighbors.
