National Geographic
Menu

A Brief History of Light-Years (Or Why Astronomy Is a Form of Time Travel)

Recently astronomers announced the discovery of the youngest black hole yet found, which we see as an object that’s roughly 30 years old. But the news created a bit of a stir, because the black hole lies in a galaxy that’s about 50 million light-years away.

Understanding the controversy means knowing the definition of a light-year. From Merriam-Webster:

Main Entry: light-year

Function: noun

Etymology: light

: a unit of length in interstellar astronomy equal to the distance that light travels in one year in a vacuum, or 5,878,000,000,000 miles

In other words, a light-year isn’t a unit of time, but a unit of distance that’s based on light having a speed limit.

hubble-depp-field-light.jpg

Seeing across the light-years: The Hubble Ultra Deep Field

—Image courtesy NASA, ESA, and R. Thompson (Univ. Arizona)

Scientists had been debating whether light has a finite speed since the time of the ancient Greeks. But it wasn’t until 1676 that Danish physicist Ole Christensen Rømer was able to prove it.

Rømer figured it out by watching Jupiter and measuring how long it took for the moon Io to reappear after its orbit took it behind the planet.

Because of the two planets’ relative distances from the sun, Earth and Jupiter orbit at different speeds, which is part of the reason the distance between the two planets varies over time.

Several years of data showed Rømer that Io reappeared faster when Earth was closer to Jupiter and took longer to reappear when Earth was farther away.

solar-system-model-jupiter-earth.jpg

—Image courtesy NASA/JPL

Jupiter’s size wasn’t changing, though, so why would this be?

French astronomer Jean-Dominique Cassini, to whom Rømer was an assistant, suggested the effect might be due to the time it takes light to travel from the Jovian system to Earth.

Running with that hypothesis, Rømer used his data to calculate that light takes 22 minutes to travel a distance equal to the width of Earth’s orbit.

In truth Rømer was slightly off, because Earth’s orbital width wasn’t accurately known at the time. But if you combine modern data for Earth’s orbit with Rømer’s Jupiter data, you get pretty close to the currently accepted value for the speed of light.

Fast forward 162 years to the work of German astronomer Friedrich Wilhelm Bessel.

He was the first person to calculate the distance between Earth and a star other than the sun, 61 Cygni, using parallax, or the apparent change in position of an object when it’s seen from two different points along a baseline.

That’s a wonky way of putting it, I know, so try this: Sit at a desk and place a small cup about arm’s length in front of you. While sitting in place, focus on the cup and close one eye, then the other.

It’s subtle, but you should notice that the cup seems to jump from one position to another on the table—even though nothing is actually moving.

If you could calculate how much the cup “shifts” and combine that with the distance between your eyes, you’d be able to figure out the distance between you and the cup.

For objects as far away as stars, astronomers use Earth’s position along its orbit in place of two eyes, with the width of the orbit as a baseline.

Take a star’s position on dates separated by half a year, and you get the widest possible baseline. Even then, stars appear to shift by miniscule amounts, so this is a tricky calculation.

But Bessel managed it in 1838, calculating the distance to 61 Cygni as 10.3 light-years. By some accounts, he was also the first to use the term “light-year” as a unit of measurement in astronomy.

Today, no matter how we calculate distance in astronomy, the figures are often expressed in light-years, because it’s a great way to think about things on such huge scales.

Confusion sets in when we start talking about age.

(Related: “Einstein’s Relativity Affects Aging on Earth [Slightly].”)

The speed of light is really fast, so on Earth-scale distances, you can be pretty sure that when you meet a 30-year-old person, he or she appeared on this planet 30 years ago.

That 30-year-old black hole, meanwhile, is 50 million light-years away.

That means the light from the black hole (technically, the light from superheated material falling into the black hole) left its point of origin about 50 million years ago.

What we see from Earth is the way the black hole looked about 30 years after it formed, which is great for scientists who want to study how young black holes grow and evolve.

But if we could instantly transport to the black hole, the object we’d see would be about 50,000,030 years old.

baby-black-hole.jpg

Baby or senior citizen?

—Image courtesy NASA/CXC/SAO/D.Patnaude et al; ESO/VLT; NASA/JPL/Caltech

This makes things tricky when talking about objects from the early universe. These objects may be 13 billion years old, give or take, but we’re seeing them as they existed shortly after the big bang.

(Related: “Most Distant Object Found; Light Pierced ‘Dark Age’ Fog.”)

Depending on how you want to think about it, galaxies in pictures from Hubble’s deep observations, for example, are either among the youngest or the oldest objects yet seen.

Another interesting effect of this light-year business is that some of the structures we see today may, in fact, no longer exist—we just haven’t seen their destruction yet.

Add all this to the fact that the universe is expanding, and that expansion is accelerating. This means there may be some galaxies that started out so far away from Earth their light hasn’t reached us yet.

Since we’re moving away from these galaxies, we may be going fast enough that their light will never reach us, effectively meaning there are galaxies that we’ll never be able to see.