Picture this. You’re camping with your family and it’s a clear night. As you look up into the night sky, it feels like there are a thousand stars, and they’re bright enough to touch. You feel the impact of how small you are in the grand scheme of things. And then your mind wanders as you try to wrap your head around how big the universe is. If this question has been keeping you up at night, we have the answer.

So, how big is the universe?

Photo of a cosmic phenomenon in the night sky
Credit: NASA/JPL-Caltech

There was a time when we couldn’t give you a hard figure. But as far back as 1920, astronomers have been sharing estimates on the size of the known universe. Before we dig into hard figures, best guesses, and even erroneous ones, we need to set some ground rules.

First, the universe is constantly expanding. Any measurements given today won’t be accurate in the future. Likewise, scientists and astronomers can give only measurements based on the observable or known universe. This references what can be seen through their telescope, whether on the ground or with a satellite. Much like the expanding universe factor, dimensions based on the observable universe can be limited.

In other words, based on current research, observations and mathematical equations, the experts can estimate the universe’s size within a fair degree of certainty. But the caveat will always apply that these figures are impacted by the universe’s growth rate and the limitation of the observable universe.

What’s the number?

Photo of a cosmic phenomenon in the night sky
Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA

Slow down there! Another important note is that we aren’t measuring in miles or kilometers like we would the distance between New York and London. Instead, we use light-years when we’re discussing the distance between two bodies in space. Standard forms of measurement would be too impractical because, in space, celestial bodies are very far apart.

Speaking literally, a light-year describes the distance a beam of light can travel in one year. To help you quantify that and realize why light years are better than traditional Earth-distance measurements, one light0year is the equivalent of 6 trillion miles. If you got dizzy just hearing that, now you know why astronomers prefer light-years over miles or kilometers.

Is there an estimate?

Photo of a cosmic phenomenon in the night sky
Credit: NASA/JPL-Caltech/STScI

Initial size estimates of our universe began with measuring our galaxy, the Milky Way. In 1920 the American astronomer Harlow Shapley was one of the first experts who attempted to measure the Milky Way and came up with a diameter of 300,000 light-years. It turns out he was very wrong, as today most astronomers believe our Milky Way is somewhere between 100,000 and 150,000 light years in diameter.

For perspective, we know that the Milky Way isn’t the only galaxy in our universe — nor is it the biggest. Current counts estimate that there are at least 100 billion galaxies in the known universe and the largest discovered galaxy to date is IC 1101 with a diameter of 6 million light-years (although this figure is contested). So if our little corner of the universe is 100,000 light-years wide, and the biggest galaxy is around 6 million light-years in diameter, that can give you a hint that the known universe is quite large.

Just say how big the universe is!

Photo of a cosmic phenomenon in the night sky
Credit: NASA/JPL-Caltech/STScI/CXC

Current measurements place the observable universe at roughly 93 billion light-years in diameter. There are a variety of methods used to reach this figure, but popular options include measuring radio wavelengths, parallax measurements, main sequence fitting, and cepheid variables. Radio wavelengths are a great option within our solar system because astronomers can measure the time it takes for a radio wave to bounce off the surface of a planet or asteroid and translate that into an actual light-year reading. But for celestial bodies farther out in the universe, it’s not practical.

Beyond our solar system, parallax measurement is preferred as it relies on comparing distances to an object based on measurements from multiple angles. This method relies on telescopes and satellites to compute various distance readings over time and scientists to extrapolate accurate positions from the data. But beyond 100 light-years, even parallax measurement is inefficient.

At great distances, main sequence fitting and cepheid variables are the preferred measurement tools. Main sequence fitting relies on a basic understanding of a star’s brightness and color compared to its age to determine distance. Cepheid variables focus on the actual “twinkle” or pulsating factor to determine age and position.

So what does this all mean?

Photo of a cosmic phenomenon in the night sky
Credit: NASA JPL-Caltech

If we haven’t given you a headache yet, it means that even though astronomers and experts have a great grasp on the general size of the universe, figures can change as our methods for analyzing data improve. And for the average Joe, just know that the universe is huge, and we’re in one little corner of it!