How Vast Is the Observable Universe?


By Robert Hazen, Ph.D.George Mason University

The past hundred years of astronomy have been characterized by an ever-expanding view of the observable universe. We see many different kinds of stars; billions of stars, from brown dwarfs to super-giants. We also see exploding stars and supernovas, neutron stars, and black holes. Some stars in our own Milky Way galaxy are up to 100,000 light-years away.

Image of the Milky Way Galaxy with stars.
Celestial objects in the Milky Way are light-years away from us. (Image: sripfoto/Shutterstock)

From a Grapefruit to the Sun

There are galaxies, clusters of galaxies, some of them billions of light-years away. However, it’s crucial when we throw around terms like galaxies, and clusters of galaxies, and galactic voids, to get at least some sense of how vast these objects really are.

Let’s start with a simple box about a tenth of a meter on each side. A box of this sort could hold, for example, a grapefruit. That’s about four inches on a side. To truly grasp the scale of the universe, take this box and increase each edge length by a factor of 100. What you end up with is a box that has a million times the volume because it’s 100 times 100 times 100. 

You start with a four-inch-edge cube; you end up with one that has 400 inches on each edge. The larger box, then, is ten meters on a side, and this would fit nicely around a small two-story building. It’s effortless to imagine that first increment. The second increment of a factor of 100 takes us up to a cubic kilometer, and a cubic kilometer would hold a small town and a volume of air above it or soil below it. 

The third increment would be a cube 100 kilometers on each side. Next, the fourth increment, a 10,000-kilometer cube, would not quite hold the entire planet Earth. The Earth is about 13,000 kilometers in diameter so you would be cutting off tiny portions of the Earth in that box.

But the fifth increment, that’s a box a million kilometers on a side, would hold the Earth and the entire orbit of the Moon. Alternatively, this box would fit the Sun quite nicely.

Here we’ve come five increments; five increments of a factor of 100 on edge. But we still have a very, very long way to go in our exploration of the universe. 

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Light Distance

We’re now beginning to approach the epic distances where it’s much easier to speak of light distances.

Light travels 300,000 kilometers a second, so 300,000 kilometers is a light-second. Then 60 times 300,000 is about 18 million, so 18 million kilometers is a light-minute. Then 60 times 18 million is more than a billion kilometers in a light-hour; a light-day is about 1.1 billion kilometers, and a light-year is about 10 trillion kilometers.

From the Sun to the Solar System

Recall that that fifth increment took us to a box that held the Earth and the Moon. The next factor of 100 is a box that’s 100 million kilometers on each side. That’s about six light seconds on a side, and that box holds the Earth and either Mars when it’s at its closest approach or Venus when it’s at its closest approach. Alternatively, that box could hold the Sun and almost encompass the entire orbit of the planet Mercury, which averages about 600,000 kilometers from the Sun. 

A 3D illustration of the solar system and planets
A box of 38.5 light-days on a side would contain the solar system and all the main objects gravitationally bound to the Sun. (Image: Vadim Sadovski/Shutterstock)

The seventh increment would be ten billion kilometers on edge or nine light-hours on each edge. This vast volume, if centered on the Sun, would encompass almost all the planets. Then we have a box of 38.5 light-days on a side; this would contain the solar system and all the main objects gravitationally bound to the Sun, and virtually nothing else.

Learn more about the nebular hypothesis.

The Size of the Milky Way

The next box, the ninth box, 10.5 light-years on a side, could contain both the solar system and the brightest neighboring stars. We’d also, in that box, have about 100 other stars. Then we get to the tenth box, 1,000 light-years on each side. 

This box only encompasses a tiny portion of the Milky Way galaxy; the Milky Way galaxy is 100,000 light-years across. Here we have a box of 1,000 light-years on each side. It would include countless thousands of stars, but only a tiny fraction of our Milky Way. 

Here we’ve reached another increment of five—that is, another factor of ten billion in distance from that original scale. We still have a long way to go because we’ve only talked about one percent of the size of the Milky Way. We’re at the scale of the galaxies now. The next increment takes us to 100,000 light-years on a side, almost holding all the stars in the Milky Way.

Learn more about how information is collected from electromagnetic radiation.

Unimaginable Vastness of the Observable Universe

From the twelfth box, we begin to see the epic scales of the universe. The twelfth box is ten million light-years on a side; it holds the Local Group of galaxies and dozens of more galaxies. Then a billion light-years on each side, in the thirteenth box which extends well beyond the Local Supercluster, it holds hundreds of millions of galaxies, grouped in clusters, with voids, and vast regions with few galaxies. 

Finally, the entire universe is only about a factor of ten larger than that. That is, the known universe is roughly 20 or 25 billion light-years across. Such a volume is unimaginable, but hopefully, this has given you a sense of the scale. We’ve gone from a grapefruit-size object through 14.5 increments—that’s 29 orders of magnitude difference in size.

Common Questions about the Vastness of the Observable Universe

Q: How many light-seconds is 300,000 kilometers?

Light travels at a speed of 300,000 kilometers per second. So, 300,000 kilometers is equal to one light-second.

Q: What celestial objects could be contained in a box that’s a million kilometers on each side?

A box that is a million kilometers on a side would hold the Earth and the entire orbit of the Moon. Alternatively, this box would fit the Sun.

Q: What is the approximate size of the known universe?

The known universe is roughly 20 or 25 billion light-years across.

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