Space is big. That’s why we call it space.
But how big is “big”?
That’s relative. When an astronomer says something is nearby, they might mean a few million kilometers (if they’re talking asteroids) or a few tens of trillions (for stars) or a few tens of quintillions (for galaxies).
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No matter the destination, it’s a long walk. We make it easier on ourselves by using huge units to measure distance, such as a light-year, the distance traveled in a year by light—the fastest thing in the universe. A light-year is about 10 trillion kilometers. But that’s still pretty abstract to the casual person reading about “nearby” exoplanets or “distant” galaxies.
One way to help you grasp this scale is to take it step by step.
The moon is the closest astronomical object to us in the entire universe. On average it’s about 380,000 km from Earth. That’s already a pretty long way; nearly 30 Earths could fit side-by-side over that distance! Or think of it this way: the Apollo astronauts, traveling faster than any human before them, still took three days to reach the moon’s vicinity.
And that’s the closest heavenly body.
The sun is about 400 times farther away from us than the moon is: 150 million km. How far is that? If you could pave a road between Earth and the sun, at highway speeds, it would take you about 170 years to drive there. Better pack a lunch. A commercial jet would be better—it would take a mere 17 years.
When working with objects inside the solar system, it’s convenient to use the Earth-sun distance as a sort of cosmic meter stick. We call it the astronomical unit, or AU, and it’s defined by the International Astronomical Union (the keepers of all astronomical numbers, names and other agreed-upon conventions) to be exactly 149,597,870.7 km. In these terms, Mercury is about 0.4 AU from the sun and Venus about 0.7. Their distances from Earth depend on where all the planets are in their orbit—and increase when respective planets are on opposite sides of the sun—so Venus actually ranges from about 0.3 to 1.7 AU from Earth.
Neptune, the farthest major planet from the sun, is 4.5 billion km out, or 30 AU. Pluto’s at roughly the same distance, and it’s a long way from us. The New Horizons spacecraft took more than nine years to get there despite moving at speeds of more 50,000 km per hour.
These numbers are still difficult to grasp. When I used to travel to schools to give demonstrations to kids about astronomy, one of my favorite props was the solar system rope: a hefty 50-foot cord that represented the average sun-Pluto distance. The students were given printouts of planet photographs, and we’d place them at the proper scaled distance from the sun. The inner four planets were so close together that the kids were practically on top of one another, but the outer planets were spread out a long way; we had to either find a long hallway or go outside for the demo.
That demo proved so popular that I created a spreadsheet allowing anyone to calculate the solar system to scale. It’s based on the size of the sun, so you can change it from the default of one meter to, say, the size of a grape and find out how big and how far-off the planets become. (You have to download it to change the values; the link goes to a read-only version.) It’s fun—and eye-opening.
But it’s useful, too, to consider the separation between objects in terms of their size. For example, the sun is 1.4 million km wide. The nearest star system to the sun is Alpha Centauri, which is 41 trillion km away. If we divide the two numbers, Alpha Centauri is about 30 million “suns” away. Stars are very small compared to the distance between them, and that is one reason why you really don’t need to worry about one ever colliding with our sun!
That’s also why we use light-years to measure these distances; it’s a more palatable unit when dealing with interstellar journeys. Alpha Centauri is 4.3 light-years away. The Orion Nebula is about 1,250 light-years from the sun. The center of the Milky Way is 26,000 light-years away, and the galaxy itself is a flattish disk some 120,000 light-years across.
The nearest big galaxy to the Milky Way is Andromeda, which is 2.5 million light years from us. That’s an interesting number because it’s “only” 20 times the size of the Milky Way itself. On the scale of galaxy sizes, galaxies are actually pretty close together.
Inside galaxies, collisions between stars are extremely rare because they’re so far apart relative to their size. But galaxies themselves are more crowded, so it’s not too big a surprise that galaxy collisions are not only common, they’re ubiquitous. The Milky Way grew to its huge size by colliding and merging with other galaxies, and in fact every big galaxy has undergone multiple collisions.
The Milky Way and Andromeda are the two biggest galaxies in a clutch of about 100 galaxies that we call the Local Group. It’s about 10 million light-years across. Bigger and more populous groups exist called galaxy clusters. The nearest big one is the Virgo Cluster, with well more than 1,000 galaxies in it, located about 50 million light-years from us, though smaller groups exist that are closer to us.
Galaxy clusters are held together by the gravity of their members and can be tens of millions of light-years wide. But we’re not done! Clusters can clump up in the cosmos to form clusters of clusters called superclusters. The Virgo Cluster and the Local Group are part of one called the Laniakea supercluster, which may have more than 100,000 galaxies in it and stretches for 500 million light-years.
The universe is 13.8 billion years old, so you might think the most distant objects we can see are roughly that distance away in light-years. But the cosmos is expanding, and in the time it’s taken for the light from distant objects to reach us, that expansion has swept them farther from us. Because of this, the observable universe is estimated to be more like 90 or so billion light-years across!
After all that, I’ll let you in on a secret: even astronomers can’t truly grasp these scales. We work with them and we can do the math and physics with them, but our ape brain still struggles to comprehend even the distance to the moon—and the universe is 2 million trillion times bigger than that.
So yeah—space is big. And it’s true that we are very, very small. These scales can seem crushing. But I’ll leave you with this: while the cosmos is immense beyond what we can grasp, using math and physics and our brain, we can actually understand it.
And that makes us pretty big, too.