As astronomers have discovered more and more galaxies since the 1920s, they have acquired one fundamental piece of knowledge: The universe is really big! Lets imagine that you could climb into a spaceship and travel out into the universe, seeing more and more distant things as you went on. Lets also imagine that the spaceship could travel at the speed of light the fastest speed we know of in the cosmos. Thats about 186,000 miles per second, the speed at which photons particles of light are striking your eyes, enabling you to read this article. (Photons can travel that fast because they have no mass; spaceships have mass, so we know that spaceships couldnt move that fast. But, for the sake of understanding the size of the universe, lets pretend that our spaceship could.)

In our spaceship, lets set out from the Milky Way Galaxy, our home. The closest galaxy we can encounter is the Sagittarius Dwarf Spheroidal Galaxy, a tiny galaxy that orbits ours. If we move at the speed of light, it would take us 70,000 years to reach this galaxy. Another way of thinking about these enormous distances is to understand how long the light that we now see from other galaxies has been traveling through space to reach us. The light from the Sagittarius Dwarf Spheroidal Galaxy has traveled since humans made their earliest bits of art inside caves in South Africa. If we traveled for 163,000 years in our spaceship, we could arrive at the Large Magellanic Cloud, our galaxys largest satellite. Traveling for 200,000 years would carry us to the Small Magellanic Cloud, another satellite of our Milky Way. The light you see from this galaxy tonight has traveled through space since our earliest human ancestors closely linked to our species walked the African plains.

But those are dwarf galaxies that are very close to us. The largest nearby galaxy is the Andromeda Galaxy, which would take us 2.5 million years to reach in our spaceship. The light you see from this galaxy tonight has traveled through space since some of our earliest human ancestors were here on Earth.

And these are just some of the galaxies closest to us. Traveling outward, you would find countless examples of strange and beautiful galaxies at all manner of distances. These would include spirals like IC 239, M100, M106, NGC 210, NGC 2683, NGC 2841, NGC 3310, NGC 3338, NGC 4565, and NGC 6946. You would encounter fields of multiple galaxies like those in the Leo Trio (M65, M66, and NGC 3628), M81 and M82, and the galaxy group Hickson 31. Some galaxies that seem to be connected, like NGC 3314, would grow away from each other as you approached and their visual alignment disappeared. You would encounter numerous weird, distorted galaxies the result of interactions or disruptions by black holes like Arp 188, ESO 243-49, NGC 474, NGC 660, NGC 2685, NGC 4622, NGC 5291, NGC 7714, and UGC 697.

You can see how enormous the cosmos is and understand that, fundamentally, it is filled with galaxies. The Virgo Cluster galaxies would take 50 million years to reach in our light-speed spaceship. More distant galaxies are arranged in clusters and superclusters that we can see from Earth, and some lie hundreds of millions or billions of light-years away. Reaching the most distant galaxies that we can see would take us more than 13 billion years, traveling at the speed of light.

Living our lives on this third planet from the Sun in our solar system, its easy to ignore how unbelievably immense the universe is. But moving farther and farther out into the universe to explore galaxies allows us to understand how the universe came to be, and where its going.

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Explore the world of galaxies - Astronomy Magazine