Galaxies and the universe

Light is fast, but space is huge, so light still needs time to cross it. Light travels about 300,000 kilometers every second. A light year is the distance light covers in one whole year, about 9.5 trillion kilometers. This is why astronomers love the light year: it turns gigantic distances into numbers we can say out loud.

When you look at a star 100 light years away, the light hitting your eyes tonight left that star 100 years ago. You are seeing the star as it was a century ago, not as it is right now. The same idea works nearby too: sunlight takes about 8 minutes to reach Earth, so you always see the Sun as it was 8 minutes ago.

This also means there is a limit to how fast we can learn about anything in space. No message, picture, or signal can travel faster than light. Looking deeper into space always means looking further back in time.

Imagine the solar system is your house. Then our galaxy, the Milky Way, is like your whole town, filled with hundreds of billions of stars instead of houses. And the universe is like a planet covered with towns, because it contains enormous numbers of galaxies, stretching as far as our best telescopes can see.

The distances jump hugely at each step. Inside our solar system, we measure distances in millions or billions of kilometers. But the nearest stars are light years away, and other galaxies are millions of light years away. That is why astronomers switch units: kilometers for planets, light years for stars and galaxies.

The universe means everything that exists that we can observe. Every galaxy we can spot is part of it, and each one holds billions of stars of its own.

Our galaxy, the Milky Way, is shaped like a giant flat pinwheel. It has a round bulge of stars in the center, a flat disk around it, and long spiral arms winding through the disk like the arms of a pinwheel. The whole disk is about 100,000 light years across.

Our Sun is one ordinary star among hundreds of billions. It sits in the disk, partway out from the center, roughly 26,000 light years from the middle, inside one of the smaller spiral arms.

Mapping our own galaxy is tricky because we live inside it, like trying to draw a map of a forest while standing among the trees. Astronomers figured out the spiral shape by listening to radio signals from cool hydrogen gas, which can be detected even when dust blocks our view. Those signals revealed where the spiral arms lie.

Galaxies are not scattered through space like sprinkles on a cupcake. They gather together. The Milky Way belongs to a small gathering called the Local Group, which holds a few dozen galaxies, including the big Andromeda galaxy.

Bigger gatherings are called clusters, holding hundreds or thousands of galaxies, and clusters group into even larger superclusters. Between these gatherings lie huge empty regions called voids, where almost no galaxies are found.

On the very largest scales, the universe looks surprisingly even, like a sponge or a web: walls and strings of galaxies wrapped around great empty spaces. Astronomers call the idea that the universe looks roughly the same everywhere, when you zoom out far enough, the cosmological principle. The distances here are mind-bending: neighboring galaxy groups are separated by many millions of light years.

People dream about traveling to other stars to look for life. But the distances make this incredibly hard. The closest star system to us, Alpha Centauri, is about 4.37 light years away. Remember, one light year is the distance light travels in a full year.

Even a super-fast spaceship moving at 90 percent of the speed of light would need nearly 5 years to get there. The fastest spacecraft humans have actually built move far slower than that. At those real speeds, the same trip would take tens of thousands of years.

This is why exploring beyond our solar system takes enormous amounts of time. Space is so big that even light, the fastest thing there is, needs years to hop from one star to the next. Reaching another galaxy would take millions of years. When scientists plan missions, they have to think in decades, generations, or even longer.