If you’re on a dark countryside hill some night, look up at the sky. Arcing overhead, a faint band of light may appear that looks like milk spilled across the sky. The ancient Romans called the band via lacteal, which means “milky road” or “milky way.”

The band of light that you see isn’t actually milk, of course—it’s a galaxy. A galaxy is a huge bunch of stars clustered together in space. Our solar system—which includes the sun, Earth, and seven other planets—is part of this galaxy, called … you guessed it … the Milky Way.

Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity. Nearly all large galaxies are thought to also contain supermassive black holes at their centers. In our own galaxy, the Milky Way, the sun is just one of about 100 to 400 billion stars that spin around Sagittarius A*, a supermassive black hole that contains as much mass as four million suns.

The Universe is all of space including everything that exists. Our planet, all other planets in existence, stars, and galaxies are all part of the universe. It was born more than thirteen billion years ago in the Big Bang. Scientists say that it is at least ten billion light-years in diameter. In this sense, the Universe means the Cosmos. The Universe is all time and space, i.e., spacetime. It includes all the galaxies, stars, planets, moons, and all other forms of energy and matter.

Science is the intellectual and practical activity encompassing the systematic study of the structure and behaviour of the physical and natural world through observation and experiment.