Milky Way

We've found that there are more stars in the Universe than there are grains of sand on all of the beaches on planet Earth. It's worth pausing to imagine being on one of those beaches with its soft golden sand sweeping off into the distance. Bright sunny day, of course. In fact, let's make it the Caribbean. No expense spared. You reach down, hands cupped, and gather up two handfuls of that golden sand. Then you let the sand fall through your fingers, the grains glistening as they catch the sunlight. Each grain is a star. And each star is a sun, like our own local star, the Sun. You saunter on a few more steps, and again you gather up the sand and let it fall. And so on, over all the beaches on Earth. So much sand, so many stars.

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On a large scale, swarms of such stars dwell in galaxies, effortlessly wheeling their way through the vastness of deep space. Each galaxy contains millions, if not billions, of stars. Under cover of the night sky, some galaxies can be seen with the naked eye. It's worth remembering that the very word ‘galaxy’ derives from the Greek term galaxias, or ‘milky circle’, for its appearance to the eye.

Imagine yourself again, on our same trip, under the starry reach of the Caribbean sky. You look up, the star field is dazzling. But here and there is the odd, nebulous smudge of a galaxy. The great galaxy in the constellation Andromeda, for instance: one of our near neighbours, and yet even this nearby citadel of stars is resolvable only by telescope.

So that's how galaxies look to the naked eye: like tiny, pale thumbprints in the sky. And yet the Milky Way Galaxy alone holds between 200 and 400 billion stars, and has special significance since it is the home galaxy of planet Earth. The capitalised word ‘Galaxy’ refers particularly to our own Milky Way, to distinguish it from the hundred-billion or so other galaxies in the observable Universe.

Galaxies such as our Milky Way are known as spiral galaxies because they have a core at the centre, surrounded by a disk of gas, dust and stars forming spiral arms. Many astronomers believe it is in spiral arms that star formation continues today. And where there are new stars, they may be new planets, and new life…