What’s a galaxy?

Every now and again, in casual conversation, someone asks something which catches me off guard briefly. Earlier today over a coffee, I was asked just such a question. After years of doing what I do and hearing all sorts of people speak at great length about them, hearing someone, in jest or otherwise, ask what exactly a galaxy is was… refreshing. And interesting. I mean, the word is batted about by pop culture no end. From chocolate bars to mobile phones to Super Mario, the idea of galaxies (if nothing else) seems to be everywhere. So let’s go back to basics here. What exactly is a galaxy anyway?

A galaxy, simply speaking, is an empire of stars. Billions and billions of stars like the Sun – some identical, many small, some hundreds of times bigger – living out their entire billion year lifespans in a huge swirling cloud. A miscellany of gasses, punctuated by more points of light than you can see in the darkest night sky. In galaxies, gasses condense and form into stars. Stars are born and burn for billions of years. Then those stars die and return, as mere gas and dust, back to the clouds that spawned them. All of these stars revolve constantly around a bright core. A core containing a supermassive black hole, unfathomably massive, yet curiously small. As the Sun’s gravity holds Earth it its orbit and Earth’s gravity holds us to the ground, the gravity of a supermassive black hole is strong enough to hold trillions upon trillions of tons worth of stars in orbit around it.

And we are in one. Our Sun is part of the Milky Way galaxy, and we can no more capture an image of our own galaxy from within, than we can photograph our own planet from the ground. The image here is one of our nearest neighbouring empires, the Andromeda Galaxy. Andromeda is slightly less massive than the Milky Way, but a fair bit larger. The diffuse light making up the Andromeda galaxy in this photograph is the light of over one trillion stars. A million million stars, surrounding the supermassive black hole which lurks somewhere within the centre of the image (the “galactic bulge”, seen as a bright white patch at the galaxy’s centre).

Yes, astronomers, and everyone else for that matter, may use the word “galaxy” lightly. But we should never forget precisely what that word refers to, and how incredible it really is!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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