Every star that you see in the sky is part of the same enormous galaxy. Our solar system resides in a galaxy called the Milky Way, stuffed with between 100 billion and 400 billion other stars, many of them with planets of their own.
The Milky Way got its name from the way it looks from the ground: like a streak of spilt milk across the sky. That hazy white band is made up of stars, dust and gas. It looks like a flat stripe because we are viewing it from within its disk; if we were able to get above the Milky Way, it would look like an enormous spiral about 100,000 light years across.
The spiral is made up of four huge arms of stars connected by a straight bar at the galaxy’s centre. The arm that’s home to Earth is a smaller one called the Orion arm, where our solar system resides about 26,000 light years from galactic centre. The galaxy is rotating, and it takes us about 240 million years to complete one circle around its middle.
The centre of the galaxy is a dense and chaotic place, with stars and gas hurtling around the Milky Way’s supermassive black hole, Sagittarius A*. It has a mass more than 4 million times the mass of the sun crammed into a diameter just about 30 times the sun’s width.
Advertisement
The Milky Way is just one of more than 100 billion galaxies in the universe – but it’s the one that we call home.
