Most of us have heard of the Milky Way – the galaxy of stars in which our planet resides. But what exactly is a galaxy? And how did the Milky Way get its name?
A galaxy is a collection of stars, gas and dust that is bound together by gravity. None of this was known to our distant ancestors, of course; all they saw was a band of cloudy-looking stars stretching across the night sky. The Romans named our galaxy via lacteal (‘road of milk’) because it resembled a milky patch, while to the Ancient Greeks it was the galaxias kyklos (‘milky circle’).
The Ancient Greek philosopher Aristotle speculated that the Milky Way was created by interactions between the terrestrial and celestial spheres. About 1,800 years later, Galileo showed that it was a great group of stars, planets and other matter.
We now know that the Milky Way is vast, containing at least 100 billion stars. Imagine how many planets must be orbiting those alien suns! Light takes about 8 minutes to reach Planet Earth from the Sun, but it takes 100,000 years to cross from one end of the galaxy to the other.
The Milky Way is shaped like a whirlpool that rotates once every 200 million years. Its oldest stars are 13.4 billion years old – almost as old as the universe itself. Our own Sun is a mere 4.5 billion years old and lies about 27,000 light-years away from the Galactic Core, which contains a super-massive black hole called Sagittarius A*.
If you find the size of our galaxy overwhelming, you’ll be astounded to learn that it’s just one of many. The Milky Way is part of a cluster of 40 galaxies called the ‘Local Group’, which also includes Andromeda. This itself is part of a super-cluster called Laniakea (Hawaiian for ‘immeasurable heaven’), which is 520 million light years across and contains 100 million billion stars!
There is more to the universe than our ancestors could ever have conceived. As we gaze at the night sky, we could feel daunted by our own insignificance, or we could marvel that we are part of something far greater than we can imagine.