The next time you’re outside at night with a clear sky, look up. What do you see?
If you live in or near a city, the sky you see at night is different from the night sky that almost everyone who has ever lived saw. George Washington, Queen Elizabeth I, Shakespeare, the Pharaohs: everyone who lived before about 1920 saw a different sky than today’s city-dwellers do. When they looked up at night, they saw thousands of stars. If George Washington had decided, one clear, moonless night, to stay up all night and count all the stars in the sky, he could’ve counted 4 to 5 thousand stars.
How many stars can you see at night? If you live in a big city, like Chicago or Boston, you might only be able to see about 35 stars on the clearest, darkest night! Today, cities put a lot of light into the sky. Even when it’s not cloudy, that city-light bounces off tiny particles of water in the air to make the sky glow faintly. That’s called “skyglow,” and it drowns out the light of all but the brightest stars.
Below, two views of the same stars: one from the countryside, and one from the city.
The Milky Way
Kids in cities a hundred years ago, like kids in the countryside today, could see something else that’s invisible from the city: a pale band of light called the Milky Way. The picture at the top of the page is a horizon-to-horizon shot of the Milky Way. The Milky Way is so hard to see from the city that in 1994, when a power outage turned out all the lights in Los Angeles, dozens of people called 911 to ask about the strange thing in the sky!
Of course, the Milky Way doesn’t have anything to do with milk. It’s our galaxy. A galaxy is a lot of stars all clustered together in space, and the Sun is one of the stars in the Milky Way galaxy. The 4,000 stars that George Washington could’ve seen are only a tiny fraction of the stars in the Milky Way. Even on the darkest night, with the strongest telescope, we can’t see all of them. Astronomers think that our galaxy has hundreds of billions of stars. The light of all those stars is what gives the Milky Way its glow.
The Milky Way may have a hundred billion stars, but it’s only one galaxy. The Universe contains billions more galaxies, each with its own billions of stars. Just how many stars might be out there in the whole universe?
Astronomers think that just the part of the universe that we can see has more than a hundred billion galaxies. A hundred billion times a hundred billion is 10 000 000 000 000 000 000 000 stars. That’s a 1 with 22 zeros behind it. There are at least that many stars in the universe–and probably more. But most of them are too far away to be seen, even with the strongest telescope on the darkest night.