card
083

Universe

OLogy Series
place
card
083

Universe

OLogy Series
place

You have to be a really big thinker to imagine the size of the Universe. It's so big that light from the most distant galaxies takes over 10 billion years just to reach us on Earth! That's twice as long as Earth's been around. Everything we've ever observed in space is part of our Universe. We don't know what's beyond it or if there are other Universes out there.

The Center of the Universe
The first person to come up with a scientific theory about the size and shape of the Universe was a Greek astronomer named Ptolemy, who lived in the 2nd century. Ptolemy's Universe placed Earth at the center, while the other planets, the Sun, and the stars revolved around it. Most people accepted this idea, but in the 16th century, Copernicus (right), a Polish scientist, said that Earth was not in the center of the Universe. Based on his observations, he said that the Earth and the other planets revolved around the Sun. Early in the 20th century, astronomers realized that the Sun wasn't the center of the Universe. They discovered that the Sun is just one of billions of stars in the Milky Way, nowhere near the center of the galaxy. There are about 100 billion galaxies that make up the Universe, each one containing billions of stars.

Distances in the Universe are so vast that they're measured in light-years. In one year, light travels:

almost 600 miles

almost 600,000 miles

almost 6,000,000,000 miles

Are you right?

Correct!

Galaxies are so large that it takes light thousands of years to cross them.

Charles Liu, astrophysicist

The basic forces . . . that govern our lives on Earth and that govern the workings of the Universe are one and the same . . . We live in one Universe.

In the early years, everything in the Universe was just gas.

Fact
or
Fiction
?

Fact

As the Universe expanded and cooled, gravity formed stars and galaxies out of this gas. Gravity continues to form stars in galaxies today.

Size: the observable Universe is over 10,000 megaparsecs across
Age: about 13 billion years
Significance: everything that we know of is within the Universe (along with lots of other stuff we don't know anything about)
Origin: Big Bang
Composition: around 100 billion galaxies in groups and clusters

Image credits: The image of the universe © American Museum of Natural History and the National Center for Supercomputing Applications, 1999; Charles Liu: courtesy of AMNH.