Pocket Daring Book for Girls: Wisdom & Wonder

Chapter One

Galaxies

Galaxies are enormous, organized systems of stars, star clusters, dust, and gas. As vast as it seems to us here on Earth, our galaxy, the Milky Way, is just one of billions of galaxies in the universe. Exactly how many galaxies exist isn't known—there may be as many as 100 billion galaxies in just the part of the universe scientists can actually observe—and scientists also aren't sure about exactly where galaxies came from and how they were formed. What we do know is that galaxies can contain anywhere from several million to several trillion stars (our own galaxy, the Milky Way, contains somewhere in the neighborhood of 100 billion stars, including the sun); that they can be separated by as little as few thousands of light years to millions of light years in distance; and that they come in three basic shapes: spiral, elliptical, and irregular.

Our galaxy's nickname, the Milky Way, comes from the Greek kyklos galaktikos, or "milky circle," which likens its appearance in the distant sky to glistening drops of spilled milk. But why milk? Greek mythology tells the interesting tale. In one of the many colorful stories the Greeks used to explain the natural world, Zeus (the king of the gods and the god of thunder) tricked Hera (goddess of women and marriage) into breastfeeding his mortal son Heracles by placing the baby on her breast while she was sleeping. His plan was to have the baby drink Hera's milk and thus become a god like him. But Hera awoke and pushed the baby away, causing the milk to spray across the night sky.

Spiral

Our Milky Way galaxy is a spiral galaxy, its twisting, whorling shape resembling water circling around a drain, a hurricane as seen from a satellite, or a child's pinwheel blowing in the breeze. Spiral galaxies usually have an "eye" at the center (a disk with a bulging center made up of stars, planets, dust, and gas) and spiraling arms extending outward in a spinning motion. Everything rotates around the galactic center at speeds of hundreds of kilometers per second. A faster or slower rotation can affect a galaxy's shape—such as a kind of spiral galaxy called a "sombrero galaxy," due to its flattened, spread-out appearance. The bulge at the center of the galactic disk is where older stars usually reside, while newer stars often form in the galaxy's arms. The newer stars are often quite large, and very bright, but they don't last very long: their sheer size causes them to burn out quickly. Smaller stars that aren't quite as luminous last longer.

Elliptical

Elliptical galaxies often have an elongated, football-like shape. Unlike spiral galaxies they do not have a disk at their center. Elliptical galaxies are also usually smaller than spiral galaxies, and may contain anywhere from a few thousand stars to billions of stars. Most of the stars in an elliptical galaxies are very old and often clustered together, which makes the center appear as though it is one giant star. It is very rare that new stars form in these types of galaxies. Very large elliptical galaxies, called Giant elliptical galaxies, are the largest galaxies in the universe that we know of, and can be as much as two million light years in length.

Irregular

Irregular galaxies are just what they sound like: irregularly shaped galaxies that are neither spiral nor elliptical. They can appear misshapen or formless. This may be due to repeated collisions with other galaxies, or it may be that they have always been shaped that way.

The Milky Way around the world

In the Baltic languages, the Milky Way is called the "Bird's Path." In ancient China it was called "Heavenly River of Han," and in contemporary China and other parts of Asia it is called "Silver River." In Japan, the Milky Way is called the "Silver River System" or the "River of Heaven." In Sweden, the Milky Way is called Vintergatan, or "Winter Street."

The term Milky Way first appeared in the English language in 1380 in a poem by Geoffrey Chaucer titled "The House of Fame." (The poem is written in Middle English, which, as you can see in the spelling below, differs from the modern English we use today.)

"See yonder, lo, the Galaxy
Which men clepeth the Milky Wey,
For hit is whyt."