36 Galaxies Far, Far Away-Low

GALAXIES FAR, FAR AWAY FOCUS Question What can galaxies tell us about the universe?

What Is a Galaxy? If you look at a dark sky on a clear night, you will see thousands of stars. These stars make up a very small part of the billions of stars in our home galaxy, the Milky Way. Galaxies are enormous groups of stars, planets, dust and gas held together by gravity. Astronomers estimate that there are at least 100 billion galaxies in the universe. Some "tiny" galaxies contain only a few hundred or a few thousand stars. Giant galaxies have trillions of stars. Some galaxies are forming right now, but others are more than 13 billion years old. The light that reaches us from the most distant galaxies has traveled through space for billions of years.

Exploring Galaxies In 1923, astronomer Edwin Hubble observed the Andromeda Nebula using the Hooker Telescope in California. He discovered that the Andromeda Nebula was not a nebula, but rather a galaxy made up of billions of stars. Before this discovery, our own Milky Way was the only known galaxy. Today, astronomers use even more powerful telescopes on land and in space to study galaxies.

Looking Back in Time Astronomers describe distances in space in terms of light-years. One light-year is the distance light travels in one year. For example, the Andromeda galaxy is about 2.5 million light-years from Earth. It takes the light from Andromeda about 2.5 million years to travel to Earth! Light travels 300,000 kilometers every second, so a light- year is a very long distance. This means that when we look far into space, we are also looking back in time. When we look at Andromeda, we see what it looked like 2.5 million years ago. We won't know what it looks like today for another 2.5 million years! Think About It The Perseus galaxy cluster is 300 million light-years from Earth. Imagine that a creature living in the Perseus cluster is looking at Earth with a telescope. How might Earth look different compared to how it looks now?

Astronomers are not just interested in what galaxies look like now. They also want to learn how galaxies form and how they evolve, or change over time. One tool that looks out into space—and back in time—is the Hubble Space Telescope (HST). The HST orbits about 569 kilometers (354 mi.) above Earth's surface, and looks out into deep space. Astronomers use the HST to see objects that are more than 13 billion light-years away. The HST and other space telescopes can show how parts of the universe looked 13 billion years ago, shortly after it formed. Do You Know? Astronomers think that four billion years from now. gravity will cause the Milky Way galaxy and the Andromeda galaxy to merge into a single super galaxy.

Types of Galaxies Each galaxy is unique. Galaxies vary in size, shape, age, location, and how they move. One way astronomers classify galaxies is by their shape. There are three types of galaxies: spiral, elliptical, and irregular. Spiral galaxies have a bright center with arms of stars, dust, and gas spinning around the center. Spiral galaxies rotate through space around a central bulge of stars. Our home galaxy, the Milky Way, is a spiral galaxy. It contains roughly 400 billion stars.