Sorting Galaxies and Finding Home

By the end of this lesson, you will be able to sort galaxies into three shapes and describe where Earth's solar system sits inside the Milky Way.

A galaxy is a huge group of stars, gas, dust, and planets held together by gravity. Galaxies are not all shaped alike, so astronomers sort them into three main groups. Spiral galaxies look like a glowing pinwheel, with a bright bulge in the middle and curving arms of stars wrapping around it. Elliptical galaxies look like smooth, fuzzy footballs or round blobs of light, with no arms at all. Irregular galaxies have no neat shape, often because they were pulled out of shape by a nearby galaxy's gravity.

Think about a football stadium on a Friday night in Texas. The spiral galaxy is like the marching band in a perfect pinwheel formation on the field. The elliptical galaxy is like the crowd packed smoothly into the stands, no pattern, just a big rounded mass. The irregular galaxy is like the tailgate parking lot afterward, people scattered everywhere with no shape you can name.

Our home galaxy is the Milky Way, and it is a spiral galaxy. It is so wide that light, the fastest thing there is, takes about 100,000 years to cross it. It holds hundreds of billions of stars, and our Sun is only one of them.

Our solar system, which is the Sun plus everything orbiting it, is not at the center of the Milky Way. We sit about halfway out from the center, tucked inside one of the spiral arms. From here, the Sun takes roughly 230 million years to travel once around the galaxy's center. When you see the hazy band of light called the Milky Way on a dark Texas night, you are looking edgewise through your own galaxy's disk.

Try it: If you could see our galaxy from far above, would our solar system be in the bright center, in a spiral arm, or outside the galaxy completely?