Spiral galaxies are just one type of galaxy. Two other types are elliptical (egg-shaped) galaxies—like the one that will result from the merger of the Milky Way with Andromeda—and lenticular galaxies, which are mostly flat but do not have spirals. Learn more about the types and evolution of galaxies.

Galaxies with a majestic two-armed structure are called grand-design spiral galaxies. An example is M51, aka the “Whirlpool Galaxy.” Watch this video to learn more about how the influence of a companion galaxy created M51’s spiral structure. The Milky Way is not a grand-design galaxy, but rather seems to have additional small and large spiral arm segments.

Figuring out the shape of the Milky Way and where the Earth is located within it has proved incredibly challenging. It’s a bit like standing within a dense forest and trying to create a map of it without being able to move. Although researchers first floated the idea of spirals in the Milky Way back in the 1850s, the first clear observations of nearby spiral arm segments didn’t happen until the 1950s. Astronomers still haven’t arrived at a consensus on how many spiral arms our galaxy has, where they are located, and how they are oriented relative to each other. Read more about our best current understanding of Earth’s place in the galaxy. The observations suggesting that the Milky Way’s spirals may only last 80 to 100 million years came from the Gaia space telescope, a spacecraft launched by the European Space Agency in 2013 with the explicit goal of creating “the largest and most precise 3D map of the Milky Way.” Read more about the mission and some of the other insights scientists have gained from its data.

Want to get out your pencil and do some calculations? See if you can figure out which spiral arm our solar system might have been in when most dinosaurs went extinct. The angular pattern speed of the Milky Way’s spiral structure—the rate at which it is rotating around the center of the galaxy—is estimated to be 5.2×10-16 radians per second. Traveling in the same direction as the spiral pattern, our solar system takes about 230 million years to orbit the center of the galaxy. Using this map, estimate where our solar system was in relation to the spiral structure 65 million years ago. The answer you arrive at is not definitive—since neither the pattern speed nor the structure of the arms on the map are known with precision, future telescope observations could shift our understanding of the history of the Milky Way’s spiral arms and our journey between them.

You don’t have to be a professional astronomer to contribute to our collective knowledge about galaxies. Galaxy Zoo is a community science project in which members of the public classify the shapes of galaxies, which helps astronomers test their theories of how galaxies form and evolve. Galaxy Zoo classifications have provided evidence that spiral arms may break apart and reform, and Galaxy Zoo volunteers have even discovered new astronomical objects that professional astronomers had overlooked. Hundreds of thousands of people have contributed to Galaxy Zoo so far, and you can join them. The website also includes educator resources for using Galaxy Zoo in the classroom. 

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