Spiral galaxies are surrounded by halos, mixtures of old stars, star clusters, and dark matter – invisible material that does not emit or reflect light but still has a gravitational pull on other matter. The spiral arms can be wound tightly or loosely, and some cannot be seen from Earth because we view the galaxy from the side, edge on. These galaxies resemble giant rotating pinwheels with a pancake-like disk of stars and a central bulge or tight concentration of stars. Our Milky Way is one example of a broad class of galaxies defined by the presence of spiral arms. Chu (University of Illinois, Urbana) and STScI CFHT Image: Canada-France-Hawaii Telescope/J.-C. This stunning view of spiral galaxy M101, also known as the Pinwheel galaxy, was captured by the Hubble Space Telescope.
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