How Are the Arms of Spiral Galaxies Formed?

The theory of density waves

Credit: NASA & ESA — A majestic view of Messier 101 captured by the Hubble Space Telescope

Located about 25 million light-years from Earth in the constellation Ursa Major, Messier 101, or M101 for short, is a magnificent spiral galaxy, which, at 170,000 light-years in diameter, is over one and a half times the Milky Way’s size.

M101 is also known as the Pinwheel Galaxy, a name well representing its most…