Rather, they are concentrations of gas and dust where stars form, produced by disturbances within the Milky Way, or on occasions by a jolt from outside, such as a supernova or the passage of the Solar System through one of the dusty gas clouds that congregate in spiral arms.
The ghostly arms are not permanent features of a disc galaxy like the Milky Way. At present, our Solar System is in its twentieth orbit of the Milky Way near the inner edge of a spiral feature known as the Orion Arm or, less poetically, the Local Arm.
