What is the Milky Way?

The Milky Way is a barred spiral galaxy — one of an estimated two trillion galaxies in the observable universe. It spans roughly 100,000 light-years in diameter and is about 1,000 light-years thick at the disk. It contains between 200 and 400 billion stars, and potentially 100 billion planets. Our Solar System orbits the galactic center at about 828,000 km/h, completing one orbit (a "galactic year") in approximately 225–250 million years. At the center lies Sagittarius A*, a supermassive black hole with the mass of about 4 million Suns. The Milky Way is part of the Local Group of galaxies and is on a collision course with the Andromeda Galaxy — expected to merge in about 4.5 billion years.