Milky Way, Jupiter and Saturn, Algonquin Park, Ontario, Canada, July 31, 2019

The Milky Way is our home galaxy, a spiral system with a disk of stars spanning more than 100,000 light years across. From Earth, the Milky Way appears as a faint band of light because we are looking through the galaxy from within its disk, seen edge-on from our position inside it.

The galaxy is estimated to contain between 100 and 400 billion stars, yet from even a perfect dark-sky location the unaided eye can see only a few thousand stars at one time. The nearest star beyond the Sun, Proxima Centauri, is about 4.24 light years away. At the centre of the Milky Way is Sagittarius A*, a supermassive black hole with a mass equivalent to roughly 4.3 million Suns.

The Milky Way does not rotate like a solid wheel. Stars and gas orbit the Galactic Centre at different rates depending on their distance from it. Our solar system takes about 240 million years to complete one orbit around the galaxy, a timescale often called a galactic year. Since the Sun formed about 4.6 billion years ago, it has completed only twenty such orbits.

Beyond the Milky Way, Hubble-based estimates suggest that the observable universe may contain two trillion galaxies, many too faint or distant to be seen with present-day telescopes. In this photograph, Jupiter and Saturn appear within the same field of view as the Milky Way, placing a quiet Ontario shoreline in relation to planetary, galactic, and extragalactic scale.