6. The Milky Way
Summary The Milky Way, our host galaxy, is a barred spiral galaxy and contains at least 100 billion stars. The Sun and our Solar System orbit the galactic center every about 250 million years. The Milky Way and the neighboring larger Andromeda Galaxy will collide with each other in about 4 billion years.
Keywords Black Holes; Galaxies; Interstellar Medium; Stars
An artist’s impression of the top view of the Milky Way. Several spiral arms reach out from the galactic bar-shaped center to the edge of the galaxy. The Sun is located in the Orion Spur, a minor spiral arm. (© NASA/JPL-Caltech/ESO/R. Hurt / Wikimedia Commons / Public Domain)
Our host galaxy is the Milky Way, a barred spiral galaxy which contains at least 100 billion stars and probably at least 100 billion planets, and which has a diameter of around 100,000 light-years. The galactic disk is surrounded by a spherical halo which consists of dark matter, old stars and globular clusters. The Milky Way’s name comes from the fact that its disk component, in which the Sun and consequently Earth are located, is visible as a milky band in the night sky to the human eye. Only about 5000 stars are visible to the naked human eye on Earth, with most of them having a distance of less than 1000 light-years, which means that the overwhelming majority of the Milky Way’s stars is not visible without an assistive technology. It is known since 1923 through measurements of American astronomer Edwin Hubble that the Milky Way is not the only galaxy in the Universe, as was widely believed up to then.
The closest star system to the Sun is Alpha Centauri at a distance of 4.4 light-years. It consists of three stars which orbit each other, with at least one exoplanet. Voyager 1, the 1977 launched space probe which travelled the furthest distance from Earth, travelled around one light day, which corresponds to 0.1% of the distance to Alpha Centauri. While the Sun currently moves along the edge of the Local Interstellar Cloud, which is a region in the galactic interstellar medium with a moderate density and is about 30 light-years across, the Local Insterstellar Cloud is located inside the Local Bubble, which is a relatively dilute region in the galactic interstellar medium and is about 300 light-years across. The Local Bubble was likely created by star explosions which blew matter away and occurred within the last 20 million years. It lies within the Orion Spur, a minor spiral arm of the Milky Way which is about 3500 light-years across and 10,000 light-years in length.
The galactic center of the Milky Way in the infrared light domain. While interstellar dust clouds let pass the infrared light from the galactic center, they block light at frequencies visible to the naked human eye. (© NASA/JPL-Caltech/S. Stolovy (SSC/Caltech) / Wikimedia Commons / Public Domain)
The distance of the Sun to the galactic center is about 26,000 light-years. The Sun including our Solar System is moving at a speed of 230 kilometers per second around the galactic center and orbits it each about 250 million years. The latter time period is also called a galactic year. Its central body is a supermassive black hole with a mass of around four million solar masses, which is also an intense and compact radio source.
The Milky Way is orbited by two smaller galaxies, that are the Large Magellanic Cloud and the Small Magellanic Cloud which both are at distances of about 150,000 to 200,000 light-years, and additionally by a few dwarf galaxies. The Milky Way is the second largest galaxy in the Local Group of galaxies, with the largest one being the spiral-shaped Andromeda Galaxy at a distance of 2.5 million light-years. The Andromeda Galaxy, the Large and the Small Magellanic Clouds can be seen by the naked human eye from the Southern Hemisphere. The Milky Way and the Andromeda Galaxy will collide with each other in about 4 billion years. They will ultimately merge and possibly form a giant elliptical galaxy.