Our galaxy is thought to be at least 100,000 light years across and have more than 100 billion stars.
The Milky Way is a spiral galaxy with a bulge at its centre and a surrounding spiral disc of stars, gas and dust.
The entire galaxy rotates around a central axis. Since the dinosaurs died out about 65 million years ago, the Sun is estimated to have travelled about a third the way around the Milky Way's centre.
Image: A Spitzer Space Telescope infrared image of hundreds of thousands of stars in the Milky Way's core (credit: NASA/JPL-Caltech)
Astronomers Pete Lawrence and Paul Abel discuss some of the highlights of summer night skies in the northern hemisphere. The summer months are an ideal time to view the Milky Way galaxy and to look at the constellation of Sagittarius which contains a pattern of stars that looks a bit like a teapot.
Astronomer Pete Lawrence explains how to find our galaxy in the summer night sky.
Astronomer Pete Lawrence explains how stargazers in the northern hemisphere can find the Milky Way in the summer night sky using a pattern of stars called the summer triangle.
Sir Patrick Moore notes that when we look at the prominent winter constellation Orion, we do not see bright galaxies. Dr Chris Lintott explains that during the winter months, we are looking through a stunted spiral arm of the Milky Way known as the Orion Spur. Because we are looking through our galaxy, it is difficult to see galaxies out in the wider Universe. But our view of the night sky changes in the spring and autumn.
In the late 1960s, astronomers use radio waves to build up a picture of our galaxy, the Milky Way.
The Earth makes a dangerous journey through our galaxy, the Milky Way, periodically passing through areas of dense stars. The influence of other stars' gravity sends comets in the outer Solar System hurtling towards Earth.
The Milky Way is the galaxy that contains our Solar System.[nb 1] This name derives from its appearance as a dim "milky" glowing band arching across the night sky, in which the naked eye cannot distinguish individual stars. The term "Milky Way" is a translation of the Classical Latin via lactea, from the Hellenistic Greek γαλαξίας κύκλος (pr. galaxías kýklos, "milky circle"). The Milky Way appears like a band because it is a disk-shaped structure being viewed from inside. The fact that this faint band of light is made up of stars was proven in 1610 when Galileo Galilei used his telescope to resolve it into individual stars. In the 1920s, observations by astronomer Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 200–400 billion stars. It may contain at least as many planets. The Solar System is located within the disk, around two thirds of the way out from the Galactic Center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion Arm. The stars in the inner ≈10,000 light-years form a bulge and one or more bars. The very center is marked by an intense radio source named Sagittarius A* which is likely to be a supermassive black hole. Stars and gas throughout the Galaxy rotate about the center at approximately the same speed, which contradicts the laws of Keplerian dynamics. This indicates that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation; this mass is known as dark matter. The rotational period is about 200 million years at the position of the Sun. The Galaxy as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest known star in the Galaxy is about 13.2 billion years old, nearly as old as the Universe. Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a subcomponent of the Virgo Supercluster.
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