Thomas Wright, an Englishman, was the first to explain the appearance of the Milky Way and define its shape, in his book 'An Original Theory', or 'New Hypothesis of the Universe' published in 1750. He proposed that the stars were arranged in a flat disk and the band of radiance arose because we view it from inside.
The philosopher Immanuel Kant further developed Wrights' ideas, suggesting that the galaxy took on a disk shape due to its rotation.
The astronomer Sir William Herschel and others placed the Sun near the centre of the galaxy and it was not until the 1920s when Harlow Shapley's idea that the Sun lay at the edge of the galactic disk was accepted. He used observations of star's apparent magnitude and absolute magnitude to determine the distance to globular star clusters, allowing him to work out the 3D distribution of these globular star clusters in space. He found that they formed a spherical halo around the galaxy and were concentrated at a point far from the Sun, which he was sure was the Milky Way's centre.
Our galaxy is so called because it appears as a milky band of light in the sky. It is a spiral-shaped system of stars, gas (mostly hydrogen), dust and dark matter that orbits a common centre and are bound together by gravity.
The Sun is one of c. 200 000 000 000 stars in the galaxy. The Sun and planets are about half way out, 26,000 light years (ly) from the galactic centre and about 20 ly above the plane of our galaxy. (The Earth & planets in our solar system don't orbit within the plane of the galaxy but are tipped by about 63°).
The Milky Way began forming about 12 billion years ago, not long after the Big Bang. It is part of the Local Group - about 50 galaxies, including Andromeda, Magellanic Clouds etc. The Local Group is part of a larger gathering of galaxies called the Virgo Supercluster.
The Milky Way has its long arms of material spiralling out from a concentrated, oblong core. It has 2 major spiral arms, Scutum-Centaurus and Perseus from the galactic core (each full of young and ancient stars), and 2 minor arms - Sagittarius and Norma, which branch off and contain young stars. The Sun is in the much smaller Orion Arm located between the Sagittarius and the Perseus arms.
The centre is hard to see because of clouds of gas and dust. It is believed to contain a supermassive black hole referred to as Sagittarius A* (near to the border of the constellations Sagittarius and Scorpio) and contains a mass of about 4.3 million Suns.
The Milky Way rotates every 200 million years. It is made of at least 100 million stars. They all orbit at about 220 km/s. The stars form a large disk of diameter about 25 000 kiloparsecs (about 80 000 ly). It takes 230 to 250 million years for our solar system to go round the galactic centre (galactic year).
The most common stars in the galaxy are red dwarfs - cool stars about a tenth the mass of the Sun.
The Milky Way has grown by merging with other galaxies and is currently acquiring stars from a small galaxy called the Sagittarius Dwarf Spheroidal, as well as material from the Magellanic Clouds. The Milky Way is surrounded by and enormous halo of hot gas that extends for hundreds or thousands of light-years. This halo is spinning rapidly.
The solar apex is the direction the Sun travels through space in the Milky Way. Its general direction is towards Vega near the constellation of Hercules.
Observing the Milky Way
Many think of it as stretching from Scorpio to Cygnus. The constellations found in the Milky Way include Perseus, Cassiopeia, Lacerta, Cygnus, Aquila, Sagittarius, Ophiuchus and Scorpius. The best time to observe the bright galactic centre is mid-March to mid-October. From Sept to April the Sun obscures the bright galactic centre. In the northern hemisphere the galactic centre is visible in the southern half of the sky. Late summer is one of the best times when there is a faint white glow in an arc from the southern to the north-eastern horizon.
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