How It Works
Explore the Milky Way
It’s just one of two trillion galaxies in the observable universe, but it’s our home. Join us as we journey across the Milky Way
The Milky Way is our galaxy, home to our Solar System. It formed more than 13 billion years ago, less than a billion years after the Big Bang. The galaxy is estimated to be about 100,000 light years in diameter and 1,000 light years thick. It is part of a system of about 50 galaxies known as the Local Group, which is part of the Virgo Supercluster. Containing as many as 100 billion planets and 400 billion stars, the Milky Way is a spiral galaxy. It has a centre, known as a ‘bulge’, which is surrounded by a flat disc comprising several loose arms that contain stars and their orbiting bodies as well as gases and dust. The centre is thought to contain a massive black hole and a complex radio source known as ‘Sagittarius A*’. Around the outside of the Milky Way there is a halo containing dark matter and a very small percentage of the galaxy’s total number of stars. Astronomers have observed that the Milky Way is actually a special type of spiral galaxy called a barred spiral, meaning that it has a bar-shaped distribution of stars running across its centre.
Aristotle first wrote of the Milky Way in the mid-300s BCE. He broke from other Greek philosophers who believed that the milky streak in the sky might be stars. Aristotle thought that it was a sort of fiery emission coming from a cluster of very large stars, and that it resided in the Earth’s atmosphere. Ancient astronomers continued to speculate about the true nature of the Milky Way until Galileo determined in 1610 that it comprised a massive number of stars.
In 1755 Immanuel Kant realised that the Milky Way rotated and was held together by gravity. 30 years later William Herschel attempted to depict the shape of the Milky Way and the Sun’s location in it by counting and recording the position of visible stars. Finally, Edwin Hubble determined in the 1920s that there were nebulae beyond the Milky Way, proving that there were other galaxies in the universe. Hubble is also
responsible for coming up with the classification system for galaxies that we use today, which includes spiral, elliptical and irregular galaxies.
For all our observations, the Milky Way is still mysterious. Determining its actual size and our location in it has been difficult since we can only observe from within; Herschel and astronomers before him believed that our Solar System was in its centre because of the apparently equal distribution of stars in our sky, for example.
Several different indirect methods have been used to calculate the actual size of the Milky Way. This includes using the period-luminosity relation of certain stars. The luminosity, or brightness of some stars pulse in a predictable pattern, which can be measured along with their apparent magnitude to estimate distance. In the early 20th century an astronomer named Harlow Shapley used some of these measurements to extrapolate the distances of globular clusters outside the
This showed that the Sun was not at the centre of the galaxy and provided a rough, although inaccurate estimate of the Milky Way’s diameter. Today we can map the galaxy using telescopes that pick up light and radio waves emitted by gases and molecules floating in space.
The Milky Way isn’t a static object – the arms rotate about the centre, and it is also moving in the direction of a large gravitational anomaly known as the Great Attractor. Our galaxy also has its own orbiting galaxies. The two largest of these satellite galaxies, the Small Magellanic Cloud and the Large Magellanic Cloud, create a vibrational warp in the Milky Way’s disc as they orbit due to the presence of dark matter.
Because of light and other types of atmospheric pollution, it’s difficult to view the Milky Way from Earth with the naked eye; it’s best viewed in very rural areas under clear skies, and looks like a faint milky band of clouds stretching across the night sky.
Light pollution maps are available online, and local astronomy clubs can help locate the best place to go to see it.