It’s one of about 100 billion other galaxies in the universe, but it’s our home – join us as we explore the Milky Way Galaxy!
The Milky Way is our galaxy, home to our solar system. It formed a little more than 13 billion years ago, just a few 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 50 galaxies known as the Local Group, which is part of the Virgo Supercluster. Containing as many as 50 billion planets and 400 billion stars, the Milky Way is a spiral galaxy. It has a centre known as a ‘bulge’, surrounded by a flat disk comprising several loose arms that contain stars and their orbiting bodies, as well as gases and dust. The centre contains 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. Some astronomers believe 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 with 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. 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 Twenties 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; 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 its 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 Milky Way. 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 ca n map t he 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 galaxies, the Small Magellanic Cloud and the Large Magellanic Cloud, create a vibrational warp in the Milky Way’s disk 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 -indeed, it’s best viewed in very rural areas under clear skies – and it also 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.
Galaxy on the move
The spinning galaxy rotates differentially (the closer to the centre an object is, the less time it takes to complete an orbit). The Sun travels around the galactic centre at 800,000km/h (100.000 mph), taking 225 million years to make one orbit.
The galaxy rotates differentially, which means that objects closer to its core orbit the core faster than the stars in the arms of the galaxy. The Milky Way rotates at about 170km per second. Our solar system, located around 30,000 light years away from the galactic core, completes an orbit once every 225 million years. The Milky Way is also moving through space at about 630km per second, relative to the cosmic background radiation – the Big Bang’s remnants. It moves in the direction of a gravitational anomaly in the universe, known as the Great Attractor.
When galaxies collide
Astronomers are predicting that in about 3 to 5 billion years, the Andromeda galaxy may collide with the Milky Way. The violent crash will result in a blob-like elliptical galaxy, dubbed ‘Milkomeda’. Currently Andromeda is about 2.5 million light years from the Milky Way, but it is moving towards our galaxy at 120km per second. The possibility of stars and planets within the galaxies actually colliding is highly unlikely, but the different gravitational fields will jostle them out of their current locations. Our solar system could even be ejected during the collision, but that probably wouldn’t affect the planets much. By the time of the proposed collision, Earth will probably already be devoid of life due to the Sun’s increasing heat.
Facts about Milky Way and Earth
Halo – The Milky Way has a halo surrounding it that is about 200,000 light years in diameter. The halo contains dark matter and nearly 150 globular clusters (collections of stars rotating around a core}, many of which are rotating in the opposite direction to the Milky Way. It also contains some of the oldest stars in the universe.
Bulge – The bulge at the centre contains globular clusters and old, red stars (called population II objects). It is between 70,000 and 100,000 light years in diameter. Astronomers believe that there is a massive black hole at the very centre, including a complex radio source called Sagittarius A.
Disk – Most of the Milky Way comprises a large, flattened disk that rotates. The disk is made up of spiral arms (at least four and possibly six) which contain somewhere between 200 and 400 million relatively young stare. It also contains star clusters, nebulae and matter that will eventually give birth to more stars.
1,000 light years Milky Way – The edge of the Milky Way is about 1,000 light years away from Earth. Keep in mind that our solar system is already on the outer edges of the Milky Way itself, about 28,000 light years from its centre.