The Milky Way is a spiral galaxy, which is a part of the Local Group of galaxies. Though it is one of the billions of galaxies present in the universe, it has special significance, due to the existence of human form. Moreover, it is also the home galaxy of our Solar system. While viewing from Earth, the plane of galaxy appears like a band of light in the night sky. This acted as the inspiration behind the name of the Milky Way galaxy, which derived from the Greek word, Galaxias. For some more information on what is Milky Way, read on.
Size and Age
With regard to its size and age, Milky Way galaxy is approximately 100,000 light-years (ly) i.e. 9.5x1017 km in diameter. On an average, its thickness is estimated to be about 1,000 ly i.e. 9.5x1015 km. It is believed to contain minimum 200 to 400 billion stars. Beyond the stellar disk, a much thicker disk of gas can also be seen, which has a thickness of around 12,000 ly i.e. 1.1x1017 km. However, nothing can be said about the age of the Milky Way. The oldest star that has been found in the galaxy is estimated to be 13.2 billion years old.
Composition and Structure
The Milky Way consists of a bar-shaped core region, which is surrounded by a disk of gas, dust and stars. This forms four distinct arm structures, spiraling outward in a logarithmic spiral shape. In effect, the mass distribution within the Galaxy resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms. Moreover, in 1990s, astronomers began to suspect that the Milky Way is a barred spiral galaxy rather than an ordinary spiral galaxy.
Their speculations were confirmed by the Spitzer Space Telescope observations in 2005. As per the observations, the Galaxy's central bar was found to be larger than suspected. In effect, the Milky Way's mass is thought to be about 5.8x1011 solar masses, comprising of billion of stars. Its integrated absolute visual magnitude is deemed to be −20.9. In fact, the majority of mass of the Galaxy is believed to be dark matter. This forms a dark matter halo of about 600 - 3000 billion M, which spreads out evenly.