The simple answer to ‘how big is the Milky Way?’ is that the stellar disk of the Milky Way galaxy is approximately 100,000 light years in diameter. The stellar disk is estimated to be 1,000 light years thick and the entire gaseous disk is around 10,000 light years thick, increasing to 30,000 light years at the nucleus. Knowing that a light year is about 9.5 x 1012km, so the diameter of the Milky Way galaxy is about 9.5 x 1017 km in diameter.
The Milky Way is a barred spiral galaxy that is part of the Local Group of galaxies. It is merely one of billions of galaxies in the observable universe. All the stars in the night sky are part of the Milky Way Galaxy. The galaxy appears as a hazy band of white light arching around the entire celestial sphere. The light originates from stars and other material that lie within the galactic plane. Dark regions within the band(the Great Rift and the Coalsack) are areas where light from distant stars is blocked by dark nebulae. The Milky Way has a relatively low surface brightness because of the interstellar medium that fills the galactic disk.
The galactic center lies in the direction of Sagittarius. From Sagittarius, the Milky Way appears to pass westward through many constellations. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane.
In 2007, star HE 1523-0901 was estimated to be about 13.2 billion years old. As the oldest known object in the Milky Way at that time, it placed a lower limit on the age of the Milky Way. This estimate was determined using the UV-Visual Echelle Spectrograph to measure the presence of thorium and other elements. The age of other stars can be estimated in the same way. Measurements of thin disk stars yield an estimate that the thin disk formed about 8.8 billion years ago. These measurements suggest there was a difference of almost 5 billion years between the formation of the Galactic halo and the thin disk.
The galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust, and stars that form four distinct arms spiralling outward. Astronomers began to suspect that the Milky Way is a barred spiral galaxy in the 1990s. This was confirmed by Spitzer Space Telescope observations in 2005. The mass of the galaxy is about 5.8×1011 solar masses. Most of the mass of the galaxy is thought to be dark matter. This dark matter forms a dark matter halo that is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic center. The overall mass of the entire galaxy is estimated at 600–1000 billion M. Is that a large enough number for you?
We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.