What is a galaxy?
A galaxy is a cluster of stars, nebulae, dark matter, and other astronomical objects. Most galaxies are tens of thousands of light years in diameter, and contain billions of stars. Galaxies come in three primary shapes; spiral galaxies are thin disks, with spiral arms surrounding a central hub; elliptical galaxies are uniform, oval-shaped agglomerations; and irregular galaxies have little or no definite structure. A spiral galaxy centers around a hub, which is roughly spherical and bulges outward from the disk. The hub is the gravitational center of the galaxy, and usually contains a supermassive black hole. Surrounding the hub are spiral arms, which are bunched-up waves of stars and gas orbiting the center. The spiral arms usually have more gas and dust, and they contain most of the newly formed stars, which make them appear blue in color imagery. A spiral galaxy may have a horizontal bar passing through its hub; our own galaxy, the Milky Way, is believed to contain a bar. Elliptical
A galaxy is a huge region of space that contains hundreds of billions of stars, planets, glowing nebulae, gas, dust, and empty space. Galaxies may also contain a black hole, a single point of infinite mass and gravity, at their center. The fifth billion or so existing galaxies are believed to contain most of the observable mass in the universe; this does not include invisible dark matter, which may make up as much as 90% of the mass in the universe.
The Milky Way represents only a small part of the whole matter in the universe. It consists of an accumulation of a few hundred billion stars, forming a disk-like structure which is held together by the gravitational pull. This galactic disk presents a curved area in its center, visible in the direction of the Sagittarius constellation, where the so-called galactic nucleus is located. The galactic nucleus consists in a large population of stars densely packed into a relatively small volume, orbiting at incredible speeds a supermassive black hole, dead in the center of the galaxy. Similarly to the water draining through a sink hole, the galactic disk presents whirlpool-like features, meaning that stars are distributed in galactic arms forming a spiral structure. At least that is the case of our Milky Way, as other galaxies might take a globular or elliptical shape. Our solar system is located in one of these galactic arms called Orion Cygnus, between the Perseus and the Carina-Sagittari
