A Galaxy, or nebula, is any large-scale system of stars, interstellar gas, dust, and plasma within the universe. The average galaxy contains more than 100 billion solar masses and ranges in diameter from 1,500 to 300,000 light-years, 90% of which is actually composed of largely unknown substance called dark matter Every galaxy is controlled and held together by one central gravitational force. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds.
There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter.
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Types of galaxies
Galaxies are of mainly three types: ellipticals, spirals, and irregulars.
Ellipticals- These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter
Spiral and barred Spiral
Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. Our own galaxy is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec in thickness. It contains about two hundred billion (2×1011) stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.
MilkyWay
The Milky Way, or simply the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies. It is one of billions of galaxies in the observable universe.
Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the band of light that the galaxy forms in the night sky, while the galaxy should receive the full name Milky Way Galaxy, or alternatively the Galaxy.
The Sun (and therefore the Earth and the Solar System) may be found close to the inner rim of the galaxy's Orion Arm, in the Local Fluff inside the Local Bubble, and in the Gould Belt, at a distance of 7.62±0.32 kpc (~25,000±1,000 ly) from the Galactic Center. The Sun is currently 5–30 parsecs from the central plane of the galactic disc
