Milky Way

The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 100–400 billion stars. It may contain at least as many planets. Surrounded by several smaller satellite galaxies, the Milky Way is part of the Local Group of galaxies, which forms a sub-component of the Virgo Super-cluster.

The Milky Way contains all known space, though in total very little of it has been charted. This spiral galaxy appears like a band because it is a disk-shaped structure being viewed from inside.

The very center is marked by an intense radio source named Sagittarius A, which is a super-massive black hole. Stars and gas throughout the Galaxy rotate about the center at approximately the same speed, which contradicts the laws of Keplerian Dynamics. This indicates that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation; this mass is known as dark matter.

Size Composition
The stellar disk of the Milky Way is approximately 100,000 light-years in diameter, and is, on average, about 1,000 light-years thick.

The Milky Way contains at least 100-billion stars and may have up to 400-billion stars.

Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars, while the thickness of the gas layer ranges from hundreds of light-years for the colder gas to thousands of light years for warmer gas.

The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars drops smoothly with distance from the center of the Galaxy, the Galactic Core. Beyond a radius of roughly 40,000 light-years, the number of stars per cubic parsec drops much faster with radius, for reasons that are not understood. Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic center is about 180,000 light-years.

Structure
The Milky Way consists of a bar-shaped core region surrounded by a disk of gas, dust and stars. The gas, dust and stars are organized in roughly logarithmic spiral arm structures. The mass distribution within the Galaxy closely resembles the SBc Hubble-classification, which is a spiral galaxy with relatively loosely wound arms.

Galactic Center
The Galactic Center is the rotational center of the Milky Way. It is located at a distance of 27,000 light-years from the Earth. There is a super-massive black hole at the Galactic Center of the Milky Way, Sagittarius A. The motion of material around the center indicates that Sagittarius A* harbors enough mass to equal of 4.1–4.5 million times the mass of a G-type star.

Spiral Arms
Outside the gravitational influence of the Galactic bars, astronomers generally organize the interstellar medium and stars in the disk of the Milky Way into four spiral arms. All of these arms contain more interstellar gas and dust than the galactic average as well as a high concentration of star formation. Counts of stars in near infrared light indicate that two arms contain approximately 30% more red giant stars than would be expected in the absence of a spiral arm, while two contain no more red giant stars than regions outside of arms.

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences.

As in most spiral galaxies, each spiral arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms range from ≈7° to ≈25°. Until recently, there were thought to be four major spiral arms which all start near the Galaxy's center. These are named as follows, with the positions of the arms shown in the image at right:
 * 3 KPC Arm
 * Norma Arm
 * Crux Arm
 * Carina Arm
 * Orion Arm
 * Perseus Arm
 * Outer Arm
 * Scutum Arm
 * Sagittarius Arm

The Milky Way obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.

Quadrants
The Milky Way is split into four quadrants, spreading out from the galactic center. A Quadrant is a region of space 50,000 light-years x 50,000 light-years.


 * Alpha Quadrant
 * Beta Quadrant
 * Gamma Quadrant
 * Delta Quadrant

Formation
The Milky Way began as one or several small over-densities in the mass distribution in the Universe shortly after the Big Bang. Some of these over-densities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. These stars and clusters now comprise the stellar halo of the Galaxy. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including most of the modern ones, exist in this disk.

Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Galaxy's growth) and accretion of gas directly from the Galactic halo. The Milky Way is currently accreting material from two of its nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high velocity clouds like the Smith Cloud. However, properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has suffered no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbor the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.

Etymology
This name derives from its appearance as a dim "milky" glowing band arching across the night sky, in which the naked eye cannot distinguish individual stars. The term "Milky Way" is a translation of the Classical Latin via lactea, from the Greek γαλαξίας κύκλος (pr. galaxías kýklos, "milky circle").