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The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia (gala, galactos means "milk")) is a hazy band of white light across the celestial sphere, formed by stars within the disc of its namesake galaxy, which is also simply called the Galaxy as we are in it. The Milky Way appears brightest in the direction of Sagittarius, where the galactic center lies. Relative to the celestial equator, the Milky Way passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux. This reflects the fact that the Earth's plane of orbitThe ecliptic is the geometric plane that contains the orbit of the Earth. The orbits of most planets in the Solar System lie very close to it. Seen from the Earth, this is a bisecting great circle, superimposed upon the celestial sphere, which contains th is highly inclined to the galactic planeThe galactic plane is the plane in which the majority of a flattened galaxy's mass lies. The most commonly discussed example is the plane on which the Earth's galaxy, the Milky Way, lies, which is also called the Milky Way's plane or plane of the Milky Wa, as is the SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt's equator. The galactic pole lies at right ascensionRight ascension (RA; symbol α: Greek letter alpha) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination. RA is comparable to l 12 h 51,42 m, declinationIn astronomy declination (dec is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Dec is comparable to latitude, projected unto the celestial sphere, and is measured in degrees north of 27° 7,8' ( epoch 2000.0In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. In the case of celestial coordinates, the position at other times can be computed by taking into account precession and proper motion. In the cas; this is a conventional value adopted by the IAUThe International Astronomical Union (IAU) unites national astronomical societies from around the world. It is a member of the International Council of Science (ICSU). It is the recognised authority for the naming of stars, planets, asteroids and other ce in 1959Events January-February January 1 Cultivars of plants named after this date must be named in a modern language, not in Latin. January 1 Cuba: Fulgencio Batista flees Havana when forces of Fidel Castro advance January 2 CBS Radio cuts four soap operas: Bac). The fact that the Milky Way divides our night sky into two roughly equal hemispheres reflects the fact that the solar system lies close to the galactic plane.

The Milky Way galaxy is a large spiral galaxy of Hubble type SBbc (loosely wound barred spiral) with a total mass of about solar masses (M_☉), comprising 200-400 billion stars (see [3]). The galactic disk has a diameter of about 100,000 light-years (see for a list of comparable distances). The distance from the Sun to the galactic center is about 27,700 light-years.

The stars in the Galaxy's disk rotate around the Galaxy's center, which is suspected to harbour a supermassive black hole. Sagittarius A* is agreed to be the most plausible candidate for the location of this supermassive black hole. It takes the solar system about 226 million years to complete one orbit, and so has completed about 25 orbits during its lifetime. The orbital speed is 217 km/s, i.e. 1 light-year in ca. 1400 years, and 1 AU in 8 days. The orbital speed of stars in the Milky Way does not depend much on the distance to the center: it is always between 200 and 250 km/s [1]. Hence the orbital period is approximatedly proportional to the distance from the star to the Galaxy's center (without the power 1.5 which applies in the case of a central mass). The disk has a bulge at the center.

There are believed to be four major spiral arms and at least two smaller ones which all start at the Galaxy's center. These are named as follows, counting outwards from the centre along a radius through our solar system:

• Norma Arm, or 3 kpc Arm
• Scutum-Crux Arm, or Centaurus Arm
• Sagittarius Arm, or Sagittarius-Carina Arm
• Orion Arm, or Local Arm - labeled "0"; a minor spiral arm. The Earth's solar system may be found close to the inner rim of the Local Arm, in the Local Bubble, 8.0 ± 0.5 kpc from the galactic center.
• Perseus Arm
• Cygnus Arm, or Outer Arm

The distance between the local arm and the next arm out, the Perseus arm, is about 6,500 light-years (see [2]). Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with pitch approximately 12 degrees (see [1]).

The disk is surrounded by a spheroid halo of old stars and globular clusters. While the disk contains gas and dust obscuring the view in some wavelengths, the halo does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.

The Milky Way, the Andromeda Galaxy and the Triangulum Galaxy are the major members of the Local Group, a group of some 35 closely bound galaxies; The Local Group is part of the Virgo Supercluster. Canis Major is a dwarf galaxy, which is being pulled into the Milky Way.