Milky Way Galaxy

Our home galaxy, also known simply as the Galaxy, a large type Sb or Sbc spiral galaxy containing some 200 to 400 billion stars (or possibly many more if brown dwarfs are included). Comprised of a disk, a bulge, and a halo, it has a total mass, including an uncertain but large amount of dark matter in the halo, of 750 billion to 1 trillion M_sun. The galactic disk is home to the various spiral arms of the Galaxy, including the Orion Arm in which the Sun is located (27,700 light-years from the center), the Outer Arm and Perseus Arm (both outside our own arm), the Sagittarius-Carina Arm (immediately inward of the Sun), and the Scutum Arm, Crux Arm, and three-kiloparsec Arm (all even closer to the center). Within these arms are many ordinary, intermediate-age disk stars, such as the Sun, together with the more showy extreme Population I objects, in the form of young, hot stars, stellar associations, open clusters, diffuse nebulae, and the bulk of the interstellar matter from which future stars will form. The galactic bulge and the much larger galactic halo contain Population II objects—mostly old stars and roughly 200 globular clusters, of which about 150 are known. These globulars are strongly concentrated toward the galactic nucleus.

Diameter of disk:	About 100,000 light-years
Thickness of disk:	2,300 to 2,600 light-years
Thickness of bulge:	13,000 light-years
Mass:	At least 100 billion Msun
Proportion of gas/dust:	5 to 10%
Mean density:	0.1 Msun per cubic parsec
Total luminosity:	About 1044 ergs/s
Magnetic field:	3 to 5 × 10−6 gauss

The nucleus of the Milky Way contains a complex of gas, dust, stars, supernova remnants, magnetic filaments, and, almost certainly, a massive black hole at the very center; it lies in the direction of Sagittarius, around R.A. 17h 46m and Dec. −28° 56′. Lying dead center in the Galaxy is the Sagittarius A Complex, which is believed to be associated with a black hole of about 3 million M_sun, material in orbit around this object, and a nearby supernova remnant. Surrounding the galactic center are narrow threads known as non-thermal filaments (NTFs), the most prominent of which are called the Arc, the Pelican, and the Snake. These seem to consist of magnetic flux tubes filled with relativistic electrons, beaming synchrotron radiation, that have been swept up from adjacent molecular clouds and hurled along the field lines at incredible speeds. Another unusual structure in the nucleus is cataloged as 359.1-00.5 and appears to be a superbubble with a cluster of as many as 200 newborn stars at its heart.

The Milky Way Galaxy has a number of satellite galaxies, as shown in the table “Satellite Galaxies of the Milky Way,” (also see individual entries for these galaxies) and is the second largest (after the Andromeda Galaxy), but possibly most massive, member of the Local Group.

Satellite Galaxies of the Milky Way

Galaxy	Distance (light-years)	Year of Discovery	Absolute Magnitude	Diameter (light-years)
Sagittarius Dwarf Elliptical	78,000	1994	−13.4	>10 000?
Large Magellanic Cloud (LMC)	160,000	—	−18.1	20,000
Small Magellanic Cloud (SMC)	180,000	—	−16.2	15,000
Ursa Minor Dwarf	220,000	1954	−8.9	1,000
Draco Dwarf	270,000	1954	−8.8	500
Sculptor Dwarf	285,000	1938	−11.1	1,000
Sextans Dwarf	290,000	1990	−9.5	3,000
Carina Dwarf	330,000	1977	−9.3	500
Fornax Dwarf	450,000	1938	−13.2	3,000
Leo II	670,000	1950	−9.6	500
Leo I	830,000	1950	−11.9	1,000

Milky Way Galaxy The innermost region of the Milky Way, only a few light-years across, seen in infrared light with the Very Large Telescope. The compact objects are stars and there is also diffuse emission from interstellar dust. The two small arrows mark the position of the black hole, SgrA*, believed to exist at the very center of our Galaxy. European Southern Observatory