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Edge-on view of the Milky Way and the Magellanic Clouds in infrared light

This infrared all-sky view, created by the 2MASS Survey, gives us a hint of what the Milky Way would look like if we could see it from the outside in an edge-on perspective. The image is dominated by the galaxy's blazing central Bulge and thin Disk. The latter is pervaded by dark dusty nebulae. At lower right are the Milky Way's two principal satellite galaxies, the Large and Small Magellanic Clouds. Image Credit: 2MASS/J. Carpenter, T. H. Jarrett, & R. Hurt.


The Milky Way is the second-largest member of our neighborhood galaxy cluster, known as the Local Group. It is also the center of mass for a cluster of satellite galaxies numbering about two dozen. By far the largest of our satellites are the Large and Small Magellanic Clouds. Each was originally a small spiral galaxy that has been severely disrupted by tidal interaction with the Milky Way. Our remaining satellite galaxies include Bootes, Canis Major Dwarf, Canes Venatici I and II, Carina, Coma Berenices, Draco, Fornax, Hercules, Leo I-IV (Leo III = Leo A), Leo T, Phoenix Dwarf, Sagittarius Dwarf Elliptical, Sculptor Dwarf, Sextans Dwarf, Ursa Major I and II, and the Ursa Minor Dwarf Spheroid (Van den Bergh 2000, Simon & Geha 2007, Irwin et al. 2007, Atlas of the Universe, Frommert & Kronberg).

Most of the Milky Way's satellites are classified as dwarf spheroid or dwarf elliptical galaxies. Only Leo A, Leo T, and Phoenix, in addition to the Small Magellanic Cloud, are typically described as dwarf irregular galaxies. The Large Magellanic Cloud is also irregular in form, but disagreement persists as to whether it is a dwarf or a full-size galaxy. As their name implies, irregular galaxies have little ordered structure. They are typically rich in hydrogen clouds and dust, and they contain many active nurseries where hot blue stars are born (see the 2MASS Survey).

Another member of the the Local Group, the dwarf irregular galaxy NGC 6822 (also known as Barnard's Galaxy) is sometimes included among the satellites of the Milky Way. NGC 6822 is similar to the Small Magellanic Cloud in size and structure, but it is more remote than most of the Milky Way's confirmed satellites, lying at a distance of about 500 kiloparsecs (1.63 million light years; see the 2MASS Survey).


The Large Magellanic Cloud, as photographed by Wei-Hao Wang

The Large Magellanic Cloud, courtesy and copyright Wei-Hao Wang

The Large Magellanic Cloud (LMC) is located at a distance of 50 kiloparsecs (163,000 light years; 2MASS Survey). We see it almost face-on (Van der Marel 2001). Within the Local Group it ranks fourth in mass, after Triangulum. The LMC was originally a small spiral galaxy, but it has been distorted by tidal interactions with the Milky Way and the Small Magellanic Cloud. Indistinct fragments hint at its former spiral arms, while an off-center bar-like structure suggests that it was once a barred spiral like the Milky Way. Overall, the LMC retains a flat disk shape, but the disk is elliptical rather than circular because of tidal effects (Van der Marel 2001). The Astronomy Picture of the Day Web site provides a diameter of "about 15,000 light-years" for the visible disk of the LMC, implying a radius of about 2.3 kiloparsecs. This is a little less than one-sixth the radius of the Milky Way.


The Tarantula Nebula (also 30 Doradus) is a star-forming region more extensive than the Milky Way's Great Nebula in Orion. Image courtesy European Southern Observatory. See larger image. LH 95, a star-forming region photographed by the Hubble Space Telescope. Image courtesy D. Gouliermis/ STScI/ AURA/ ESA/ NASA. See larger image. N180B, a star-forming region containing hot O and B stars, photographed by the Hubble Space Telescope. Image courtesy NASA/ ESA/ STScI/ AURA/ Y-H Chu/ Y. Naze. See larger image.



The Small Magellanic Cloud

The Small Magellanic Cloud, courtesy ESA/Hubble/Digitized Sky Survey/Davide De Martin

The Small Magellanic Cloud (SMC) orbits a little farther away than the LMC, at a distance of about 60 kiloparsecs (195,600 light years; 2MASS).



N 81, a star-forming nebula only 12 light years (less than 4 parsecs) in diameter. Courtesy Heydari-Malayeri et al./ NASA/ JPL-Caltech/ Hubble Space Telescope. See larger image. NGC 346, a young star-forming complex in which newborn stars are still tightly clustered. Courtesy A. Nota et al./ STScI/ ESA/ NASA. See larger image. NGC 602, a star-forming region in which newborn stars are scattering and evaporating their native clouds of dusty hydrogen. Courtesy NASA/ ESA/ STScI/ AURA. See larger image.







All text is copyright Raymond Harris 2006-2008. Image credits appear in the accompanying caption.