The Small Magellanic Cloud (SMC) is an irregular dwarf galaxy and a companion to our own Milky Way Galaxy. Unlike spiral and elliptical galaxies, irregular galaxies lack any appearance of organized structure. Like its neighboring Large Magellanic Cloud (LMC), the SMC appears as a huge and diffuse cloud in the southern nighttime sky. Both of these galaxies are named in honor of the explorer Ferdinand Magellan, who noted their presence in becoming the first to sail around the world nearly 500 years ago.
Note that the field of view of the images in this gallery is 4.4 degrees on a side, or about nine times the diameter of a full Moon. While considered dwarf galaxies, their close proximity means that the Magellanic Clouds subtend a large angle on the sky. The Magellanic Clouds and the Milky Way are members of the Local Group, a collection of about 30 galaxies loosely bound by their mutual gravitation.
Visible: DSS and Visible: AAO/Malin
The black-and-white visible-light photograph (above left) is about 3 degrees on a side, and has been padded with blank space to match the size of the other images in this gallery. The diffuse and fuzzy nature of the SMC makes it impossible to define a clear center to the galaxy. The brightest object, towards the northeast (upper left) of the photo, is an emission nebula. This is merely one of many pinkish nebulae seen in the color image (above right), which is a composite of three images (using B, V and R filters) taken with long exposures of 40-60 minutes. The red (or R) filter easily identifies the nebulae as supergiant H II regions, where hydrogen is ionized by the ultraviolet and visible light from newborn stars. The bright circular feature at the northern (top) edge of the field is NGC 362, a foreground globular star cluster located in our own Milky Way.
Mid-Infrared: MSX and Mid-Infrared: IRAS
This pair of pictures was obtained at longer IR wavelengths. The mid-infrared photo (above left) was obtained at 6-11 microns, or about ten times the wavelengths of visible light. The image was taken with a small infrared telescope aboard the Midcourse Space Experiment (MSX), a military satellite that orbited the Earth in 1996-1997. The satellite spent most of time its time studying infrared backgrounds near the limb of the Earth, but also devoted roughly ten percent of its observing time to mapping the plane of the Milky Way Galaxy and other selected regions of astronomical interest. Because the exposure time was rather short, only the brightest individual stars and nebulae in the SMC can be seen.
The 100-micron far-infrared image was obtained by the Infrared Astronomical Satellite (IRAS) in 1983. This false-color photograph adopts an unusual color-coding scheme, with white and blue denoting the brightest areas; red and violet correspond to regions of faint emission. White contours have been superimposed on the photograph, much like on a topographical map, and connect areas with the same far-infrared intensity. At these long wavelengths, there are a handful of distinct peaks in the infrared emission. Two of them are located in the western (right-side) portion of the galaxy and are due to dust being heated by young stars in the SMC. Note that the position of the largest peak (to the southwest) is in the same location as the densest concentration of stars seen in the MSX mid-infrared image.
Far-Infrared: IRAS, Visible: AAO/Malin and Ultraviolet: ASTRO-1 UIT
Now compare the far-infrared image (above left) with the previously examined visible-light photograph (above center). You will see that the peaks in the infrared (white and blue) are matched by the distribution of pink emission nebula in the visible-light photo! The visible and ultraviolet light being emitted by newborn stars in these regions not only ionizes the hydrogen gas (creating the pink nebula), but also heats up the embedded dust grains and causes them to glow in the infrared. Young and massive stars emit large amounts of ultraviolet light, a fact that is confirmed in the UV image (above right), where the peaks once again correspond to the dusty and gaseous nebulae.
Radio: Parkes and Far-Infrared: IRAS
This pair of images contrasts the radio emission from the SMC (above left) with the previously studied far-infrared. The radio data were obtained at a wavelength of 21.4 cm using the 64-m diameter Parkes Radio Telescope in Australia. The distribution of radio and far-infrared emission in galaxies is often similar, since they portray different evolutionary phases of the same stellar population: massive stars. The infrared light results from heating of dust grains by young stars, while the radio luminosity results from synchrotron emission resulting from supernova explosions. Despite the vastly different color schemes, you should be able to see a general similarity between the pattern of red knots in the radio image and the brightest regions of far-IR emission.
X-Ray: ROSAT and Ultraviolet: ASTRO-1 UIT
The mosaic of x-ray photographs, with circular fields of view, reveals only a small handful of sources. Some of the sources are x-ray binaries, a special type of binary star system, in which one of the members is a neutron star. The intense gravity of the neutron star sucks gaseous material off of the companion star and heats it to millions of degrees, thereby creating x-rays. [For additional information about x-ray binaries, please visit this site Other sources of x-ray emission in the SMC include supernova remnants. In addition, distant background quasars could account for some of the x-ray sources that appear in the field of view.