The Milky Way Galaxy
The Milky Way Galaxy, as seen from the inside (from our location, of course).
The bulge near center is the direction of the center of the Galaxy.
(Serge Brunier, apod051004)
The position of the Sun (yellow dot below center), relative to our Galaxy.
(R. Hurt (SSC), JPL-Caltech, NASA, GLIMPSE Team, apod050825)
Satellites of Our Galaxy
Our galaxy has a number of smaller neighbors, which are believed to be satellites of our galaxy. Some of them may "orbit" our galaxy for some time, but eventually, gravitational interactions between our galaxy and the various satellites will distort their structure, tear them to pieces, and cause those pieces to be absorbed by our galaxy. In fact, there are a number of star streams in the outlying portions of our galaxy which are believed to be satellite galaxies which are in the process of being absorbed. Such galactic "cannibalism" is thought to be the way in which galaxies grow, and the long-term fate of our own galaxy may well be a cataclysmic collision with the closest large galaxy, M31, in which each galaxy is torn to pieces, and becomes part of the galaxy which results from those pieces combining to form a new, still larger structure.
The largest satellites of our galaxy are easily visible with the naked eye in southern skies, as apparently detached regions of the Milky Way. They are called the Magellanic Clouds, after Ferdinand Magellan, who noted them during his circumnavigation of the globe, in the mid 1500's. All other satellites are too small, and too faint, to be seen without a telescope, and in some cases, are difficult to distinguish from the starfields within our galaxy, even with a telescope.
The Magellanic Clouds
 This pre-dawn view of southern skies is partially obscured by clouds in our own atmosphere, but gives an otherwise good comparison of the relative size and appearance of the Magellanic Clouds, dwarf companions of our Milky Way Galaxy, named for the 16th century explorer, Ferdinand Magellan. When seen high in the sky, the clouds look like outlying portions of our own galaxy; but they lie beyond all but the furthest reaches of its halo. The Small Magellanic Cloud, or SMC (upper right), looks smaller partly because it is further away -- 210,000 light years, versus 180,000 light-years for the Large Magellanic Cloud, or LMC (lower left) -- and partly because it is only about half the size of the LMC, which is itself only a sixth the diameter, and less than one percent the volume and mass of our own galaxy. (Chris Schur, apod060809) |
 The largest of the Milky Way's companion (satellite) galaxies, the Large Magellanic Cloud (named after Portuguese navigator Ferdinand Magellan). A dwarf irregular galaxy, the LMC is only 15,000 light years in diameter; but since it is only 180,000 light years away, it is easily visible as an apparent extension of the southern Milky Way, nearly four degrees in width. A study in stellar birth and death, the galaxy is filled with clouds of brightly glowing gas lit up by clusters of stars forming within them, the most prominent of which is the Tarantula Nebula (the largest red knot on the upper left); and is also the site of the closest supernova of modern times (SN 1987A), which not only provided a relatively close-up view of this particular sort of stellar death, but also established an upper limit for neutrino mass, ruling out neutrinos as the dark matter which makes up most of the mass of the Universe. (Wei-Hao Wang (IfA, U. Hawaii), apod060510) |
 Above: The LMC in an infrared image, created with the Spitzer Space Telescope, which shows the distribution of warm clouds of gas and dust. A spectrum of false colors is used to highlight various types of structures. A faint blue band just below center is the combined infrared radiation from the dense clouds of old stars which occupy the central bar of the galaxy, while green-tinted clouds show regions where gas is scattered unevenly between the stars, red-tinted clouds show where dust is concentrated near hot young stars, and small red dots show shells of dust and gas ejected from old stars. The original image, a composite of three hundred thousand individual images, shows more than a thousand times greater detail than any previous image of the LMC.
Below: A portion of the previous image (near left center on the full image) at maximum resolution. (NASA/JPL-Caltech/M. Meixner (STScI) & the SAGE Legacy Team, apod060510) |
Other Satellites of Our Galaxy
 One of the closest satellites of our galaxy, the dwarf elliptical galaxy SagDEG, lies less than a hundred thousand light-years from our galaxy, and is being torn to pieces by its gravity. Recent studies show that there is a torus of debris from this devastation -- the Sagittarius tidal stream -- scattered around our galaxy, as shown in the artist's conception above. Over time, larger galaxies in a cluster of galaxies, such as our Local Group, assimilate fragments from other galaxies which may retain their individual motions, if not their individual structure, for long periods of time before interactions with passing stars sufficiently disturb their motions to conceal their origins. (Note: The odd designation of SagDEG is intended to distinguish it from the much more distant dwarf irregular galaxy, SagDIG.) (David Martinez-Delgado (MPIA) & Gabriel Perez (IAC), apod050529) |
 The location of SagDEG is shown above, as the large irregular region below and to the left of the central bulge of the Milky Way. At one time, the SagDEG was probably an elliptically shaped ball of stars, but passing through our galaxy over and over (about once every hundred or two millions of years) has stretched it into an irregular mass, and scattered parts of it all around the Sagittarius tidal stream, as shown in the previous image. In addition to its visible stars (barely visible, but visible), the dwarf elliptical, like all other galaxies, must contain large amounts of relatively diffuse "dark matter", to have a mass large enough to survive such passages. (R. Ibata (UBC), R. Wyse (JHU), R. Sword (IoA), apod980216) |
 One of the most distant satellites of our galaxy, the dwarf elliptical galaxy Leo I, is visible as a large mass of faint stars on the right side of this image. Located only a fifth of a degree from Regulus, the bright star on the left, Leo I is actually ten thousand times further away, being about 800,000 light years distant, on the outer fringes of our galaxy's halo, while Regulus and its nearby companions (among others, the moderately bright star just above the diffraction spike to the right of Regulus) are less than 80 light years away. Because of its closeness to such a bright star, observations of this galaxy are difficult; but studies of the motions of the stars in the dwarf elliptical indicate that it has about twenty million solar masses, and perhaps an equal but undetectable amount of ionized gases are scattered throughout the region around it. (Russell Croman, apod060619) |
 A better view of Leo I, ignoring Regulus and as much of its influence as possible. Most of the stars in this dwarf elliptical are far older than the Sun, and have less than 1% of the Sun's already minor amounts of elements heavier than hydrogen and helium; but rather surprisingly, Leo I has quite a few stars of sufficient brightness to limit their lifetimes to only a billion or three years, implying that was an episode of star formation during that time, most likely as a result of gravitational interactions with our own galaxy, or another satellite of our galaxy. (David Malin (AAO), AATB, apod991003) |
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