Online Astronomy eText: Galaxies and the Universe
Our Galaxy and its Satellites
(also see The Local Group)
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. (ESO)
Click on the image for a far larger version, and additional discussion.

A Spitzer Space Telescope (false-color) infrared panorama toward the center of our galaxy
(Spitzer Space Telescope, Susan Stolovy (SSC/Caltech) et al., JPL-Caltech, NASA)
Click on the image for a far larger version of this and other images of the Milky Way.

The probable structure of our galaxy, based on recent surveys.
(R. Hurt (SSC), JPL-Caltech, NASA, GLIMPSE Team, apod080606)
Below, a labeled version of the same image shows the position of the Sun,
and indicates directions relative to the center of the Galaxy (galactic longitude 0).

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 most cases, are difficult to distinguish from the starfields within our galaxy even with a telescope.

 A graphic representation of the dozen known satellites of our galaxy within half a million light-years of the Milky Way (another dozen or so lie within the next half million light-years' distance). Other than the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), the satellites of our galaxy are generally referred to as the so-and-so dwarf, according to their direction. For example, the two satellites in the direction of Ursa Major are called Ursa Major I, or the Ursa Major I dwarf, and Ursa Major II (shown as UMII in the diagram), or the Ursa Major II dwarf. (To reduce the complexity of the diagram, the "dwarf" suffixes are not shown.)
 Save for our galaxy, whose approximate size (about 100,000 light-years diameter) is indicated, none of the galaxies' sizes are shown; but this makes little difference, as on this scale, only the Milky Way and the Magellanic Clouds would have noticeable size. Diagonals from the center of our galaxy are used to indicate the distances of the satellites (so long lines represent more distant satellites), while vertical lines are used to represent distances above (shown by lines and text in white) or below (shown by lines and text in yellow) the plane of our galaxy (that is, North or South of the Galactic Equator). To help visualize the plane of our galaxy, keep in mind that if seen from above, the Milky Way galaxy would appear roughly circular; the elliptical shape shown here indicates foreshortening caused by the angle of view used for the diagram.

The Magellanic Clouds
 This image shows 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 under good conditions, 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 (lower left), looks smaller because it is further away -- 210,000 light years, versus 180,000 light-years for the Large Magellanic Cloud, or LMC (upper right) -- and 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. (S. Brunier, ESO)

 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)

 The Magellanic Stream, an extensive (extremely rarefied) cloud of gas presumably removed from the Magellanic Clouds, is shown in the whole-sky image above, sweeping across nearly half the heavens, and most strongly concentrated at the Magellanic Clouds. The false-color pinkish stream, superimposed onto the visible-light whole-sky image, was measured by its radio-wave emission (the Milky Way is shown running horizontally through the center, as the coordinate system for this image is the galactic equator). The origin of the Stream is unknown. Originally, it was thought that it might be due to tidal forces from our Galaxy disrupting the Clouds, or by a passage through our galaxy's halo stripping gases from them; but its unexpected extent has led to other theories, including an interaction of the two Clouds with each other (about 2 1/2 billion years ago), which led to a burst of star formation (similar to that seen in M82), and subsequent ejection of the gas into intergalactic space. (David L. Nidever et al., NRAO/AUI/NSF & A. Mellinger, LAB Survey, Parkes Obs., Westerbork Obs., Arecibo Obs., apod100125)

Other Satellites of Our Galaxy (work in progress)
 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)

Leo I Dwarf Galaxy
(2000.0) RA 10:08, Dec +12 18 (galactic longitude 226, galactic latitude +49)
Discovered 1950 (A. G. Wilson)
Distance 800,000 light years (about 10% uncertainty)
Dwarf spheroidal galaxy (E;dSph), approximately 2000 light-years diameter
Mass (very) approximately 15 to 30 million Msun
 One of the most distant satellites of our galaxy, the dwarf elliptical galaxy Leo I, is visible as a pale mass of very 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 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 as yet undetectable amount of ionized gases scattered around it. (Scott Anttila, Wikimedia Commons)

 A better view of Leo I, digitally compensating for Regulus' influence. (This is a composite of two images, with more detail in the top image, as indicated by a diagonal 'line' running across the lower part of the image.) 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 extending from around 6 to 2 billion years ago, most likely as a result of gravitational interactions with our galaxy. (WikiSky snapshot based on Sloan Digital Sky Survey)

Leo II Dwarf Galaxy
(2000.0) RA 11:13, Dec +22 09 (galactic longitude 220, galactic latitude +67)
Discovered 1950 (R. G. Harrington, A. G. Wilson)
Distance 700,000 light years (about 10% uncertainty)
Dwarf spheroidal galaxy (E0;dSph), approximately 2000 light-years diameter
Mass approximately 30 million Msun

 A dwarf elliptical galaxy on the outskirts of our galaxy's realm of influence, Leo II consists primarily of very old, very metal-poor stars, implying that it has mostly escaped disruption and cannibalism by our galaxy. However, its core contains numerous stars formed within the last 8 billion years, indicating some promotion of star formation within that time.
 Because of its distance and the relative faintness of its mostly older population of stars, Leo II is difficult to notice with even large telescopes; but tens of thousands of its stars have been studied in some detail, in an attempt to unravel its structure and history. (WikiSky, Wikimedia Commons)