Our galaxy, the Milky Way, is surrounded by a swarm of dwarf galaxies each composed of 100,000 to billions of stars. Many of these galaxies are easy to see; for example, the Magellanic Clouds are visible to the naked eye for observers in the Southern Hemisphere. Many of the dwarfs are far more difficult to spot; some of these were discovered telescopically in the nineteenth century. One class of nearby dwarf galaxy eluded discovery for even longer. In the late 1930s, Harlow Shapley and Walter Baade reported the discovery of a new type of ‘star system’ in the constellation Sculptor. Barely visible on their original plates, this new object was the first example of a dwarf spheroidal (dSph) galaxy ever found. It is now known to be one of nine such galaxies orbiting the Milky Way.

By convention, the nearby dSph galaxies are usually named after the constellation in which they are located. The nine dSph galaxies closest to the Milky Way are all very likely gravitationally bound to our Galaxy. The remaining dSph galaxies reside within the Local Group of galaxies, and all but one are satellites swarming around the nearby the Andromeda Galaxy. In recent years dSph galaxies have been discovered in large numbers within other nearby Groups Of Galaxies and Galaxy Clusters. When these numbers of known dSph galaxies are extrapolated to the universe as a whole, it becomes apparent that dSph galaxies are by far the single most common type of galaxy by number.

If dSph galaxies are so common, how did they elude discovery for so long? Even in the darkest, most remote sites on Earth, the sky glows faintly with optical radiation. This light comes mostly from atomic and molecular emission from the upper parts of our own atmosphere, from sunlight reflected off dust scattered throughout the solar system and within our atmosphere, and from individually invisible, but very numerous faint galaxies and stars. Most galaxies have surface brightnesses comparable to that of the night sky; that is, the total light they emit over the total area of the galaxy as seen in the sky is comparable to the amount of light the sky itself emits. These high surface brightness galaxies—Spiral and Elliptical Galaxies—are the ones most commonly pictured in popular astronomy books. They represent what most people think about when they imagine a ‘typical’ galaxy. In contrast, dSph galaxies have far lower surface brightnesses, glowing faintly with a surface brightness as low as only 1% that of the night sky. These galaxies are the closest examples of true Low Surface Brightness systems.

Finding such ghostly objects would be similar to spotting a 40 W lightbulb in front of a large searchlight or discerninga1m rise in a 100 m tall plateau from far away. Sculptor was found in the 1930s only because it is one of the brightest nearby dSph galaxies, it is located in a particularly dark part of the sky, and many photographic plates were available by that time to confirm its discovery. Many of the most recently discovered dSph galaxies were not found by eye, but from sensitive electronic measurements of photographic images of large regions of the sky. Because they are so difficult to spot, it seems certain that many more nearby dSph galaxies remain to be discovered.

As their name implies, dSph galaxies are among the smallest, least luminous galaxies known. Ursa Minor, for example, shines with a total luminosity of about 300,000 times the luminosity of the Sun. By comparison, the Magellanic Clouds—dwarf companions of our Milky Way emit as much light as 2 billion Suns. Even some of the Milky Way’s Globular Clusters emit more light than the faintest dSph galaxies. Most dSph systems show a centrally concentrated structure with a core region of nearly constant, but extremely low, surface brightness. The outer parts of dSph galaxies slowly fade into the night sky; in many cases, no well-defined outer boundary can be measured. The central cores range from 1000– 3000 Light-Years in diameter, while the ill-defined outer boundaries typically extend 3–10 times further out; the dimensions of the Local Group dSph galaxies are representative. The ultra-low core surface brightnesses and small sizes are the features that principally distinguish dSph systems from all other classes of galaxies.

Many dSph galaxies also exhibit well-defined central nuclei that are far more compact than the galactic cores. In many cases, the optical appearance of the nuclei resemble that of large globular clusters. Nuclei in dSph galaxies are most common in the more luminous systems, but, even among the brightest dSph systems, not all possess nuclei. Only one dSph galaxy in the Local Group unambiguously contains a nucleus: NGC 205, a companion of the Andromeda Galaxy. These galaxies range in shape on the sky from nearly circular to highly elliptical. In terms of the Hubble Classification system, they range from E0 to E7. It is not unusual for the core of a given galaxy to exhibit a different elliptical shape and orientation than the outer region of the same system .

Most of the radiation from dSph galaxies is emitted in the form of starlight in the optical portion of the Electromagnetic Spectrum. The lack of strong emission lines, infrared, or radio radiation suggests that these galaxies are generally devoid of an interstellar medium composed of gas or dust floating freely between the stars. Recent observations have revealed a possible, though complicated, relation between some nearby dSph galaxies and clouds of neutral hydrogen gas visible only at radio wavelengths. These galaxies also emit very little light that can be attributed to extremely young stars or star-forming regions. This suggests that dSph galaxies are currently dormant systems that formed most of their stars more than about 1 billion years ago.

Because most distant dSph galaxies appear as featureless smudges of light, we must turn to the closest dSph satellite galaxies within the Local Group to try to understand the true nature of these little, but extremely common, systems. At the time of the initial discovery of Sculptor by Shapley and Baade, it was clear that dSph galaxies were composed of stars similar to those found in globular clusters. Moreover, dSph galaxies and globular clusters were the only sorts of objects known to inhabit the outermost regions of the halo of our Galaxy. These features and the simple structure of both types of star systems seemed to indicate that dSph galaxies were simply puffed up versions of otherwise normal globular clusters.

This belief persisted for many years, but in time enough evidence accumulated to begin to suggest that something far more complicated was going on within dSph galaxies. The Ursa Minor galaxy, for example, appeared to be so extended that it could not possibly be gravitationally bound. The implication was that we were witnessing the disruption of this dwarf galaxy as it passes close by the Milky Way. The problem with this was that independent calculations of the tidal forces suggested that Ursa Minor was sufficiently far from the Milky Way that it should not yet be falling apart. As astronomers began to carry out detailed studies of the individual stars in the closest dSph systems, they confirmed that the stars in dSph galaxies are deficient in heavy elements compared with the Sun. Oddly, the global properties of the stars in color–magnitude diagrams of dSph galaxies did not confirm these low element abundances. This seemingly contradictory behavior is known as the second parameter problem and it is present in nearly every dSph system. These dwarfs also contain luminous Carbon Stars and unusual Variable Stars that are not found in globular clusters.

In the early 1980s the first studies of dSph galaxies employing modern electronic detectors and large-aperture telescopes revealed two unexpected features that helped explain these puzzles. The first was that the stars with individual dSph galaxies appear to be moving far too fast relative to one another to remain bound within such small galaxies. That is, if we estimate the mass of each galaxy based on the number of visible stars, we find that every one of these galaxies appears to be disintegrating before our very eyes, not just Ursa Minor! Because this seemed highly improbable, the assumption that we see all the mass in dSph galaxies was questioned. One alternative model is that the galaxies are much more massive than expected from their visible matter because they contain Dark Matter. Dynamical models that include dark matter do adequately explain the internal motions of the stars in all dSph systems. In the most extreme cases, only 1% of the mass of the galaxy is visible. The rest of the mass is composed of dark matter whose nature, unfortunately, remains a complete mystery. If this is true, dSph galaxies have the largest proportion of dark mass of any known type of galaxy and can therefore claim to be the ‘darkest’ galaxies known. Curiously, dark matter models also suggest that no dSph galaxy has a mass less than about 10 million times the mass of the Sun.

The second revolutionary observation proved that, unlike globular clusters, dSph galaxies contain significant numbers of ‘intermediate-age stars’. Thus, these galaxies appear to have formed stars over a large fraction of the lifetime of the universe while globular clusters generally formed nearly all of their stars very soon after the universe first formed. Modern studies of nearly all the nearby dSph galaxies reveal evidence of a complicated star formation history, each galaxy being different from all the others much like the ages of children may differ greatly from family to family even within a small town or neighborhood. How and why individual dSph galaxies have been able to support extended and episodic star formation histories remains unclear.

Every galactic group or cluster that has been adequately studied has revealed large numbers of dSph galaxies. Most of these galaxies appear to be quite similar to the ones found in the Local Group, but some interesting variations are also found. For example, the nearby M81 group contains a few dSph galaxies that are considerably more extended and have even lower central surface brightnesses than the known local systems. Nearby galaxy clusters such as the Virgo, Fornax, Centaurs and Coma Clusterss contain hundreds to thousands of individual dSph galaxies (figure 2). Dwarfs with nuclei are particularly common in these environments, especially among the brighter galaxies. In most cases, the nuclei appear to contain stars that are distinctly different—possibly older or with different elemental abundances—from the majority of stars in the parent dSph galaxies.

Dwarf spheroidal galaxies appear to concentrate more towards the centers of galactic groups and clusters than any other type of galaxy, including spiral galaxies, large elliptical galaxies or the relatively dust-free S0 Disk Galaxies. This is also observed in the Local Group where the vast majority of all dwarf companions nearest the Milky Way and the Andromeda Galaxy are dSph systems. Astronomers speculate that proximity of dwarfs to larger galaxies or the centers of large clusters helps remove gas from the smaller systems as a result of tidal forces or perhaps from interactions with isolated gas clouds also orbiting the larger systems. Either process can truncate star formation in the affected systems, allowing them to fade to very low surface brightnesses and ultimately resulting in the formation of dSph galaxies. Alternatively, dwarf galaxies near larger companions or near the dense centers of clusters may accelerate their star formation due to interactions with their larger neighbors. Such galaxies would run out of gas and appear today to contain mostly middle-aged stars.

Two observations do suggest that dSph galaxies may interact tidally with their massive parents. First, none of these galaxies are found extremely close to a large galaxy, implying that tidal forces destroy dSph systems that venture too near to their parents. In the case of the Milky Way, the limiting distance appears to be about 150 000 light-years. Second, we actually see one example of the disruption occurring today. The Sagittarius dSph is unique among all known dSph galaxies in its enormous size and low density, both indicative of tidal disruption. This case really does appear to be one of a dSph galaxy in its final death throes. Models of its interaction with the Milky Way suggest that within about one billion years, Sagittarius will have dissolved away as a distinct galaxy, its stars scattered through the halo of our Galaxy.

Beyond about 100 million light years, dSph galaxies appear too small and too faint to be identified easily. However, their presence may still be inferred indirectly. Deep counts of galaxies have revealed a population of so-called faint blue galaxies located at such great distances that we are seeing the systems as they appeared many billions of years ago. Some of these could conceivably be dSph galaxies caught during one of their many strong episodes of star formation. Such events make the galaxies brighter and much bluer and consistent with the properties of at least some of the faint blue galaxies observed. Faint Irregular Galaxies appear to have global properties similar to dSph galaxies, but with the addition of young stars and a significant interstellar medium. Perhaps these represent modern-day examples of galaxies that have undergone evolution from a young, blue phase and are changing into typical dSph galaxies. Because dSph galaxies are so common and because most popular models of Galaxy Formation in the early universe suggest that small galaxies formed first, it may even be possible that dSph galaxies represent the basic unit of galaxy formation. If so, the dSph systems we see today may represent fossils of objects similar to the very first galaxies ever formed.