By no means all galaxies are spiral in shape. This illustration - of NGC 5253 in Centaurus - shows another less common galactic form, the irregular galaxy.
Vast accumulations of matter in which the force of gravity holds together great numbers of stars, as well as clouds of gas and dust. Typical galaxies are a hundred thousand light years across or more, and the largest can reach diameters of a million light years. Most fall into one of several regular structures, most commonly elliptical or spiral, but many other configurations are also seen.
Galaxies are not evenly distributed throughout the universe, but are bound by gravity into groupings. The smallest such aggregations are known as galaxy groups, each comprising of dozens of galaxies. The Sun's own Galaxy belongs to such a group, known as the Local Group, along with the Andromeda Galaxy, the Triangulum Galaxy, and many other smaller members. Galaxy groups are organised into clusters, and clusters into superclusters that can contain tens of thousands of individual galaxies. Between the clusters and superclusters are regions almost devoid of galaxies, known as cosmic voids.
History of Discovery
As seen from Earth, galaxies are typically too faint to be seen with the naked eye, with only a handful of exceptions. By far the most notable of these exceptions is the Milky Way, and we now know that this band of light that runs around the sky is due to the fact that the Sun lies within the plane of a spiral galaxy, and the Milky Way is the appearance of that galaxy as seen from within. Ancient Greek and Arab astronomers hypothesised that the Milky Way was formed from numerous distant stars as early as Democritus in the fifth century BCE, but before the invention of the telescope, they had no way to confirm this speculation.
It was not until Galileo turned his telescope on the bright band in the sky that he was able to confirm that it was indeed a formation of dense and very distant stars. Soon afterward, it became obvious that a faint elongated shape in Andromeda represented something remarkable, but the telescopes of the time could not distinguish its form in detail. So the relationship between the two - the Milky Way and the object in Andromeda, which were two spiral galaxies viewed from very different perspectives - was not yet understood.
It was not until the middle of the eighteenth century that the modern concept of a galaxy beyond our own began to develop, an idea encapsulated by Immanuel Kant in his phrase 'island universe'. By the early twentieth century, the true distance and scale of this objects began to be appreciated. In 1923, Edwin Hubble was able to locate Cepheid variables within the Andromeda Galaxy, enabling its distance to be estimated. Thus it was conclusively demonstrated that this was a galaxy of its own, separated from the Milky Way by a distance initially estimated at about half a million light years. (More recent calculations place the Andromeda Galaxy - one of the nearest neighbours of the Milky Way - at a distance of about two and a half million light years.)
Over the last hundred years or so, it has become clear that there are more than a few 'island universes' like the Milky Way, but millions spread far and wide across the broader universe. There is no definitive answer to the question of how many galaxies the universe holds, but conservative estimates place the number at two hundred billion, and there may be as many as two trillion galaxies in total. These galaxies can take many forms, but they do tend to fall into some basic recogisable structures.
Formation and Structure
It is not known with certainty how galaxies form and develop, but it is thought that the seeds of these structures lie in ancient clusters of dark matter, into which material was drawn and collected by gravity to gradually enlarge and expand. Early galaxies take a spiral, disc-like form around a central black hole, but over time these structured galaxies can combine and merge to produce more diffuse, undifferentiated structures, the elliptical galaxies. Galaxy formation was rapid and widespread in the early universe (the Milky Way Galaxy emerged soon* after the Big Bang, for example) but the rate of formation has slowed very considerably, and new galaxies in more recent cosmic history tend to be formed from mergers of existing galaxies.
A typical spiral galaxy, adrift in a still and starless void. This is M109 in a Ursa Major, a galaxy fairly similar to our own Milky Way Galaxy. Galaxies like this can be tens or hundreds of thousands of light yearsacross, and carry thousands of millions of stars. Imagery provided by Aladin sky atlas
At the core of most - and quite possibly all - large galaxies is a supermassive black hole, drawing in material from the surrounding regions in the centre of the galaxy. If sufficient material is available, and the black hole sufficiently massive, the core can become an active galactic nucleus, blasting intense electromagnatic radiation out into space. The Milky Way hosts its own central black hole, but its central regions do not host sufficient mass to make it an active galaxy of this kind.
At least in a galaxy with a spiral structure, this nucleus lies within a region known as the galactic 'bulge', a grouping of ancient redstars, primarily of the older Population II classification. The bulge swells outward from the main plane of the galaxy, hence its name. In many spiral galaxies, including the Milky Way, the bulge is not a round structure, but instead is elongated by gravitational forces to form a 'bar' shape. From this bar, or from the nucleus itself, extend the spiral arms, coiling outward from the centre to form the galactic disc. The stars in the disc are generally bluer, younger Population Istars.
The disc of a spiral galaxy extends across a vast distance. The Milky Way is a fairly typical example, with a diameter estimated at about 106,000 light years, though their are many larger examples, especially among the elliptical galaxies. There are also numerous smaller galaxies - known as 'dwarf galaxies' - which often swarm around larger galaxies like the Milky Way or Andromeda Galaxy.
The visible stars and nebulous matter that form the main structure of a galaxy are not its only components. The movements of the constituent stars within a galaxy betray the presence of greater gravitational force than the galaxy's visible elements can account for. This effect is attributed to dark matter, a component within the galaxy that cannot be directly observed, but nonetheless exerts detectable gravitational forces on the formation and rotation of the galaxy's form.
Many galaxies exhibit a halo of distinctive clusters of stars. These globular clusters are distinct from the open clusters within the body of a galaxy, and each contains thousands stars arranged in a dense spherical grouping. The Milky Way possesses dozens of these globular clusters - one hundred and sixty at the last count - arranged within a roughly spherical region around the main Galaxy. These clusters are ancient, with most dating back to the Milky Way's early evolution, and containing old redPopulation IIstars like those in the bulge around the Galaxy'score.
Larger galaxies are rarely alone. Just as galaxies are bound into broader groups and clusters, individual large galaxies will attract other smaller satellite galaxies. For the Milky Way, the most prominent examples are the two Magellanic Clouds, each of which was once a small spiral galaxy, now twisted into an irregular form by the Milky Way'sgravity. These are by no means the only satellites of the main Galaxy, with perhaps sixty of more other minor galaxies, small and faint, now known to exist within close proximity (though not all of these are necessarily satellites of the Milky Way). Other galaxies, too, can be observed to have satellites of their own: notably, the Andromeda Galaxy has two prominent attendant elliptical galaxies in orbit around it, M32 (sometimes called Le Gentil) and M110.
Galactic Evolution
In the early universe, before the formation of the first galaxies, the density of space was not constant. Through gravity, the denser pockets began to accumulate matter, gradually growing and expanding. In the centre of many, and perhaps all, major galaxies is a black hole, but the relation of this black hole to a newly-forming galaxy is not fully understood. The question of whether the black hole emerged as part of the formation process, or developed later through an accumulation of smaller black holes within the new galaxy, is one that is currently being actively studied.
The stars that make a galaxy shine are formed in molecular clouds within the galaxy. Areas in which these clouds are sufficiently dense will draw matter into denser and denser regions, eventually giving rise to protostars. The increasing forces in the cores of these objects eventually cause fusion to begin to occur, igniting them into true stars.
star formation does not occur in all galaxies at the same rate. In diffuse elliptical or lenticular galaxies, molecular clouds are rare, and few new stars are formed. Conversely, in irregular or spiral galaxies like the Milky Way, molecular clouds are comparatively plentiful, and such a galaxy typically contains many star-forming regions. In some galaxies, especially those that have been disturbed by interaction with others, variations in density are magnified and the rate of star formation is increased. This gives rise to the phenomenon of a starburst galaxy, one in which stars are produced at many times the typical rate. An example of this is the Cigar Galaxy, M82, in which interactions with the neighbouring Bode's Galaxy (M81) have triggered a vast starburst region within the central regions of the galaxy.
Galaxies cannot continue to form new stars indefinitely, and they will eventually reach a point where they no longer contain the gases needed to create new stars. This transition to a quiescent state is known as galaxy quenching. Quenching is a relatively rapid process on a galactic scale, seeing the rate of star formation collapse within a galaxy over a period of approximately one billion years. This is too rapid to be explained by the galaxy simply expending its reserves of gas, and there seem to be other mechanisms at play, a process that is still under investigation. There are signs that the Milky Way Galaxy may itself be undergoing this quenching phase, though star formation will not come to an end for many millions of years.
Galaxies can take a huge array of different forms, but most if these forms fall into one a few standard types. At the highest level, galaxies can be subdivided into two major categories: blue and red. Blue galaxies are those in which stars are still actively forming, and are mostly spiral in structure. Red galaxies or (also called 'early-type' galaxies) are those in which star formation has been quenched, and are typically elliptical or lenticular in shape. The name 'early-type' has its origins in older theories of galaxy formation, which suggested that red-type elliptical galaxies developed over time into blue-type spirals. This is no longer thought to be the case, but the old phrase 'early-type' has nonetheless survived.
Going beyond these two basic types, the general structure of galactic classification was devised by Edwin Hubble in 1926. On this system, there are three fundamental types of galaxy: 'E' (elliptical), 'S' (spiral) and 'SB' (barred spiral). These three types are connected in a diagram known as the 'tuning fork', with various E-type galaxies forming the handle of the fork, and the two different spiral types branching to represent the fork's tines. At the point where the 'E' handle splits into the 'S' and 'SB' branches, the 'S0" type is shown, representing lenticular galaxies, which share some aspects of both ellipticals and spirals. Galaxies that do not fit this system at all are given their own classification of 'Irr' or irregular.
Elliptical (E-type) galaxies are relatively undifferentiated galaxies of older red stars, forming into an elliptical or ovoid shape. They are classified by their appearance in the sky, with 'E0' representing a circular or near-circular galaxy, and 'E7' representing a narrowed ellipse (these classifications are based on observations from Earth, and may not represent the galaxy's true structure). Elliptical galaxies are thought to originate from the merging of other galaxies, and represent about 20% of all known galaxies. Some extreme examples are vast even in galactic terms, with diameters exceeding a million light years (that is, about ten times the diameter of the Milky Way).
Lenticular (S0-type) galaxies represent a transitional form. Like ellipticals, they are populated by old stars, and show little or no star formation. Like spiral galaxies, they are shaped like a flattened disc with a central bulge, but they show little or no spiral structure within the disc. Lenticular galaxies can sometimes show a bar in their central regions, in which case they are classified as SB0-type. They are thought to represent a later stage of evolution of a spiral galaxy, in which star formation has been quenched and the spiralling arms have lost their definition. Approximately 15% or more of galaxies take this lenticular form.
Spiral (S-type and SB-type) galaxies are the most common type, representing about 60% of all galaxies. Galaxies of this type have a bright central bulge, with winding arms extending outward through their discs. Commonly - in about two thirds of cases - the central bulge extends outward in a roughly linear form to create a barred spiral, with the arms extending from the ends of this bar. Unlike elliptical and lenticular galaxies, spiral galaxies form new stars regularly from the clouds of gas and dust contained within and between their spiral arms.
Together, these three types - elliptical, lenticular and spiral, account for the vast majority of major galaxies. There are other types, however, some of which overlap with the three main types, while others represent exotic and unusual classifications of their own.
Irregular (Irr-type or I-type) galaxies lack any regular pattern or symmetry, instead appearing as a random arrangement of stars and gas. Often irregular galaxies originated with a more structured form that was broken and reorganised by the gravitational forces of another galaxy. Prominent examples include the Magellanic Clouds, both of which were once spiral in form, but whose former structure is now barely detectable. Strictly, the clouds are not true irregular galaxies, but instead are given a classification that reflects their original nature, as 'Magellanicspirals' (or SBm).
Dwarf galaxies can fall into any of the other major types, but are typically either elliptical or irregular. Whereas a major galaxy like the Milky Way might contain several hundred billion stars, a dwarf galaxy contains far fewer. The smallest dwarf galaxies contain just a few thousand stars, while the largest might contain a few tens of billions. It is common to see dwarf galaxies twisted and warped by the gravitational forces of more massive nearby galaxies (as, for example, the gravity of the Milky Way disturbed the structures of the Magellanic Clouds). Dwarf galaxies are indicated by adding a 'd' to the main galactic classification.
Compact galaxies are those within which rapid star formation has resulted in rich and densely clustered stars within the galaxy. In principle, galaxies of any kind, or parts of larger galaxies, can be 'compact' in this sense, though the term is most commonly used for certain classes of dwarf galaxy. Compact dwarf galaxies have been observed to form especially readily in the central regions of large galaxy groups (for example, a number of such compact dwarfs are found in the heart of the Virgo Cluster).
Interacting galaxies appear where two or more galaxies are so closely bound by gravity that they exchange material or are physically distorted. Sometimes this effect can be relatively minor, such as a pair of galaxies being connected by a stream of stars or gas. In other cases, the results of the interaction can be truly extreme, as in the cases of the Antennae Galaxies or Mice Galaxies, where the galaxies are physically colliding with one another, creating chaotic forms and sending stellar material cascading into space.
Ring galaxies are an unusual configuration, in which the core of the galaxy is surrounded by an apparently disconnected ring of stars and dust, with little or no luminous material between the two elements. Some examples appear to have evolved naturally due to the conditions early in their formation, while others are thought to be the result of one galaxy passing through another, and disrupting its spiral structure. The classic example of a ring galaxy is Hoag's Object in Serpens, which is seen face-on from Earth so that its ring structure is clearly visible.
Active galaxies are those that host a supermassive black hole, consuming matter within the galaxy's nucleus and emitting extreme levels of radiation across the electromagnetic spectrum; such a core is known as an active galactic nucleus. In some cases, seen in visible light, an active galaxy can show a typical galactic form, albeit with an exceptionally brightnucleus relative to the rest of the galaxy. It is only in other wavelengths, such as radio, that the galaxy is revealed to be exceptionally energetic, and galaxies of this kind are broadly known as Seyfert galaxies.
In more extreme cases, the active nucleus can generate thousands of times the energy of a typical galaxy, giving rise to a quasar. In some cases, the black hole within the nucleus will produce jets of ionised matter, and when those jets happen to be oriented towards Earth, the active galaxy can appear intensely bright; such phenomena are described as blazars.
The Ultimate Fate of Galaxies
The fact that the universe is expanding means that galaxies, in general, are gradually accelerating away from one another. Almost all of the galaxies in the sky will eventually reach a point where their acceleration is so great that their light can no longer overcome it to be seen by observers within the Milky Way. These galaxies will then reach a threshold, billions of years in the future, beyond which they will permanently disappear from sight, as seen by observers on Earth.
While most galaxies vanish beyond sight, a handful will remain. The Milky Way is part of the Local Group, a collection of some forty galaxies that are gravitationally bound to one another (also including the Andromeda Galaxy and the Triangulum Galaxy, as well as many smaller members). In the far future, however, the arrangement of these grouped galaxies will be far different. Already, their mutual gravitational forces are drawing the members together, and in the extreme future - hundreds of billions of years from now - those same gravitational forces will cause all of the galaxies in the group to coalesce. Ultimately the Milky Way and its fellow members of the Local Group will be left as a single vast galaxy, surrounded on all directions by empty black space from which all other galaxies have departed.
* 'Soon' in this context is a relative term. Current estimates suggest that the universe itself is some 13.8 billion years old, and the Milky Way 13.6 billion. Thus the Galaxy formed about two hundred million years after the Big Bang, a little over one per cent of the universe's age, which can be considered 'soon' on these extreme timescales.
