Galaxies and Galaxy Classification

   On any clear moonless night, even the slightest glance upwards will reveal that stars are not just randomly strewn across the sky. From reasonably dark rural locations, a vast soft glow can often be seen stretching across the sky from one horizon to the other--especially during the winter and summer months. This is the Milky Way, an immense gathering of gas, dust, and more than 100 billion gravitationally bound stars including our very own Sun! Appearing quite amorphous from our particular point of view, the Milky Way is actually an intricate structure of grand design known as a spiral galaxy. The word galaxy itself being a derivative of the Greek word “galaktos”, meaning milk! The existence of such enormous collections of stars and matter is concrete evidence that mass and energy are not evenly distributed throughout the universe. As a matter of fact, it is now well established that even galaxies themselves tend to congregate into huge clusters often containing more than a thousand members…

   When looking up at the nighttime sky, every star visible to the naked eye resides within our local realm of the Milky Way Galaxy--which we call home. As we peer outwards beyond the edges of our very own galaxy, however, we are allowed a much different perspective on a vast number of other galactic forms. Because they exist in countless numbers, galaxies by far represent the most abundant query for the amateur deep-sky enthusiast. For reasons not yet fully understood, galaxies are nearly always found gathered into compact groups and large clusters. Since many such “galaxy clusters” contain upwards of a thousand individual members, and literally thousands of such clusters are known to exist, it is quite easy to see how the number of galaxies visible from our unique perspective is simply staggering. Even the Milky Way itself is the dominant member of a rather small assembly known as the Local Group of Galaxies. Other prominent members of the Local Group include the relatively bright Andromeda Galaxy (Messier 31), the Pinwheel Galaxy of Triangulum (Messier 33), and the nearby Magellanic Clouds.

    The only thing more perplexing than the vast number of galaxies visible in the sky is their great plethora of observed shapes and sizes. Ranging from only a few hundred thousand to several trillion times the mass of our very own sun, such objects are very diverse in their mass and it’s distribution. Galaxies can generally be classified into four fundamental categories based largely upon their morphological appearance: Elliptical (E), Lenticular (SO), Spiral (S), and Irregular (I). Each of these major categories is further divided into many different subcategories based upon particular patterns of light distribution . Although this may seem somewhat straight-forward, the classification process is naturally quite complicated, often resulting in confusion for those not familiar with the terminology and the galactic classification scheme. No matter what size, shape, or morphology, one thing is for certain--galaxies are the fundamental blocks on which the universe and modern astrophysics are built. Directly below is the classic "tuning fork" diagram, an extremely simplified way of visualizing galaxy classification. For a more in depth look into the world of modern galaxy classification, please see the chart at the very bottom of this page.

Hubble's Classification Scheme

Ellipticals (E):

    Based upon size alone, elliptical galaxies represent the most diverse class of galaxy presently known. “Ellipticals” range in size from the cluster-dominating giants (cD/E+ types) containing trillions of stars to the more common dwarfs (dE) so often seen associated with larger, more massive galaxies. Being predominantly composed of highly evolved, relatively cool K and M-class stars, ellipticals typically contain very little interstellar gas and dust. They, therefore, appear quite ruddy or orangish in color photographs and do not exhibit bright nebulae nor clusters of young, hot stars. Another defining characteristic of elliptical galaxies is their light distribution/brightness profile. When the surface brightness of such a galaxy is plotted using isophotes, the distribution of light from the central core to the outer envelope follows a fairly specific pattern, which almost matches that of a common globular star cluster! This characteristic inverse relationship between luminosity and it’s radial distribution in elliptical galaxies is known as De Vaucouleur’s R() Law.

    Elliptical galaxies range in shape from perfectly round (E0) to nearly spindle-like (E7). The exact formula used to calculate the number after “E” for a particular subclass (En) is n=10*(1-b/a), where as b/a is the ratio of the galaxy’s minor axis to it’s major axis. Of course, intermediate types are classified by the amount of elongation observed in the galaxy, with higher numbers following the “E” representing higher degrees of ellipticity. Since they are generally subclassed by their apparent elongation, the exact shapes of elliptical galaxies has long been an area of much controversy. For example, a football-like(E5) or cigar-shaped (E7) galaxy viewed from one end of it’s major axis would appear essentially circular against a 2-dimensional background like the sky. Such an observation would typically lead one to suspect an actual tri-axial object (such as an ellipsoidal E5 type galaxy) to be spherical (E0) in nature. So the amount of ellipticity measured by Earthbound telescopes is highly dependant on the galaxy's orientation in space as well as it's true morphology. The one defining characteristic of all elliptical galaxies, however, is the lack of a distinct “disc“ of gas, dust, and stars. Below are merely three examples of the wide variety of elliptical galaxies known. They are Messier 89 (E0-1), Messier 86 (E3), and NGC 2768 (E6), respectively:

Messier 89 Messier 86NGC 2678
Lenticulars (SO):

   Once considered an intermediate galaxy type bridging the gap between spirals and ellipticals, “lenticulars” have now become well established as a distinct class on their own. Like ellipticals, lenticulars are quite diverse in their size and apparent shape. Some appear nearly spherical, others very lens-like, while a few seem to simply not fit into any other category of galaxy! The one common characteristic of all lenticulars, however, is that they all contain two very distinctive physical components--a spherical central bulge and a flattened outer disk. In other words, their brightness profiles do not follow the well-governed, even dispersion of light so characteristically observed in elliptical galaxies. In this way, lenticulars actually have more in common with majestic spirals like our very own Milky Way. The vast majority of lenticulars, however, seem to be oddly composed of cooler, more evolved stars much like those found in giant ellipticals. Even though their disks may often contain obvious quantities of interstellar dust, lenticulars seem to be completely void of younger, bluer stars and the glowing HII regions that signify areas of recent star formation!

   Most often found associated with rich galaxy clusters, lenticulars are divided into two basic families: barred (SBO) and ordinary (SAO or just simply SO). A third family of intermediate objects between barred and ordinary lenticulars, also known as transitional or “mixed” lenticulars, is recognized as (SABO). For reasons not completely understood, disk-type galaxies sometime form "bars" of material protruding from the core region. Such bar-like structures can appear as nothing more than a pair of opposing lobes (often noted as SBO-), or they can present themselves as huge lateral structures spanning the entire width of the galaxy (similarly noted as SBO+). Ordinary lenticulars (SO) having relatively smooth disks are typically subcategorized by the prominence of their disks. Such objects having very conspicuous disks are usually designated as SO+, while those having larger nuclear bulges but less obvious disks are known as SO-. As always in astronomical classification schemes, there are many intermediate types, and the lenticular class sometimes seems to be the “catch-all” class where many bizarre and peculiar disk-type galaxies are often “tucked away” for future study. Following are three distinct samples from the lenticular class of galaxy. They are NGC 3115 (SO-), NGC 4429 (SAO+), and NGC 936 (SBO+), respectively:

NGC 3115 NGC 4429 NGC 936
Spirals (S):

   Arguably the most majestic structures in the galactic realm and possibly the entire known universe, spiral galaxies are the pinnacle of astronomical artwork. Comprised of an obvious central bulge and flattened disk much like lenticulars, spirals have the wonderful distinction of sporting two or more winding arms of material often wrapping the core in an alternating tapestry of light and darkness. For it is within the spiral disks of galaxies like the Milky Way that stars like our very own Sun are born. Color photographs of spiral galaxies are wonderful presentations of the actual stellar physics occuring in such objects. The cores of spirals are fairly consistent with those of other galaxy types--composed mostly of relatively cool, orange and reddish colored stars that are generally older than the Sun. The disk of a spiral, however, typically contains very large quantities of interstellar gas and dust. Since these are the major constituents of star formation, it should come as no surprise that spiral disks are often riddled with pinkly glowing HII regions and young clusters of hot, blue stars!

   Despite a common misconception, only a very small percentage of a spiral galaxy’s mass is associated with it’s luminous arms. On the contrary, it is the exaggerated rate of star formation and copious amounts of interstellar dust that make these spiral features visible at all. It is along the edges of these winding arms of matter that the interstellar medium is compressed enough to induce star formation, resulting in the creation of the galaxy’s most luminous stars. It is also along these arms that the highest concentration of interstellar dust is often found. With such luminous and dark features existing within close proximity to one another, it is quite easy to see how the pattern of a spiral galaxy is actually defined by the concentration of matter within its flattened disk. Throw in a bit of axial rotation around a common central core, and viola--you have a very fundamental model of a spiral galaxy!

   The classification system commonly used to define spiral galaxies is based upon the scheme originated by pioneer galaxy researcher, Edwin Hubble himself. Just as other disk-type galaxies, spirals are divided into two distinct families: barred (SB) and ordinary (SA or just simply S). A third family of intermediate objects between barred and ordinary spirals, also known as transitional or “mixed” spirals, are recognized as (SAB). Based upon the tightness of the spiral pattern, such galaxies are given designations ranging from Sa to Sb, Sc, Sd, and Sm for those objects having the most inconspicuous spiral features. Sa-types are characterized as having the most tightly wound, nearly unresolvable spiral arms accompanied by relatively large central bulges. Meanwhile, Sd-type spirals have highly resolved but loosely wound spiral patterns typically accompanied by very diminutive central cores. Of course, all of these subclasses also pertain to barred spirals (ex: SBa or SBd), and there exists many intermediate classes in which spirals that seem to fall between two particular classes can be placed (ex: Sab or Sdm). Furthermore, many researchers also note particular spirals based upon the design of their spiral pattern. For example, galaxies demonstrating obvious symmetry, being composed of long continuous spiral arms emerging from a distinct core are known as “grand design” spirals. Meanwhile, those spirals having a somewhat clumpy appearance, whose discs seem to be composed of many relatively short or fragmented arms are subjectively known as “flocculent”. The chart below demonstrates only a few of the many variations of the classification scheme for spiral galaxies:

Spiral Galaxy Variations

Irregulars (I):

    Often considered to be the "oddballs" of the Hubble classification system, irregular galaxies represent a very complex and diverse group of objects. Galaxies are usually categorized as irregular if they do not fit nicely within any of the other classes, such as elliptical, lenticular, or spiral. Many irregulars do show obvious disk-like structures, however, they are often very disrupted or somewhat chaotic in appearance. Some are Magellanic by nature, appearing much like the Milky Way's nearest irregular neighbors (the Small and Large Magellanic Clouds). Many irregular galaxies show distinctive filamentary structure, while some appear completely amorphous. A few of these enigmatic objects actually appear much like extragalactic HII regions, being composed of less than 10 billion solar masses of material, of which most is apparently undergoing some form of rapid star formation!

    One such type in particular, known as a Blue Compact Dwarf (BCD), resides in a category all it’s on. Photographically, BCD’s often display a small faint outer envelope dominated by a very prominent core region. However, the luminous cores of such objects appear more like gigantic ionized HII regions than normal galactic nuclei. Hence the reason that BCD’s are often referred to as HII Galaxies. Though their exact nature remains a mystery, BCD’s are thought to be either newly-forming galaxies or normal dwarf galaxies undergoing an intense outburst of star formation. Based upon their spectra, several BCD’s are thought to contain “super clusters” of more than 10,000 young O-type stars!

    Despite their great diversity, nearly all irregulars seem to be composed mostly of young, blue stars that have only recently been created. Hence, color photographs of such galaxies often reveal numerous pink HII regions and many gatherings of young, blue stars. Pictured below are three fine examples of the bizarre world of irregular galaxies. These are Messier 82 (IO), NGC 4449 (IBm/HII), and IC 1613 (IBm), respectively:

Messier 82NGC 4449IC 1613
Peculiars (P/pec):

    Generally speaking, one or more peculiarities can be applied to all of the major galaxy types. Ellipticals are occasionally noted as doubtful ( ? ), Lenticulars sometimes sport outer rings ( R ), extremely edge-on spirals often appear as spindles (sp), and even Irregular galaxies can be tagged as uncertain ( : ). The peculiar (pec) suffix is also often applied to galaxies that are known to either be interacting or actually merging with other galaxies in space. So called “mergers” are quite arguably some of the most peculiar specimens in the entire galactic realm! Another peculiar type are the Wolf-Rayet Galaxies (WRG‘s), which have bizarre emission-line spectra resembling that of massive Wolf-Rayet ejections. WRG’s are thought to be normal galaxies that are undergoing a recent outburst of star-formation, resulting in the production of thousands of super massive O-type Wolf-Rayet stars which dominate the galaxy‘s otherwise normal spectrum. Below are most certainly a few of the more peculiar galaxies presently known. They are NGC 5128 (SO pec), NGC 3718 (SBa pec), and NGC 520 (I pec/merger):

NGC 5128 NGC 3718 NGC 520

Modern Galaxy Classification

Classes       Families   Varieties    Stages            Type

Ellipticals E Compact cE Dwarf dE Ellipticity (0-6) E0 (intermediate) E1.5 "cD" E+ Lenticulars S0 Ordinary SA0 Barred SB0 Mixed SAB0 Inner ring S(r)0 S-shaped S(s)0 Mixed S(rs)0 Early S0- Intermediate S0 Late S0+ Spirals S Ordinary SA Barred SB Mixed SAB Inner Ring S(r) S-shaped S(s) Mixed S(rs) 0/a S0/a a Sa ab Sab b Sb bc Sbc c Sc cd Scd d Sd dm Sdm m Sm Irregulars I Ordinary IA Barred IB Mixed IAB S-shaped I(s) Magellanic Im Non-Magellanic I0 Compact cI Peculiars P (Peculiarities can apply to all types) pec Uncertain : Doubtful ? Spindle sp Outer Ring (R) Pseudo Outer ring (R') Polar Ring (PR)

Notice: All images used in this article are reproduced from the Digital Sky Survey