There are four general types of galaxies, classified by their geometric shapes (morphologies) and distributions of the stars that comprise them. These are 1) Spirals (the most common), 2) Ellipticals, 3) Dwarfs, and 4) Irregular. Most astronomers add a fifth type - Lenticular - intermediate between Spirals and Ellipticals. The major forms are indicated, with their symbols, in this diagram (the Dwarfs and the Irregular or Peculiar groups are not included but are discussed below). In Hubble's time, opinion favored a left to right evolutionary trend, i.e, ellipticals may (but do not necessarily) morph into spirals. Today, whatever changes occur are from right to left. As mentioned on this page, one process involves collision of two spirals that removes the arms, builds up the central core, and leads to an elliptical.
The Hubble classification has remained essentially intact to the present but has been refined to include some non-mainstream types. This is an up-to-date version:
Before looking at each type in detail, it should be mentioned that another classification scheme is currently evolving and gaining favor with some astronomers. This is based on the amount, distribution, and activity within the gas and dust that comprises the interstellar part of a galaxy. The dust behaves in a diagnostic way in infrared light - it both absorbs and emits light in those wavelengths, thus bringing out characteristics not seen in visible light. Elliptical galaxies have low dust contents. Spiral galaxies contain far more dust than the other types: Here is an HST optical image of NGC5746 which suggests some dust and a Spitzer Space Telescope (IRAS) spacecraft image of that galaxy which shows an abundance of dust, shown in red, owing to its thermal radiation in the infrared.
In our discussions of galaxies, we will stick with the standard classification based on visible light morphology. Spiral galaxies, which seem at present to be the dominant type, consist of stars arranged in a flattened disc wherein younger (blue) stars are strung out in several prominent spiraling arms that emanate from a central nucleus or bulge that is comprised of a denser collection of older (yellow to orange) stars. Compared to the entire Universe - with both galactic and intergalactic components filling the space - this central core is about 100 billion times the density of the Universe as a whole (this also applies to elliptical galaxies described below). Typical spiral galaxies, such as those shown below, are about 100,000 light years in diameter; disc thicknesses are less than 10,000 l.y. The disc shape results from a greater degree of collapse in one direction and a significant transfer of angular momentum to the disc arms as a effect of tidal (gravitational) interaction with nearby galaxies (clots of dark matter). Spiral galaxies slowly rotate; the galaxy containing the Sun completes one full revolution about its center in 200 million years. Stars closer to the center move faster than those further out, which contributes to the bending that makes up the spiral arms. This general diagram (artist's conception) of a spiral galaxy shows its principal parts; note the central region labeled "bulge" - this is often associated with AGNs described below:
One of the Sa types, that is characterized by a minimum of spiral arms, is this one, found in the Draco constellation:
This HST image shows the well-organized spiral galaxy NGC4414 (NGC refers to New General Catalog, one of several systematic listings of stars and galaxies observed through telescopes), one with multiple arms in which much gas and dust still remains:
Another spiral, with prominent dust and red to blue stars in its arms, is M51, the Whirlpool galaxy.
NGC1232 is one of the most "perfect" spiral galaxies yet imaged; it lies 100 million light years from Earth. It has 6 distinct spiral arms, each separated by regions of low star density. Some consider it a twin to our Milky Way. Here it is imaged by the European Southern Observatory (ESO) telescope, using UV, Blue, and Red band images to make this color composite:
A different impression of NGC1232 is given by this ESO Visible-Infrared telescope image, which indicates that the galaxy is warmly glowing:
The relative "thinness" of a spiral galaxy is evident when it is oriented so as to be seen "edge-on", that is a side view looking parallel to its spiral plane. NGC4013, 55 million l.y. away, shows this perspective. Note the large amounts of cosmic dust which masks most of its stars.
The dust in the outer arms is apparent as a band in this edge-on view of the Sombrero galaxy:
Spiral galaxy M64 has an anomalous outer ring of dust. The stars in the inner arms are rotating in one direction (clockwise) whereas the outer dust is rotating in the opposite direction. One explanation for this seemingly contradictory behavior is that the dust is not part of the original galaxy but has been captured and continues its previous rotational direction; star formation is especially frequent in the shear zone between the two rotating subsystems:
One of the most famed galaxies is Andromeda (M31) - largely because it is closest to Earth (about 2.2 million light years), can be seen by the naked eye, and thus its structure is visible in telescopes used by amateur astronomers. Here is an HST image of Andromeda:
This next image of Andromeda was made by Spitzer's infrared cameras at two wavelengths. By utilizing different spectral bands, star distribution can be separated from dust. The blues represent large active stars; the reds are caused by dust (containing polycyclic aromatic hydrocarbons) that are distributed in spiral bands from which stars will later grow:
A spiral galaxy can contain up to 2-3 hundred billion individual stars; a few have even larger numbers. Around this type of galaxy are lesser numbers of stars, scattered and isolated or in globular clusters (but still well into the millions) arranged in a "halo" that extends for thousands of light years above and below the plane of the disc (see below). However, the bulk of the mass within the halo, with its important gravitational effects, is not luminous and is now presumed present as Cold Dark Matter (CDM; discussed again on page 20-9). Thus, the halo is often referred to as the Dark Halo. Its importance in galactic evolution and stability is discussed near the bottom of this page.
Spirals can develop unusual distributions of stars outside the disc; in the next example a ring has formed around NGC4650A that could be part of a second galaxy that has collided with the obvious spiral, stripping off stars from that galaxy's spiral arms.
However, such protusions perpendicular to the galactic plane can show a compositional difference. In this view of galaxy M82 (see page 20-4 for additional images of this galaxy), the reddish material moving away from the plane is excited Hydrogen in much richer amounts than within the galaxy which here shows as bright blue from its myriads of stars.
A recent combination of a Hubble image and a ground-based telescope image shows the red areas in the above image of M82 are actually composed of jets of Hydrogen moving at humongous speeds of 1.6 million kilometers/hr (1 million mph)
M82 (sometimes called the Cigar Galaxy because of its shape as seen head on) is the prototype of what is known as a Starburst galaxy. Stars are being produced at much higher rates than normal (commonly this results from a collision with another galaxy [page 20-4]). In this rendition, M82 appears as a nearly continuous white which suggests much above average brightness that would ensue from ultrahigh star formation rates:
Recent studies have shown that a Starburst galaxy may experience long periods of normal star formation rates, then it undergoes much higher rates for 100 to 200 million years, followed by relative quietude. Here are three Starburst galaxies involved in this study:
Many spiral galaxies, including our Milky Way, have an increased number of stars emanating in a narrow zone directionally from their centers. These are known as barred galaxies. Two renditions of NGC1097, a type example of barred galaxies, are shown below. The bar effect depends to some extent on the orientation of the galaxy as viewed. The greater population of stars in the bar segment represents greater production outside the core, with the stars being drawn out as the spiral arms develop. The importance of barred galaxies is considered on the next page.
About 2% of spiral galaxies contain an especially bright central region (an AGN, see page 20-5). These, known as Seyfert galaxies, are marked by a notable concentration of dispersed ionized Hydrogen gas, excited to high levels luminosity, i.e., the brightness is not just from stars alone (those present tend to be blue [relatively young]).
This central region emits radiation that gives rise to strong, broad spectral lines. This spectral signature is similar to, but distinguishable from, a typical quasar (see page 20-6). The cause of the glow may, as is also the case for quasars, be a Black Hole at the galaxy nucleus (there is growing evidence that Black Holes are generally present at the center of spiral galaxies). This glow probably emanates both from a much denser concentration of stars and from excited gases. The Seyfert class is one that has an Active Galactic Nucleus (AGN), whose trademark is that it is a strong radio wave source (however, most radio galaxies are elliptical). The core of an active Seyfert galaxy (in the Constellation Circinus) at a distance of 13 million light years from Earth is a very bright AGN. The greens and reds are excited states of Hydrogen gas presumably heated by radiation from the Black Hole.
A large AGN dominates this next galaxy (the Pinwheel) which contains a thick circlet of stars (a Starburst) just beyond the dense interior concentration of stars and outwardly scatterings of dispersed stars in the galactic plane but without well developed arms.
AGNs are a minority in both spiral and elliptical galaxies, but they are the source of extreme energy output. Within them almost exclusively are the quasars (see page 20-6) that are the visible manifestations of matter falling into Black Holes. There is growing evidence that supermassive B.H.'s are at the center of most (perhaps all) larger galaxies that have a bright central bulge. AGNs reached their peak around 4 billion years after the Big Bang, having taken some time to build up to the condition in which huge energy outputs result from their numbers of quasars, and are less frequent in younger galaxies. Although still not proved from observations, many astronomers believe that one or more AGN episodes took place in both elliptical and spiral galaxies at some stage(s) of their histories. The relationships between AGNs and Starbursts (described later), and their mutual association with Black Holes, will be established on page 20-4.