The Nature and Evolution of Galaxies
Prior to the 1920s, astronomers considered the Universe to consist of a single huge clustering of stars that was named the Milky Way (M.W.). As visualized with the naked eye in a clear, moonless desert night, the M.W. appears as a band running across the celestial sphere containg about a thousand visible stars. That number increases significantly when a time-exposure photo of the Milky Way using color filters is made, such as this one:
Using his newly crafted telescope, Galileo was the first to realize that the Milky Way contains many more stars that the eye can see unaided
This excellent photo montage, made in 1926 by E. Houck and A. Goode using a blue filter and a total exposure time of 45 minutes while moving their camera in snyc with the Earth's rotation, portrays the denseness of stars in the Milky Way, implying the number of stars were in the millions.
But as the Big Bang concept took hold, it was realized that expansion rates would carry distant "stars" well beyond the M.W.'s sphere of influence. In the late 1920s, Edwin Hubble was the first to present strong evidence that these stars were actually other galaxies. (However, historically Immanuel Kant in the 18th Century proposed that the Milky Way band was a vast collection of stars in a stretched out band and that some of the stars visible in the telescopes of his day were indeed other Milky Ways.) Thus the Universe became much bigger and contains a myriad (billions) of galaxies that make up the visible entities filling expanding space. Still, when one spots points of light in the heavens with the naked eye, what is seen are several planets, a large number of nearby stars, and a very few galaxies close by.
A galaxy is an organized concentration
or clumping of stars held together by mutual gravitational interaction in an aggregate containing millions to billions
of discrete individual stellar objects grouped into specific geometric arrangements (spiral; elliptical; irregular). A feel for this huge number of stars within a galaxy is given by this set of images:
The galaxy is NGC300, which is a neighbor to the Milky Way, being about 6.5 million light years away. The image in the upper left is a telescope view. In the upper right is an ACS Hubble image of this galaxy. Careful processing has resolved the many stars in the rectangular inset from that galaxy. While they appear to have a very high density, this is in a sense an illustion because, while the picture is 2-dimensional, the field of view is 3-dimensional, with stars at various slight differences in distances along the telescope's line of sight. Actually, the distance between stars is very much greater than their diameters.
Typical maximum dimensions of a galaxy range from 80,000 to 150,000 light years in space-time diameter. The central disk of the most photogenic type - the Spiral Galaxy - is about 10000 light years in thickness. Galaxies contain huge numbers of individual stars - a common number cited is 100 billion stars, but some have less and others up to a trillion (this estimate is based on the growing knowledge of the abundance of red and brown dwarf stars). At least 10 billion - probably many more - galaxies may have developed in the observable Universe. Of course, these numbers are estimates made by sampling regions of space close to us; attempts to accurately inventory all galaxies and stars by some counting approach are currently not feasible, and would suffer from incompleteness owing to the probable existence of stars/galaxies beyond observable limits.
While the question is not fully settled as to whether stars must have formed before (as contrasted to "during") the first galaxies, there is a growing consensus that a group of very massive, gradually heated stars emerged before any galaxies. These stars, almost entirely Hydrogen and Helium, organized rapidly, burned for a short time (around 3 million years), underwent collapse and exploded as supernovae. Being the first "furnaces"to produce heavier (atomic weight) elements, the destroyed stars yielded materials (including carbon, calcium and oxygen) that became incorporated in the first galaxies to form. This topic - element formation in stars - was treated in more detail on page 20-7.
Just as there are billions of stars in a single galaxy, such as the Milky Way, there are many billions of galaxies throughout the observable Universe. One of the largest Sky surveys to date uses the APM (Automatic Plate Measurement) technique to image galaxies between Magnitude 17 to 22 - out to intermediate spatial depths. Here is a composite made by the University of Nottingham that contains at least 2 million galaxies over a 100° portion of the sky outward from the Earth's South Pole. Individual galaxies can be resolved in enlargements but in this rendition the sky seems filled with galaxies (most rendered in red) - obeying the Cosmological Principle (the Universe is isotropic at large scales).
(A parenthetical aside: This image seems to show a crowded sky, with light sources [galaxies] almost touching. That would support Olbers Paradox: the sky should be uniformly bright if the Universe is infinite, since any ray looking outward from Earth would eventually intersect a major light source. But this is obviously not so -- the night sky is dark. The reason: the Universe is not infinite - at least the observable part; the distances between galaxies is huge with respect to individual galaxies; and, the majority of radiation reaching Earth is outside the Visible light spectral range [Cosmic Background Radiation; X-ray sources, etc.])
A synopsis of galaxy formation - for the spiral type - is evident in this simplistic model, which is generalized:
In this model, which has aspects that go back to Laplace in the 18th Century, a cloud of Hydrogen/helium gas begins to form myriads of stars. As this continues, the cloud may contract somewhat and the assemblage of stars begin to rotate around a common center. This cloud has also be called a "halo" and as such may be the precursor to the Halo that surrounds the Milky Way and other galaxies (see below). With rotation, there is a tendency for the cloud to assume a more oblate ellipsoidal shape and begin to spin. The spinning produces strings of stars in at least several distinct arms. When well developed, the stars have organized into a spiral galaxy.
Now to some more details: As the dispersing mix of primordial H and He (He comprises about 10% of the various atomic species present) atoms, photons, and other particles continued
to expand (thereby progressively decreasing in density), it eventually cooled
to temperatures around a few degrees Kelvin (see Cosmic Background Radiation
on page 20-9). Large-scale variations (called fluctuations or seed perturbations)
in mass and energy density, whose origin can be traced to the early moments
of the Big Bang, occurred at random throughout the enlarging Universe. These regions where the density was greater eventually grew (as described below) into protogalaxies and then galaxies.
As early as the first 100 million years (m.y.) (cosmic time; measured from the moment of
the Big Bang) and perhaps as far back as just after the Decoupling Era, but especially
in the first 1 to 2 billion years, protogalaxies (incipient or first stage assemblages of the Hydrogen-rich gas that evolve into galaxies)
began by means of gravitational attraction to develop as denser regions throughout the expanding Universe. This process was guided by gravity-driven irregularities or ripples in the almost homogeneous distribution of particles in the early stage expansion of the Universe. These denser strands or pockets of matter evolved over time as stars formed and collected into fullblown galaxies, with most now observed having formed during the first four billion years. The principal hallmark of galaxies is that they consist of billions of stars (whose nature and development are described on page 20-2).
Amazingly, despite the vast number of stars in a galaxy, most of the Universe's space is nearly empty of luminous matter, making up intragalactic and, even more so, intergalactic open regions. Likewise,
individual stars in a galaxy are widely separated (a scale analogy: if a star
is represented by a marble just 1 centimeter in diameter, the average distance to its nearest neighbor stars is around 300 kilometers [~200 miles]). All stars together (totaled for all galaxies) comprise just about 1 part per million by size within the space dimensions calculated for the known Universe: thus in the total volume of observable space, "void" dominates
and luminous objects are an exceedingly small part (far less than one might
expect by looking through a telescope in which much of the field of view seems
occupied by points of light [galaxies or galactic clusters], since there are
huge distances between them in the direction of viewing). In terms of mass, stars likewise constitute less than 2% of the total presently calculated for the Universe.
While most galaxies are very old, some are younger and a small fraction
may even have started forming in the last few hundred million years. One example (below) of an embryonic galaxy is Hubble-X , in the constellation Sagittarius (NGC6822), which is about 1.6 billion light years from Earth. Evidence based on star characteristics indicates the cloud started producing stars only about 4 million years ago, but a well-defined galactic shape is yet to emerge.
However, in general within galaxies the majority of larger stars has since expired (by supernova explosions, etc.) even as new stars (including those of masses up to 100 times that of the Sun) continually form (some recently, in Universe time) from the debris and gases remaining in the intragalactic materials that persist throughout the history of the galaxy. Other materials are drawn in as encountered during a galaxy's travels in space.
The starting point of galaxy formation requires accumulation of Hydrogen-rich gas, with some Helium, in a great cloud (many millions of light years in dimension). This stellar nursery may have been similar to what are referred to as a "Molecular Cloud" or the large "Giant Molecular Cloud" (GMC) because much of its Hydrogen is combined as H2 (see below). However, the typical galactic cloud would have been much larger - containing billions of stars and being at least a few million light years across - than these molecular clouds which usually contain stars only in the millions or less that are observed today in and around existing galaxies.
Some of these clouds are huge. The largest found to date is nearly 200 million light years in size and consists of several lobes of gas within which galaxies appear to be forming. This rendition of its appearance through a telescope (this appears to be an artist's conception) shows its shape:
This image below, made from radio telescope data (see 20-4), shows a huge cloud of cold Hydrogen gas (green) in the Hickman Compact Group:
For this accumulation (build-up) of gases to happen there must initially be localized regions of the expanding Universe whose density is slightly greater than the generally uniform distribution of matter and photons that, most cosmologists believe, was the outcome of the processes operating during the earliest stages of Big Bang expansion. Studies of cosmic background radiation (see page 20-9) indicate these density disparities may have been as small as 1 part in one hundred thousand. The slight differences in density also give rise to slightly greater gravitational forces which act to draw material towards these local perturbations.
As more matter accrues within a growing cloud, its internal gravity continues to increase and draw in still more gases.
The molecular cloud eventually reaches a density that requires it to then undergo local clumping of gases into clots that grow into still denser concentrations to become
stars (these smaller clots can exist for much of the galaxy's life but are the sites of further star formation).
The next HST image shows huge clots of gas and dust in a more advanced stage of development in which stars will eventually form en masse as part of a spiral or globular galaxy (see below):
Many star-forming clouds are very rich in dust, in addition to the Hydrogen gas, which make them appear as discrete dark clouds. As we shall see on page 20-7, these clouds contain various amounts of heavier elements (but still only a small fraction of the total number of Hydrogen and Helium atoms present) produced within the first stars and dispersed when these exploded as supernovae. A prime example of vast dust cloud "nurseries" from which stars are born are shown in these next images. The first shows much of the Eagle Nebula (M16), which we first depicted on page 20-2:
The great protuberances of dust-gas within the Eagle Nebula are called pillars, evident in the above image. Below that, is the trio of 3 pillars in this nebula; this image is now near the top of the list of most "spectacular" of all HST images captured so far. (See page 20-11 for three more views of this nebula.)
This assemblage of gas and dust, in which new stars have or will be formed, is not as large as some others that have been detected. This is evident in the Spitzer Space Telescope image of the W5 nebula (in the Cassiopeia constellation) in which the Eagle nebula image is shown to scale (inset) for comparison with the much larger W5 in which many nascent stars are present.
Studies of GMCs prove informative regarding the processes involved in building stars within galaxies. To some extent, they are miniature versions of the super-clouds that evolve into galaxies (a galaxy also can grow by capturing or contacting more GMCs). Fortunately there are several GMCs close (1500 light years) to Earth that serve as a "laboratory" for observing star formation processes and subgalactic growth. These are found within the famed Orion ("The Hunter") constellation that occurs near the celestial equator near the star Taurus (for location see star chart labeled "Southern Horizon, Winter" near the middle of page 20-2). Probably the most studied of all nebulae is the dominant feature within this constellation that is known as the Orion Nebula. It consists of two prominent nebulae M42 (larger) and M43 above it (in some images this appears to be a discrete entity but high resolution images show it to be continuous with M42).
This ground telescope view shows the larger M42 and smaller M43 as drawn apart, with the former including the section known as the Trapezium. Below that is an image made by the HST:
And, M43 by itself:
The interior of M42 as seen in visible light shows the clouds of Hydrogen and dust typical of GMCs; the same area in infrared light brings out the principal stars in this region.
Within M42 are four very large, bright stars that comprise a geometric figure known as a trapezium. Some of the brightest large stars in the Trapezium of M42 are shown here optically:
The next view shows the clouds around the 4-star Trapezium and the stars themselves:
The Trapezium region is a major "nursery" for stars forming and evolving, as shown in this 6 panel montage:
That these clouds are thermally active, especially where the clots are organizing into protostars, is evident in this ratio image made from thermal bands, as follows - 20µm/10µm - in this view of a cloud near the center of the Orion galaxy, made using the TIMMI2 (second Thermal IR MultiMode Instrument) on the 3.5 m telescope operated by the European Southern Observatory:
B33 is better known as Barnard's nebula which, when enlarged in this HST image, produces one of the most "popular" of images from that telescope, given the nickname of "Horsehead Nebula". Here are three views:
Returning now to more general considerations, one model ("top down") of early galaxy evolution considers a cloud
to fragment into star groupings as it develops from hot dark (radiating but
not luminous) gaseous matter. Another galactic model ("bottom up")
begins the process with localized multi-star formation from cold dark (low levels
of EM radiance) matter, with subsequent aggregation into fewer stars that grow
mainly by collision (sometimes described as "cannibalism") with one another. Recent observations
suggest the bottom up model describes the predominant process.
In the first billion years or so (the oldest galaxy found so far became organized about 400,000 million years after the Big Bang) of the Universe, as galaxies developed, models for their spatial configuration may have looked something like this computer-generated simulation of filaments within which gases of varying density (high = yellow; lower = blue) lead to organization into individual or clusters (see below) of galaxies. :
Another similated model, again highlighting filaments of Hydrogen gas, shows a similar pattern.
Several points made in the press release accompanying this illustration: 1) the development of filaments establishes connections between zones of higher Hydrogen concentration; 2) this pattern is in part related to the much smaller size of the Universe at the time, with greater density of Hydrogen; 3) as this stage progresses star formation is very rapid; 4) some stars grow to sizes of 200 or more times the mass of the Sun (roughly twice as large as the biggest stars observed today); 5) these stars burned under conditions that led to nuclear reactions that synthesized elements up to iron in atomic number (discussed on page 20-7), with iron itself being abundant; and 6) such massive stars rapidly exhausted their fuel and exploded violently as supernovae (page 20-6), so that as more advanced forms of galaxies evolved the stars comprising them contained varying amounts of the elements heavier than Hydrogen and helium (later stars and galaxies were even further enriched in these elements as burning-heavy element production continued to add the heavier elements to the gases and dust from which galaxies developed and more stars emerged and larger ones "died").
That such filaments actually exist is suggested by this HST view of a very old network of filamentous galaxies and stars in deep space.
Thus, star formation that goes hand-in-hand with galaxy evolution is a general process
that can take place wherever widespread-to-local concentrations
of dominantly Hydrogen gas produce clouds of matter of sufficient density to initiate gravitational
contractions. Typically, only a few percent of a cloud's mass will be organized into stars.
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:
From The Galactic Odd Couple by Kimberley Weaver, Scientific American, July 2003
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.
As alluded to in previous paragraphs, between star groupings in the arms and central region of spiral galaxies there remains much Hydrogen gas and dust in large clots from which more stars will form later. The gas is ionized (HII) and radiates at several discrete wavelengths. The Wide Field Imager (WFI) of the 2.2 meter MPG/ESO telescope at the southern hemisphere La Silla Observatory has imaged the spiral galaxy NGC300 with a filter that selectively passes ionized Hydrogen radiation, so that the stars are screened out leaving only the Hydrogen clots. As seen below, these clots are irregular in shape but widespread:
Sky surveys (especially with the Hubble Space Telescope) indicate that spiral galaxies contain a large number
of individual stars, clusters, and even small satellite galaxies, and considerable Hydrogen gas and dust, dispersed in galactic space around the central disk in what is called the halo region. A halo appears to be a roughly spherical envelope that surrounds galaxies in general. Haloes develop around protogalaxies and aid in the subsequent development of each type. The density of gas and dust within the halo space is overall less than that within the central disk. Globular star clusters (see below) are the most distinct entity in this distribution. This next figure is a simple diagram of the four principal components of spiral galaxies; the green marks the halo region:
The galaxy NGC 5746 possesses a distinct gas halo, seen in this HST image as a uniform blue region. Its population of halo stars appears to be low.
The genesis of the spiral type of galaxy is fairly well understood. It starts with gravitational action within a denser part of the intergalactic medium. Dark matter exerts a control over the resulting collapse of hydrogen gas in the protogalactic molecular cloud. This collapse tends to be asymmetric (uneven). Turbulence within aids in setting the contracting cloud into rotation. When dense enough, the gas begins to organize into individual stars. The cloud further contracts preferentially in one direction and a disc shape results from centrifugal flow within the star and gas assemblage. The angular speed of the stars rotating about a center varies with distance outward; this imparts a curvature to the pattern of stars within the disc. The stars tend to locate in streamers that become the arms of the spiral galaxy. This diagram is a simplified version of this model (there are variants involving collisions, shock waves, density waves, etc. that modify the basic idea):
Also enclosed by a dark matter halo is the second major galactic type, the Elliptical Galaxy,, marked by
mostly old stars (populations up to 1011 individuals). Ellipticals comprise about 15% of regular types. Such a galaxy is now believed to originate through collisions,
tidal disruption and other interactions, between small galaxies or even large spirals. leading to merging and destruction of the spiral arms (some ellipticals may have formed in the early Universe simply by a collapse mechanism still poorly understood). Elliptical galaxies rotate more slowly than spiral ones, so that the tendency to evolve into a flattened disc is thwarted. Elliptical (the majority are almost spherical) galaxies, generally more massive than spiral galaxies, usually occur in groups or clusters. Both Giant and Dwarf varieties are known. The typical elliptical galaxy contains a larger percentage of red stars than found in spiral galaxies (those have more blue or hotter stars than red); however, being more compact ellipticals are usually brighter than spirals. Recent observations of elliptical galaxies have found that there are still many younger blue stars. Elliptical galaxies, although more massive than spirals, contain much lower amounts of dust and are gas-poor which suggests that overall they contain a larger fraction of older stars than in the more abundant spirals. Here is a typical example; in this and many other images of ellipticals, the individual stars are not resolved - they are so densely packed that the galaxy image appears to be uniformly bright:
The Giant Elliptical Galaxy is probably the brightest of any category of galaxies. It can contain as many as a trillion stars. Among the best known is Messier 87 (M87) shown below as seen in visible light.
Giant Ellipticals are strong sources of radiation beyond the visible range (discussed on page 20-4). Although we are "jumping the gun" a bit, it is instructive to show M87 as an X-ray source (detected by Rosat) and as a Radio source:
As may be the case for most elliptical galaxies, which holds that many (most) of these form by collisions (see below), the Giant Galaxy type almost certainly results from multiple elliptical galaxy collisions, as depicted in this simulation:
Both spiral and elliptical galaxies can group in clusters from a few tens to hundreds of thousands of individual galaxies. Such clustering is a direct consequence of the uneven distribution of Dark Matter into halos that developed soon after the Big Bang. The image below is a giant cluster of many hundreds of elliptical galaxies (most burned to the red stage) lying at 9 billion light years from Earth. The cluster density makes for an apparently huge single entity but is actually the glow from many close-spaced ellipticals. The image is a composite of ESA's XMM-Newton X-ray space telescope and ground imagery taken through the European Space Observatory (Vis-IR) telescope in Chili.
This HST view (within the Coma Cluster) shows a Giant Elliptical Galaxy on the left and a rather diffuse Spiral Galaxy on the right; being at similar distances the relative sizes are valid:
Elliptical galaxies tend to occur in clusters of this one type, but with a few gas-poor spiral galaxies within a cluster. Spiral galaxies are more scattered in space.
Rare among these principal galaxy types is the so-called Ringed Galaxy. This example is known as Hoag's Object, found in the constellation Serpens and situated about 600,000,000 light years from Earth. In size, its diameter is 120,000 l.y., slightly larger than the Milky Way. Its central nucleus consists of densely packed yellow (old) stars which together resemble an elliptical galaxy. The ring consists mainly of younger blue stars. In the gap in between there is a dearth of stars of either type.
The origin of Ringed Galaxies is still uncertain but a stage of redistribution after the collision of two galaxies is a plausible explanation. Note the resemblance to the Pinwheel galaxy shown above; the difference is the gap within Hoag's Object.
A second example has brought out a bit of humor among the astronomer clique. Look at this Hubble image of Ringed Galaxy ARP 147 (on the right):
Those who viewed this image noted that the spiral galaxy on the left was shaped almost as a "1". This led one wag to rate this scene as "a perfect 10".
There is another galaxy type - the Lenticular Galaxy - that some consider deserving of its own category. Generally, most such galaxies are equivalent to the SO group at the base of the two spiral branches of the Hubble galaxy classification. In side view, a lenticular galaxy is just that - a double convex shape, much like an optical lens. When seen face on (as from the top), the SO type has no distinct or discernible individual spiral arms but in the part beyond the center (which may be a massive core but a few have very non-descript cores) individual stars are evident but distributed randomly and at various densities. When Lenticular galaxies are examined in this way, they show rudimentary spiral arms, suggesting they are an incipient spiral transitional to that class or are a degenerate spiral no longer typical of that class. Most of their stars are old (yellow) both in the core and the surroundings, which makes this type similar to elliptical galaxies - except for its pronounced disk shape. However, gas and dust seem in short supply, suggesting that little subsequent evolution is likely. Several examples of Lenticular Galaxies are shown in this next sequence; see their captions for details.
About 10% of galaxies are neither spiral nor elliptical but have what can be described as "irregular" shapes. As a type example, here is NGC 1569, about 7 million light years away:
Many of these irregular galaxies are actually two (or more) colliding galaxies (discussed in more detail on page 20-4). Some are described as "peculiar galaxies", a term coined by Halton Arp who compiled a catalog of these in 1966. A classic peculiar galaxy is the pair NGC 6621/6622:
Globular star clusters are intermediate between simple star clusters and galaxies - each is an aggregate of 100,000 to a
million stars. These are much like miniature elliptical galaxies but have far fewer individual stars. Like the latter many seem to have a predominance of old stars. The largest concentration of these stars is in the interior of the cluster. Typical Globular Cluster densities are several hundred stars per cubic light-year (compared with typical densities of 0.01 to 0.1 stars per (ly)3, which is the norm for most galactic space). Because of the higher densities within clusters, the frequency of star collisions (normally a rare event) is considerably greater.
Although some clusters are found around elliptical galaxies, the globular clusters mostly occur in much larger numbers within the halos of spiral galaxies, i.e., in orbits at all angles to the galactic plane within an imaginary sphere that may be 200,000 light years or more in diameter. The Milky Way contains less than 200 such clusters; Andromeda almost 500. Most globular clusters contain old (> 7 billion years) stars. Globular clusters have proved to be a primary means of determining the ages of the oldest stars in the Universe. The halo region around a galaxy also contains millions of isolated (non-clustered) individual stars, or small groupings. Below is globular cluster NGC6093:
The largest globular cluster around the Milky Way is NGC5139, estimated to contain up to 10 million stars.
Some globular clusters are more open and contain less stars, as holds for cluster M3:
The Wide Field Camera on the Hubble Space Telescope has captured a view of just how dense are the stars in globular clusters. Here is an image of a small part of the Omega Centauri globular cluster, just outside our galaxy about 13000 light years away. At least 30000 stars appear in this segment of the cluster; most of these are similar in size and luminosity to the Sun, and some of the larger ones (yellow) are Red Giants. The cluster is 12 billion years or older in age. A large number of blue-white stars formed early on have since lost their luminosity as they converted to white dwarfs and neutron stars.
Special techniques can resolve individual stars in globular clusters. M13, the Great Hercules Globular Cluster, is imaged in full below, with an inset of the central region in the upper left, and two insets on the right in which individual stars are separated to give an indication of actual spacing:
Astronomers have concluded that globular clusters formed mostly during the early stages of galaxy formation (and since most galaxies appear old, clusters seemed to be ancient cosmic features) and then became much rarer as the Universe expanded. It is now known that clusters have been forming continuously over time from denser pockets of Hydrogen in the halo regions. Young(er) clusters have been found around galaxies a few billion light years or less from the M.W. These contain stars with concentrations of heavier elements that could only have reached those levels after many episodes of stellar explosions; thus many of their stars must be young. Star clusters have been observed near galaxies that collide, indicating that one process of formation is related to interactions between merging galaxy pairs. Thus globular clusters probably formed at maximum rates in the early Universe but intermediate age and even young clusters indicate that this component of galactic systems can develop at any time.
Occasionally, telescopes locate masses of small and large stars that are still in the process of organizing into individual globular clusters. Such is the case for this grouping that is part of the Large Magellanic Cloud (a galaxy group discussed on the next page). 30-Doradus is seen here as a color composite with red contributed by x-radiation, blue from UV radiation, and green from ionized Hydrogen gas:
Recently, another class of galaxies has been discovered. Called "fuzzy" clusters because of their appearance, those few found so far occur in the plane of a galaxy rather than well out in its halo (as do most globular clusters), are larger 50-100 l.y. across (globulars are usually 15-20 l.y.), and consist of dominantly old red stars. Here is a view made by the HST and supported by Keck Telescope observations that shows a fuzzy cluster on the upper left; a farther out globular cluster appears to its right.
Still another category of globular clusters, long predicted, as at last been imaged and verified. This defining image is shown below; note the four small boxes:
These clusters are not associated with galaxies as is the usual case. They are isolated in intergalactic space, a fact that has led them to be called "orphan clusters". They contain up to a million stars. Although only a few have been detected so far, they likely are fairly common throughout the Universe. The favored explanation is that they were torn from parent galaxies by other galaxies and dragged into open space; alternatively, they may just be incipient clusters trying to build to full-scale galaxies.
In the Milky Way, there are star clusters (mainly in the halo) that consist mainly of old stars. There may be only a few thousand, up to 100000, individual stars. The grouping is controlled by mutual gravitational attraction. This type is called an Open Cluster. The Pleides, shown on page 2a, is one example. Here is another (M11, the Wild Duck Cluster, about 4 light years away):
The above galaxy types have in common the fact that they show some type of geometric organization. But objects in the Universe that are galaxy-size but seem to be irregular in shape have been given the colloquial name of "blobs". Recent studies of these blobs indicate that when at least some are resolved in infrared light, a number of spherical objects appear. This image pair gives a good example:
The Galaxies subsection is continued on Page 20-3a: Click on Next below.
