The Hubble Space Telescope
Before setting out to explore the Universe's development and history since the first moments after the Big Bang, we want to pay homage to what this writer (and many, many others) consider the greatest scientific instrument yet devised by mankind - the first of the Great Observatories: the Hubble Space Telescope (which we will often refer to as HST). No other instrument has advanced our knowledge of astronomy and the Universe as much as this splendid observatory in outer space. Perhaps no other astronomical observatory has captured the public's imagination, with its numerous sensational pictures, as has the Hubble. HST has provided many extraordinary views of stars, galaxies, dust clouds, exploding stars, and interstellar and intergalactic space, extending our view to the outermost reaches of the Universe. HST has brought about a revolution in our understanding of Astronomy and Cosmology.
This, the finest telescope up to the present, receives its name to honor Edwin Hubble, the man who confirmed much about the existence, distribution, and movement of galaxies, leading to the realization of an expanding Universe. Here he is at work in the 1920s on the 100-inch Palomar telescope:
Prior to the 1990s, surveying and studying stars and galaxies as visible entities required the use of optical telescopes at ground-based locations. This ground photo shows the Kitt Peak observatory complex in Arizona, one of the premier observatories in North America.
But such telescopes were hampered by adverse effects contributed by the atmosphere. Even when placed away from cities and on high mountains, the effects of the atmosphere, smog, any nearby lights, etc. degraded these images. The multinational Observatories atop Mauna Loa on the Big Island of Hawaii illustrates high quality terrestrial conditions for earth-based astronomy.
As the space programs developed, astronomers dreamed of placing the telescopes in space orbit where viewing conditions were optimized. HST is the outgrowth of a concept first suggested in 1946 by Lyman Spitzer who argued that any telescope placed above Earth's atmosphere would produce significantly better imagery from outer space. (Spitzer has been honored for this idea by having the last of the Great Observatories named after him; see page 20-3.) The HST was Launched from the Space Shuttle in April of 1990 after 20 years of dedicated efforts by more than 10000 scientists and engineers to get this project funded and the spacecraft built This is a photograph of the HTS in the Bay of the Shuttle.
And here is a photo of the HST in orbit, as seen from the Shuttle:
A general description of the Hubble Space Telescope and its mission is given in this review by the Space Telescope Science Institute.
This cutaway diagram shows the major features and components of the HST:
But, as scientists examined the first images they were dismayed to learn that these was both out of focus and lacked expected resolution. HST proved unable to deliver quite the sharp pictures expected because of a fundamental mistake in grinding the shape of its primary (2.4 m) mirror. The curvature was off by less than 100th of a millimeter but this error prevented focusing of light to yield sharp images.
In December of 1993 the Hubble was revisited by the Space Shuttle. At that time 5 spacewalks succeeded in installing corrective mirrors and servicing other sensors. The package was known as COSTAR (Corrective Optics Space Telescope Axial Replacement).
After the first servicing mission, the striking improvement in optical and electronic response is evident in the set of images below made by the telescope, which show the famed M100 (M denotes the Messier Catalog number) galaxy viewed by the Wide Field Planetary Camera before (bottom left) and after (bottom right) the correction. For an indication of how much HST improves astronomers' views of distant astronomical bodies, one of the best earth-based telescope images, from Kitt Peak, is shown at the top:
Another way to judge the improvement that HST provides by being above the atmosphere is to compare absorption spectra for hydrogen in the Visible and Ultraviolet coming from a quasar source as recorded by a ground based telescope and HST.
The increased sensitivity of the HST instrumentation, unimpeded by atmospheric absorption, provides more detected hydrogen lines in both the UV and Visible regions of the EM spectrum.
Information on both original instruments and those added later appears in this site prepared by the Space Telescope Science Institute. The history of HST in terms of instrument placements and servicing missions, from the early days to the present and a look to the future is given in this chart prepared by the Space Telescope Institute:
The original 5 instruments onboard HST were: the FOC (Faint Object Camera); FOS (Faint Object Spectrograph) GHRS (Goddard High Resolution Spectrograph); HSP (High Speed Photometer) and WFPC1 (Wide Field Planetary Camera); added since (by subsequent visits using the Space Shuttle) are NICMOS (Near Infrared Camera and MultiObject Spectrometer); STIS (Space Telescope Imaging Spectrograph); ACS (Advanced Camera Surveyor); FGS (Fine Guidance Sensor); and WFPC2; future additions (when Shuttle flights resume) will be the COS (Cosmic Origins Spectrograph) and WFPC3.
Thus, HST is being further improved even beyond its initial ten year life expectancy - now, hopefully, extended to 2010. A third Shuttle servicing mission was successfully completed in two stages: December 1999 and March of 2002. In addition to replacing or "repairing" existing systems on the satellite bus, a new instrument, the ACS (Advanced Camera for Surveys) was added; it represents a tenfold improvement in resolution and clarity. Below are four images of astronomical objects; their identity and description are included in the caption (remember, just click on the lower right):
During this repair mission the NICMOS (Near Infrared Camera and Multi-Object Spectroscope) sensor, out of working order for nearly three years, was repaired and upgraded. This pair of images, ACS on the left and NICMOS on the right, shows the improved quality of imaging of part of the Cone Nebula, bringing out more details of the dust that dominates this feature:
Many of the most informative HST images can be viewed on the Space Telescope Science Institute's (Baltimore, MD) Home Page . HST has imaged numerous galaxies at different distances - almost to the edge of space - from Earth that are therefore also at different time stages in the general evolution of the Universe (see below). The following illustration shows both spiral and elliptical galaxies (but not the same individuals) at 2, 5, 9, and 14 billion years after the Big Bang in a sequence that represents different stages in this development. (Note: recent determinations of the Hubble constant [see page 20-9] indicate the 14 b.y. age may be too high.)
Hubble has had a remarkable impact on the study of the Universe. In its honor, the Astronomy Picture of the Day (APOD) web site, in celebration of its 10th anniversary, has compiled a collage of a variety of the more spectacular images acquired by HST, supplemented with a few images made by other instruments. This is reproduced here; be on the lookout for many of the individual embedded images in this montage elsewhere in this Section.
However, technology and design are allowing ground-based telescopes to "catch up" with the HST, at least for those galaxies that are relatively close to Earth. The resolution and clarity of recently activated ground 'scopes is on a par with their Hubble counterparts. This results from better detectors, improved optics, and the ability of a ground telescope to dwell on the target for much longer time spans (allowing buildup of the incoming radiation to generate a bright image). This is illustrated with this pair of images which show a Highton Compact Group galaxy (HCG87) imaged by ESO's southern hemisphere telescope (left) and by the Hubble ST (right):
Thus, the need now is to have a more powerful and sophisticated telescope in space as the HST replacement. As is the usual custom, NASA and the astronomical community always seem to have new telescopes on the drawing boards. The big follow-up being planned by The Space Telescope Institute and Goddard Space Flight Center is NGST which stands for the Next Generation Space Telescope. In 2002, this telescope was formally renamed the James Webb Space Telescope (JWST), to honor the second NASA Administrator for his many accomplishments in galvanizing the space program, including his role in the Moon program. Final decisions as to its components and the contractor(s) to build it have not yet been made but a launch date has been set for no sooner than 2009. The principal scientific goal is to obtain improved information about the Universe's history between about 1 million and 2 billion years. The telescope will concentrate on the infrared region of the spectrum, with a range between 0.6 and 28 µm. Because of the spectral wavelength redshift that results from the expansion of space (see page 20-9), the visible light from these early moments in the Universe's history will have now, as received, extended into the near infrared. (For further information, check out Goddard's NGST site.)
This diagram summarizes the current and anticipated status of space telescopes' ability to see back in time towards the earliest events following the Big Bang:
The HST (along with the Chandra Telescope [X-ray region], the Spitzer Space Telescope [Infrared] and other space telescopes operating in various parts of the spectrum as described on page 20-3) have the detection capability and resolving power to look back to about the half billion years whereas the JWST will be able to detect and image events taking place about 300,000 years after the B.B. Earlier than that will be difficult to examine by visual means because of the opaqueness of the Universe at that time.
The Nature and Evolution of Galaxies
Before getting into the contents of this page and its companion, you may wish to run through a good synopsis of what galaxies are on this Internet site maintained by the Sloan Digital Sky Survey team.
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 a few thousand visible stars. Many faint stars that appeared farther out were thought to be located around the disk of the M.W. 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 galaxy and that some of the stars visible in the telescopes of his day were indeed other galaxies.) Thus the Universe became much bigger and contains a myriad (billions) of galaxies that make up the visible entities filling expanding space.
A galaxy is an organized concentration or clumping of stars held together by mutual gravitational interaction in an aggregate containing billions of discrete individual stellar objects organized into varying shapes/structures. 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 illustional, 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 magnitudes 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 much 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 the first galaxies, there is a growing consensus that a group of very massive, gradually heated stars emerged before any galaxies. These stars, composed almost entirely of 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 - the first stars - is treated in more detail on page 20-5. (NOTE: Many Cosmology texts and overviews start with a discussion of stars before galaxies; in this Tutorial that is reversed, as we favor treating these larger structures as seen at different wavelengths throughout the EM spectrum prior to a detailed analysis of how stars - the principal members of galaxies - is undertaken.)
A quick synopsis (preview) of galaxy formation: 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 stars (whose nature and development are described on page 20-5). Stars are individual spherical clots of gas (initially H and some He) that form by further gravitational collapses within broader irregularities of greater concentrations of gas and some dust forming "clouds" (also called nebula but this latter term usually refers to gas/dust concentrations within a larger cloud, within existing galaxies, or gaseous debris from supernova explosions); as this densification increases, the gases contract and heat up to interior temperatures reaching > 106 °K, sufficient to initiate nuclear reactions (the conversion of H to He) so that the stars become luminous (radiant) bodies.
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 is also called a "Giant Molecular Cloud" because much of its hydrogen is combined as H2. For this accumulation 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 tiny fraction of the hydrogen and helium species) 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 thess next images. The great protuberances of dust-gas within the Eagle Nebula are called pillars, evident in the top image. Below that, in this nebula, is the top of the Horsehead Pillar, 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.
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:
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 600,000 million years after the Big Bang) of the Universe, as galaxies organized, models for their spatial distribution 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 existed is suggested by this HST view of a very old network of filamentous galaxies and stars in deep space.
A possible analog to this early stage of galactic development, with gas clouds producing a plethora of stars, is this star burst found in galaxy NGC6832:
Thus, star formation is a general process (see page 20-5) 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). The arrows denote a progression of variation but do not necessarily connote evolution of one form to the next (true in some cases). 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.
For further clarity, this second version of the classification should be looked at; it tends to concentrate on the two dominant galaxy types - elliptical and the two types of spiral (barred; non-barred):
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 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:
This next view shows the prominent spiral arms, made up of concentrations of unusually bright (younger) stars; the arms develop from the rotational "drag" experienced by the differential velocities within the disk.
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 and a Spitzer Telescope image, in the infrared:
A spiral galaxy can contain up to a 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)
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, along with a discussion of the galactic wave model, are treated near the bottom of the next page.
About 2% of spiral galaxies contain an especially bright central region. These, known as Seyfert galaxies, are marked by a notable concentration of dispersed hydrogen gas, excited to luminosity, i.e., the brightness is not just from stars alone (those present tend to be blue [relatively young]). Here are a group of Seyfert galaxy images seen by various ground telescopes in the near infrared:
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.
A recent HST image of spiral galaxy NGC 1512 shows more details about the central or core region and the envelope of actively forming bright stars. Note the spiraling in of gases and other materials towards a presumed Black Hole at the center itself.
When viewed in the ultraviolet, galaxies often show both an inner and an outer ring of hot young stars, as illustrated here by NGC6782:
Generally, the inner part or central region of spiral galaxies, and those enroute to becoming this type, shows a range of activity in which new stars are forming in abundance from gas and molecular clouds dominated by hydrogen. This HST image of the galaxy M83 (most of which is seen in the upper left inset, with a small bright center) shows details of this central region:
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 containing a large number of individual stars, or groups of stars (including globular clusters; see below), plus gases and cold dark matter (CDM), occur dispersed in galactic space in the central part of what is called the halo region, an envelope that surrounds galaxies in general. Haloes develop around protogalaxies and aid in the subsequent development of each type. The halo contains the gases, stars, and dark matter (consisting of particles not hot enough to give off detectable radiation) that extend above and below the central disc, arranged roughly in a spherical distribution around the galactic center [see also page 20-4). 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:
Also enclosed by a dark matter halo is the second major galactic type, the Elliptical Galaxy,, marked by mostly old stars (up to 1011). Ellipticals comprise about 20% 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 (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 (these have more blue or hotter stars than red); however, being more compact ellipticals are usually brighter than spirals. Still, recent observations of elliptical galaxies have found that there are numerous 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 are two examples:
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. The image below is a giant cluster of many thousands 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 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.
Here is a neat view of the two main types of galaxies - spiral and elliptical - in a galactic cluster known as HCG87. Note the dark ring in the larger (closer) galaxy which indicate particulate matter mixed with the gases.
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:
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.
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 but in that mode of distribution isolated ellipticals make up about 20% of these two principal types.
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 \nmo longer ty pical 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 more evolution is likely. Several examples of Lenticular Galaxies are shown in this next sequence; see their captions for details.
Globular star clusters - each an aggregate of 100,000 to a million stars - 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. There densities of several hundred stars per cubic light-year (compared with typical densities of 0.01 to 0.1 stars per (l-y)3 as is observed in the Sun's region, which is the norm for most galactic space).
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. Globular clusters have proved to be a primary means of determining the ages of the oldest stars in the Universe. The halo region also contains millions of isolated individual stars, or small groupings. Below is globular cluster NGC6093:
Visual evidence of very dense concentrations of stars in the inner region of clusters is seen in this ground telescope view:
Smaller globular clusters have fewer stars in their central regions or core; thus:
The largest globular cluster around the Milky Way is NGC5139, estimated to contain up to 10 million stars.
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:
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 (as applied here the term "cluster" has a different meaning from that discussed 7 paragraphs below) 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, star clusters (mainly in the halo) consist mainly of old stars. This led to a conclusion 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.
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 at least some are resolved in infrared light into spherical objects (whose galactic nature is still unknown since the galaxies, if formed, are not resolved) which consist of huge haloes of glowing hot hydrogen gas. This image pair gives a good example:
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