The following abbreviated treatment of basic astronomical concepts is not intended to substitute for a formal course in astronomy by any means. Its purpose is to provide the uninitiated reader a basic understanding of the processes of the celestial world. In this way the reader's visual experience in the pages ahead will be enriched by a basic understanding of the nature of each object. Hopefully the information provided below will arouse further curiosity in selected areas according to the reader's interest.
The Life of Stars
Nebulae: Clouds of Creation
The Realm of the Galaxies
Catalogs and Coordinates
Fundamental to even the most basic understanding of the universe is a grasp of the vast scales of distance and size beyond the bounderies of our own small world. The transformation of the two dimensional sky to the three dimensions we know today is a great tribute to human ingenuity. The birth of this understanding began in 200BC when Eratosthenes, using basic geometric principles, calculated the circumference of the earth to within 1% accuracy. It wasn't until the 18th and 19th centuries that accumulated knowledge and observation unveiled the distance scales of our solar system. Again using the known diameter of the earth as a baseline, basic triangulation provided the means of calculating distances to the planets and closest stars. This technique is called parallax and is still used today for calculating distances out to about 150 light years.
Distances of deep sky objects (objects beyond our solar system) are measured in light years and cannot be calculated using parallax technique. A light year is the distance light travels in one year or about 5 trillion miles, or 63,241 times the distance between the earth and sun. Knowing that light can circle the earth 7 times in one second is indeed a humbling fact that begins to give the reader an idea of how truly far a light year is. These simple facts should allow for a greater appreciation of the enormous distances to even nearby deep sky objects in our own galaxy.
Many of the stars we see in the sky at night are within 100 light years of earth. The closest star to earth is alpha centuri at 4 light years. Common nebulae and star clusters within our galaxy reside several hundred to several thousand light years away. The furthest objects in our galaxy within our telescopic reach are a few tens of thousands of light years distant. The Milky Way, our parent galaxy, stretches 100,000 light years across and contains some 200 billion suns. Traveling at conventional spacecraft speeds would require over 1 billion years to traverse the galaxy!
The next step up in distance is the intergalactic scale. The closest galaxy similar to our own is the Andromeda Galaxy at 2.5 million light years. When we observe the Andromeda Galaxy through the eyepiece of a telescope we are presented a scene from the distant past. Andromeda's light began its journey toward earth approximately 2.5 million years ago, at an epoch corresponding to the dawn of human existence. Observing or photographing deep sky objects gives us an amazing opportunity to gaze back in time through a cosmic time machine. Viewed as they existed many thousands or even millions of years ago the objects remain frozen in time for us. Perhaps there is no other concept in astronomy as humbling or awe inspiring as this notion of "look-back" time. With telescope and camera we can now explore objects at far greater distances than ever before which in turn provides us with views of our universe at increasingly earlier epochs. These humbling concepts drive home the bewildering scales of intergalactic distances.
Distances to relatively nearby objects can be calculated using the principle of parallax in which an objects movement in the sky over a course of time can be used to determine its exact distance from earth. Distances to more remote objects require a "standard candle" by which astronomers can use to estimate the vast distances to objects outside our galaxy. Certain types of variable stars with fixed luminosity-brightness relationships (Cepheid Variable stars) are of paramount importance for estimating distances to galaxies within 100 million light years. Beyond 100 million light years individual stars cannot be resolved so other means of estimating distance such as bright supernovae or recessional velocities (since our universe is expanding, a galaxies recessional velocity will increase in proportion to its distance from us) are the current methods for determining distances to very remote galaxies. One of the goals of the Hubble Space Telescope and other ongoing astronomical projects is the refinement of methods to determine the distance scales of the universe.
The universe as we know it today is composed of many billions of galaxies, each one possessing countless stars and nebulae. Many of the objects in the pages ahead, especially the galaxies, are at vast distances, often exceeding our ability to easily comprehend. It may help to visualize earthly events that were occurring when these objects released their ancient photons. For example, the Virgo Galaxy Cluster released the light we see today at an epoch of time coinciding with the extinction of the dinosaurs and the rise of the mammals and primates, our ancient ancestors.
The Life of Stars
Most processes occurring in the visible universe involve stars in some way or form. In fact all elements with the exception of hydrogen, deuterium, helium and lithium (created in the Big Bang) were formed in the nuclear furnaces of stars. This includes the heavier elements like carbon and oxygen, the main constituents of living organisms. Stars are classified by astronomers using many schemes including brightness, color, temperature, size, mass, association with other stars, etc. Historically, Hipparchus (Greek astronomer, 2nd century) was the first to characterize stars by brightness. His system of magnitude is still in use today. Hipparchus divided stars by visual brightness from 1st magnitude (brightest) to 6th magnitude (faintest). Each magnitude represents a 2.5 fold change in brightness from the next magnitude. With the arrival of the telescope and later the camera, stars as faint as 30th magnitude became detectable (4 billion times fainter than could be observed visually).
Perhaps most helpful to understanding the true nature of stars is description by color and surface temperature. Stars are powered by hydrogen fusion deep within their cores where temperatures typically reach 15 million degrees Kelvin. The fusion of hydrogen to helium releases prodigious amounts of energy in the core of stars. The energy travels slowly to the stars surface before it is released in many diverse forms including heat, light, and radiation. Within our own sun (a star of average mass) a single photon takes one million years to make the trip from core to surface.
The surface temperature of the star produces the star's natural color in the same way any heated body has a color depending on its temperature. The amount of energy released at the surface (hence the star's color) is in turn related to the temperature and mass of the stars core.
The modern and most meaningful way of characterizing and analyzing stars is by "spectral class". Today we speak of stars by the letters O,B,A,F,G,K,M. (The famous mnemonic for remembering this sequence is Oh, Be, A Fine Girl, Kiss Me). A star's spectral class is defined by the physical characteristics of temperature, size and density. The hottest and most massive stars are in the "O" and "B" class. These stars are typically blue or white. Stars of intermediate temperature and mass range from type A to type G (white to yellow), and the coolest, least massive stars are types K and M (orange and red). Each spectral type is further divided into subclasses 0 to 9 depending on its temperature. Two new classes have been added (L &T) to account for the recent discovery of very low mass stars (brown dwarfs, etc.). Our sun is a type GII star and its color is yellow (temperature 5800 K.). Surface temperatures of stars can range from 40,000 degrees (type O) to 3,000 degrees (type M). The mass of stars can range from greater than 100 times to 1/8 the mass of our sun. Below this lower limit, nuclear fusion cannot occur.
Stars vary in composition. Typically stars are made of predominantly hydrogen (90%) and helium (10%). Remarkably the remainder of all elements found in nature comprise the remaining 1/10th of a percent. The dominant heavier elements found in stars include oxygen, carbon, nitrogen, and iron.
The birth of stars is one of the most studied subjects in astrophysics today but is only partially understood. Stars have their beginning in the diffuse cold molecular clouds of interstellar space. Due to a variety of factors these clouds sometimes begin to collapse by virtue of their own gravity. As the collapse proceeds, gravity becomes the driving force, escalating the collapse further and fragmenting the cloud. The cloud becomes denser and grows hotter and eventually the enormous temperatures required for nuclear fusion are reached. Thus a star is born. In the early stages the star shines by energy derived from gravity. At this stage the star is known as pre-main sequence star. When the nuclear furnace gets started, the gravitational collapse is then halted. For the remainder of a stars life a tenuous balance is struck between gravity (collapse) and nuclear fusion (expansion). A typical star like our sun fuses 600 million tons of hydrogen into helium each second. The fusion process is so efficient that each second our sun coverts 4 million tons of matter into energy.
Stars that are actively fusing hydrogen to helium (like our sun) are called "main sequence stars". In essence stars are defined by their ability to generate energy by fusion. Stars generally spend about 90% of their lives on the "main sequence". Main sequence stars show a direct relationship between their mass, size, and temperature. When stars exhaust their hydrogen fuel they begin the inevitable process of stellar death. At this point in their evolution they begin to leave the "main sequence". The lifetime of a star is directly related to its mass. The most massive stars rapidly use up their fuel and may live only a few million years. This is in contrast to lower mass stars like our sun which may enjoy a main sequence life of over 10 billion years.
When a sunlike star (0.8 to 10 solar masses) exhausts it's hydrogen core and begins to die, a new process begins. Helium fuses to carbon and later oxygen which will sustain the star for a short period of time but at the expense of further core collapse and higher core temperatures (100 million degrees), and continued expansion at the surface. The star no longer checked by gravity, becomes bloated and its surface cooler. At this stage it is referred to as a "red" giant. The bloated diameter can exceed ten times that of our sun. The star begins a futile cycle of further core collapse and surface expansion which can end in different ways depending on its mass.
A sunlike star (0.8 to 10 solar masses) becomes a red giant in its final phase of life. During this process elements and byproducts of nuclear fusion normally found in the core can be churned up to the surface. In this late phase the star begins to lose mass at a furious rate through stellar "winds" that blow from its surface. In this way the interstellar medium is enriched with heavier elements which then become the building blocks of future stars. A star can blow off half of its entire mass in stellar winds during its red giant phase.
A sunlike star ends its life as a Planetary Nebula. As the gravity of the dying red giant becomes weaker its outer envelope is expelled into space. The hot shrunken core releases abundant amounts of radiation which catch up to and collide with older winds released earlier by the dying star. The interaction of the radiation with previously expelled clouds of gas creates a dazzling display of brilliant colors. The entire complex is called a Planetary nebula. They have nothing to do with planets but received their name from William Herschel who likened them to planets because of their shape. They have a variety of complex appearances and colors depending on the nature of the gases in the cloud. Typically they consist of layers of symmetric, rings, filaments and halos and glow with fantastic colors. Recent work has shown that most planetary nebulae have a cylindrical bilobed shape which can have a variable appearance depending on the tilt of the nebula towards our line of sight. The cloud tends to expand at such rapid rates that the planetary nebula stage only lasts a few tens of thousands of years. Eventually the core collapses to a small dense sphere called a white dwarf. The white dwarf is a peculiar object of extraordinary density ( a metric ton/cubic cm) which can no longer produce energy by way of nuclear fusion. Because of their extreme density the white dwarfs do release large amounts of radiation and heat.
Supernovae and Supernovae Remnants
Higher mass stars (> 10 solar masses) have a different evolution. Initially they follow a similar path as giants. Because they have considerably higher reserves of mass than lower mass stars they are capable of much higher core temperatures and therefore can fuse heavier elements. Because they already are huge by comparison with lower mass stars they become red "supergiants" as they leave the main sequence. Their outer envelopes can extend almost the diameter of our solar system. Because of higher temperatures their cores go on to fuse heavier elements such as neon, magnesium, oxygen and later sulfur and silicon. Finally silicon and sulfur fuse to iron which cannot undergo further fusion. For these furious giants of the sky the end comes with cataclysmic destruction. Since Iron is incapable of further fusion gravity finally triumphs in the end. The star can no longer support itself. The end comes in a sudden catastrophic collapse of the stellar core followed by an enormous discharge of energy that tears through the remaining envelope and destroys the star. The incredible release of energy is so powerful that the light output exceeds that of an entire galaxy for a few days after the explosion. This is known as a type II supernova. The light output of a supernova makes them easily visible in distant galaxies. Supernovae are rare events occurring in a given galaxy maybe 2 or 3 times a century. The last one observed from earth in our own galaxy was "Kepler's Star" in 1604 which was bright enough to be visible in daylight. Because of their rarity we learn about them by observing them in other galaxies. (Supernovas can also occur alternatively in binary star systems by infalling matter on a white dwarf star causeing it to detonate as a type I supernova).
What happens to the core and outer envelope after a supernova blast? If the core is less than 2 to 3 times the mass of the sun the core collapses down to a neutron star. Neutron stars have incredible density as the core of the former star now exists in a space only a few miles wide. The density exceeds 100 million tons per cubic centimeter. Neutron stars tend to spin many times a second releasing abundant energy in the form of radiowaves, x-rays and gamma rays. If the earth is in the path of the energy the neutron star is called a pulsar. About 600 are known to exist.
If the mass of the stellar core is greater than 3 solar masses collapse continues forever. The result is an enigmatic object called a "black hole". Gravity is so strong within a black hole that light cannot even escape. We can only surmise the presence of a black hole indirectly as they cannot be directly observed.
The outer envelope and core material blown into space continues to expand in a shell called a supernova remnant. As this material smashes into surrounding gas clouds shock fronts are created and energy is released. The clouds can glow in visual wavelengths revealing brilliant colors and shapes making them appealing subjects for astrophotography.
Stars are not formed one at a time but in groups called clusters. Once the ancestral cloud is dispersed the clusters become conspicuous and are called "open clusters". The term "open" suggests they are loosely bound as each member is on average about 1 light year from its neighbor. Open clusters typically contain between 10 and 3000 stars. All the members of an open cluster have their origin from the same molecular cloud and therefore have several important things in common; they are the same age, the same initial chemical composition, the same distance from earth and move with the same velocity and in the same direction. The fact that high mass stars evolve and die more rapidly gives astronomers a method for predicting the age of a cluster. An older cluster will have a lower percentage of high mass stars compared to a younger one. The stars within an open are typically located along the spiral arms of their parent galaxy where star formation is most common. Clusters may remain together for up to a billion years but ultimately disperse from the repelling power of the members stellar winds. Our sun was once a member of an ancient open cluster but has long since left from its stellar siblings.
Globular clusters are spherical collections of mostly ancient stars numbering between tens of thousands to a million or more stars stretching 100 to 300 light years across. They are truly ancient structures with a minimum age of about 11 billion years. Most are believed to have formed at the same time as their parent galaxy. The cluster members are mostly population II stars which are highly evolved low mass main sequence stars. Any star in the cluster with a mass greater than 0.8 solar masses has already left the main sequence and become a red giant. To date there are over 160 globular clusters discovered in our Milky Way. Globular clusters reside in the spherical halo of our galaxy (and other galaxies) and orbit the center with highly eccentric elliptical orbits independent of the disk stars. Their orbits take them millions of years to complete as they wander as far as 100,000 light years from the galactic center. The central distribution explains the predominance of globular clusters in the Sagittarius-Scorpius-Ophiuchus region close to the galactic center. These constellations contain over 50% of the total number of globulars known in our galaxy. An interesting property of the population II stars in globular clusters is their low metallicity. These ancient stars are believed to have formed from the same primordial matter from which the galaxy formed in a much younger universe. The heavier elements are only formed through many cycles of star birth and supernovae which enriches the interstellar medium over many billions of years. Because these stars represent the earliest generation of stars in the universe they lack the metals present in more recent generations of stars like our sun which formed some 5 billion years ago. Globulars are divided into two groups depending on their metal content. Oosterhoff I and II groups have slightly weak and very weak metal contents respectively (Dutch Astronomer Peter Oosteroff). Globular Clusters are intriguing and important because 1) the stars are all of the same age and similar chemical compositions making them superb laboratories for the study of stellar and galaxy evolution. 2) They are the oldest star systems known. As astronomical fossils they may hold the key to unlocking the age of the universe.
In the pre-telescope era the word Nebula was used by observers to describe any "fuzzy" patch present in the night sky that wasn't sharp like a planet or star. Charles Messier the 18th century French comet hunter (and creator of the Messier catalog) rejected (but fortunately catalogued ) these "Nebulae" as comet imposters given that his primary purpose was to discover comets. With the advent of the telescope, camera, and spectroscope we now know that nebulae are vast clouds of interstellar dust and gas. These clouds are often made visible by interactions with nearby stars.
The "stuff" between stars is known as the interstellar medium. The interstellar medium condenses to form giant molecular clouds which are the essential places of star birth within galaxies. The molecular clouds are comprised of 1% dust particles (on the order of about 1 micron in size) and 99% gas. Of the gas, 90% is hydrogen and 10% helium. A small fraction of the molecular cloud consists of various molecules including water, hydrocarbons, ammonia, silicates, etc. Because of extremely cold temperatures (only a few degrees above absolute zero) elements in a molecular cloud exist in molecular form as opposed to atoms and ions which exist at higher temperatures. As molecular clouds orbit a galaxy they come into contact and interact with other clouds, spiral arms, and supernovas, all of which exert forces that trigger fragmentation and subsequent cloud collapse. Once the cloud begins to fragment individual "cores" tend to form. Under the influence of gravity the cores contract further and generate heat. As temperature and density rise "protostars" are formed which eventually give rise to fledgling stars and star clusters. The new stars in turn interact with their ancestral cloud to form the various classes of nebulae described below.
Nebulae can be distinguished by the chemical and physical processes that occur within them. "Diffuse Nebulae" (also referred to as gaseous nebulae) are diffuse clouds of interstellar gas and dust. These immense clouds can and often do provide the raw materials for starbirth. These large agglomerations often contain mixtures of various nebula types such as emission, reflection, and absorption nebulosity.
Emission Nebulae are also known as HII regions and represent bright clouds of fluorescing hydrogen gas energized by very hot young stars. Some stars (massive type O and B stars) pour large amounts of ultraviolet radiation (UV) into the surrounding interstellar medium. The UV light strips electrons from hydrogen atoms within a radius of dozens or even hundreds of light years in every direction by the process of photoionization. As the electrons randomly recombine with hydrogen nuclei, energy is emitted in the form of light within characteristic emission lines, which for hydrogen is red light (656nm). The dominant emission line for hydrogen is termed HII referring to the first ionization level of hydrogen, hence the name of these clouds. HII regions are numerous in the spiral arms of our Milky Way galaxy and other spiral galaxies and indicate regions of active stellar birth. Emission clouds can also have other emission lines like the green of double or triple ionized oxygen or the blue of hydrogen beta.
Reflection Nebulae refer to clouds of dust with embedded stars. The dust clouds consist of microscopic particles of heavier elements like oxygen, silicon, carbon. The starlight reflects from the surface of the dust particles and scatters in the shorter blue wavelengths. Reflection nebulae have the characteristic blue color of reflected starlight. Often reflection nebulae are found together in complex clouds alongside emission nebulae. The true nature of reflection nebulae was revealed by Vesto M. Slipher in 1912 when he found that the spectra of the Pleiades cloud and its stars were the same.
Absorption Nebulae (dark nebulae) are dark clouds of gas and dust made visible only by the light of bright stars or bright nebulae behind them. They do not emit or reflect light. Dark nebulae are often found within and adjacent to emission and reflection nebulae and can be places of active star formation. E.E. Barnard, one of the early great astrophotographers catalogued 349 dark nebulae in his famous article "A Photographic Atlas of Selected Regions in the Milky Way" published in 1927.
Nebulae can also be characterized by their interaction and participation in star formation. Prestellar nebulae are diffuse nebulae which have the potential to produce hundreds or even thousands of stars. Poststellar nebulae refer to Planetary nebulae and supernova remnants which were covered in the previous section on "Stars".
Special circumstances give rise to special classes of nebulae. These include Herbig-Haro Nebulae (See NGC 7129) and Wolf-Rayet Nebulae (see NGC 6888). These intriguing objects are explained further in the pages ahead.
All nebulae have finite lives. While planetary nebulae and supernova remnants may last only several thousands of years, emission nebula can exist for a million years or more before its gases are dispersed by the radiation and stellar winds of its stellar progeny. The larger molecular clouds, the progenitor clouds of stars and star clusters, can exist for tens of millions of years.
If stars are the building blocks of galaxies then galaxies should be considered the essential building blocks of our universe. Galaxies are by definition vast rotating systems of stars, gas and dust. Galactic dimensions can exceed our ability to comprehend the enormous scales of size, distance and time that define their existence. Galaxies range in size from dwarf types, only a few thousand light years across to great spiral and elliptical galaxies spanning several hundred thousand light years across. They contain anywhere from a few million to as many as one trillion stars. The true nature of a galaxy was not apparent until 1926 when Edwin Hubble published his epochal work. In the 1920's Hubble observed stars (Cepheid variables) in M31 and eventually recognized their extragalactic nature. He reported his findings in 1929 and dramatically changed our understanding of galaxies and the paradigm of the universe. The earth is located within a large spiral arm (the Orion arm) in a spiral galaxy we call the "Milky Way". The closest galaxy similar to our own is the Andromeda Galaxy (M31). The Milky Way along with two other large galaxies, M31 and M33 are among 40 other galaxies that make up the local group of galaxies. Our local group together with several other nearby groups plus the nearby virgo cluster make up the local supercluster. The supercluster contains many thousands of individual galaxies. There are probably tens of millions of superclusters in the observable universe.
Like stars and nebulae, galaxies can be classified by a variety of methods. The simplest and easiest method is by form. Galaxy types can be described as spiral, lenticular, elliptical, and irregular. Spiral galaxies make up more than 50% of all observed galaxies. Spirals show the most complex and dynamic structure with the greatest degree of organization. Within the "Hubble classification scheme" spiral galaxies are further classified by the presence of a barred versus a stellar nucleus and also by the relative proportion of central bulge to spiral pattern. Hubble arranged galaxy types in a branching classification (tuning fork) thinking that the earliest galaxies were ellipticals and that barred and unbarred spirals were more evolved and represented "later" configurations. We now know that from an evolutionary standpoint this classification is at best backward but still serves a descriptive purpose and is still often used today. There is great diversity in the structure among galaxies. The diversity of the spiral form of galaxies is one of the primary reasons for the growth of astrophotography. The vision of the spiral galaxy is an iconic figure of aesthetic beauty in astronomy.
A spiral galaxy has a basic flat shape similar to a frisbee but with a central bulge. The spiral arms exist in a relatively thin, flat disk which rotates around the central nucleus. Massive amounts of dust, gas, and stars rotate within the disk and comprise the star forming regions of the galaxy. It is within the huge spiral arms of the disk that new stars, star clusters, and nebulae form and the dynamic processes leading to the recycling of matter occur. Spirals can have dramatically different appearances depending on their orientation in space and the inclination of its disk to our line of sight. When viewed "edge on" a spiral galaxy will appear as an elongated structure split by its dark and thin equatorial disk. Viewed "face on" the spiral pattern becomes clearly visible. There are of course many variations between the two extremes depending on the angle of inclination.
The central ellipsoidal bulge consists of older star populations and is relatively devoid of interstellar matter (gas and dust). Older stars in the central bulge of spiral galaxies (yellow and red stars) are referred to as stellar population II stars formed long ago and are often as much as 10 billion years old ( literally as old as the galaxy itself). Younger stars (often massive, hot O and B type blue stars) in the spiral arms are designated population I stars. These are relatively short lived stars, many of which will end their existence in supernova explosions. Spirals also have a diffuse outer halo also devoid of gas and dust where compact agglomerations of older stars called globular clusters are found. Globular clusters occur in all types of galaxies and contain anywhere from 100,000 to millions of stars. They are among the oldest components of the galaxy. Also believed to exist in the halo is the elusive "dark matter" now thought to account for much of the invisible mass of galaxies.
Lenticular and ellipticals are galaxy types which contain little interstellar matter. New star formation has mostly ceased in these groups. Therefore they are comprised entirely of older population II stars. Ellipticals most likely form from large scale galactic mergers and may represent an end stage of galactic evolution. The Irregular types are galaxies that demonstrate a peculiar, disorganized, and sometimes chaotic appearance. Often this is the result of disruptive gravitational interactions with neighboring galaxies. Irregular types can exhibit areas of spectacular new star formation usually as a result of violent encounters with other galaxies.
Galaxies as a rule emit copious amounts of energy at every wavelength of the electromagnetic spectrum. Cold molecular clouds and interstellar matter from the disk tend to emit infrared and radio waves while high energy supernova remnants and starbursts release X-rays and gamma rays. There is recent evidence to support a "connectedness" between galaxies in the form of large scale galactic winds which can extend outward as much as 65,000 light years into the intergalactic medium. The winds represent the combined radiation from massive starbursts and supernovae. Radiation can also be released from supermassive black holes thought to be lurking in the cores of most galaxies. Energy is released when a massive black hole consumes matter that has fallen into it.
A special class of galaxies are known as the "Active Galaxies". Active galaxies are luminous galaxies that release copious amounts of energy that cannot be accounted for by stellar processes. Active galaxies include the subgroups of Seyfert Galaxies, Radio Galaxies, Quasars, and few other esoteric types. All active galaxies have in common an Active Galactic Nucleus or AGN. The AGN is almost certainly powered by a compact engine in the galactic center. Active galaxies often exhibit spectacular jets of high velocity gaseous matter from their nucleus. In all cases the source of energy is the active nucleus.
The conventional belief is that the central engine of active galaxies are super massive black holes of 1 million to 1 billion solar masses. Because of the enormous gravity of such a massive object, material in the center of galaxy spirals into the black hole forming a flattened rotating ring of matter called an "accretion disk". As material from the accretion disk falls into the black hole, a tremendous release of energy occurs in the form of jets of super heated gas that are expelled at immense relativistic speeds. The accretion process is an incredibly efficient means of converting mass to energy. The energy conversion occurs at up to 50% efficiency, far more efficient than the fusion process that occurs in stars. It is believed that when the massive black hole has devoured all the material in its vicinity the active nucleus ceases to release copious amounts of energy and then becomes a normal galaxy like the Milky Way.
A remarkable type of active galaxy is the Quasar (quasi-stellar radio source). Quasars are objects that appear as point sources (starlike) but are at high red shifts, meaning they are located at enormous distances. Although not all astronomers agree most believe they represent very distant galaxies with extremely active nuclei. Quasars are only seen at large redshifts (great distances) so we are seeing them at a very early stage of galactic evolution in the remote past of our universe. Because they are so distant their light output must be enormous making them the most luminous objects in the universe. In fact the average quasar has about 1000 times the luminosity of a mature galaxy like the Milky Way. There have been several hundred quasars observed so far ranging in distance from redshifts of .06 to 6.4. Because we are seeing quasars as they existed very long ago, at a time when our universe was much younger perhaps most galaxies went through a very active "quasar" phase before maturing and becoming relatively quiescent like the galaxies in the local universe. In fact the oldest quasars correspond to the very beginning of galaxy formation marking the end of the so called "dark ages" when the early universe was opaque to light.
Seyfert galaxies are another subtype of AGN galaxies (often spiral or irregular types) which have an extremely bright starlike nucleus which may at times outshine the entire parent galaxy. The light emitting region is compact, less than 1 light year in diameter and is assumed to be powered by a supermassive black hole. Seyferts are characterized by their very specific spectra. Radio galaxies are yet another subgroup of AGN. Most are elliptical types exhibiting huge symmetric "radiolobes" far out into space. Some of these galaxies also exhibit jet phenomenon from their nucleus. The classic radio galaxy with a conspicuous central jet is M87. Driving this jet at the center of M87 is a supermassive black hole of several billion solar masses. In all cases of AGN the central engine driving the active nucleus is a massive black hole and its surrounding accretion disk.
What is the origin of spiral structure? The most popular theory to date devised by C.C. Lin and Frank Shu in 1964 is that "density waves" propagate through the spiral disk. The stars rotate around the center of the galaxy at about twice the speed as the density wave (One rotation of the stars takes 200 million years in the Milky Way. Our sun has made 8 trips around the galactic center). The stars tend to clump together as they encounter the density wave. As the stars concentrate in space, interstellar gas becomes compressed leading to bursts of new star formation. The process of star formation can also occur by self propagation as newly formed stars further compress adjacent gas clouds triggering even more star birth. Star formation takes place almost exclusively within the spiral arms although it is unclear how much occurs by self propagation and how much is due to the presence of density waves. More than likely both contribute. There are many questions about galaxy dynamics and evolution that remain unanswered such what created the density waves in the first place?
Since the dawn of humanity people have struggled to understand the origin and ultimate fate of the universe. The last few centuries have witnessed a greater understanding of the universe with all its peculiarities. Despite a greater understanding, it has become clear that the cosmos may be even more peculiar and mysterious than we ever imagined. Our current state of knowledge defines a universe where much remains unexplained and the basic forces governing the cosmos are still shrouded in mystery. That said we do know something about how the universe got its start. The Big Bang theory is currently the accepted theory that explains the origin and evolution of the universe as we know it. It is the basic backbone of cosmology.
The Big Bang theory postulates that the universe came into existence from a small "singularity" in a single instant some 12 to 14 billion years ago. Early on it was hot and dense. It underwent inflation early on and eventually cooled to reach its current size and temperature, eventually forming the stars and galaxies we observe today. The unprecedented accuracy of recent observations leaves little doubt about the validity of the "Big Bang" origin of our universe. Three key observations laid the groundwork for the theory. The first was the observation by Edwin Hubble in 1929 that galaxies were receding from us in all directions. The second is the existence of the light elements H, He, and Li as these would be predicted to form from the fusion of protons and neutrons in the first few minutes after the Big Bang. The last is the presence of the cosmic microwave background radiation (CMB) discovered in 1965 by Arno Penzias and Robert Wilson which is believed to be the primordial remnant of energy (heat) left over from the Big Bang. This was later confirmed to be true using a sensitive satellite instrument (COBE). The fundamental questions of the ultimate fate of the universe remains to be answered but with more observational evidence we are getting closer to the truth.
The roots of modern cosmology date to the early 20th century and began with Einstein's theory of general relativity and the subsequent concept of the cosmological principle. These concepts together created a model of the universe where matter is distributed uniformly throughout the basic fabric of the universe which we know as space-time. Gravity acts on this fabric by distorting it and consequently influences the behavior of matter.
Two questions arose whose answers could potentially explain the ultimate fate of the universe. The first concerned the basic geometry of the universe. Is the fabric of space time closed and finite like the surface of a ball? Is it flat and infinite or is it saddle shaped and infinite? Current theory limits the geometry of the universe to only one of these three forms. The second question concerns the average density of matter in the universe as this will profoundly influence the geometry. The answers to these questions would have to wait until much more recent observational evidence. Recent data from the WMAP (Wilkinson Microwave Anisotropy Probe) supports a flat universe and recent observational work of very distant supernovas has concluded that the universe is flat and that expansion is actively accelerating. At our current state of knowledge the universe is predicted to expand forever, ultimately becoming a cold void in its place.
What does modern cosmological theory tell us about the age and constituents of the universe? Modern cosmological theory concludes that observable events in the universe cannot be supported by visible matter alone. Specifically the rotational velocity of galaxies demands that their total mass be at least 10 times greater than what we see. Otherwise they would become unstable and fly apart! The current explanation is that the most dominant source of gravitational force in the universe is of a mysterious form we are not familiar with. Since it isn't visible and we don't know what it is we call it "dark matter".
Data from WMAP has helped astrophysicists
determine the age as well as the critical mass and energy density
of the universe. The universe is believed to be 13.7 billion years
old. Conclusions based on the WMAP data are that the universe
is comprised of 4% visible matter, 23% dark matter, and 73% dark
energy. The dark energy is believed to be the force responsible
for driving the increasing expansion of the universe. According
to present knowledge 96% of the universe is in a form we are completely
Luckily the images in the following pages will illustrate the 4% we are familiar with.
Perhaps one of the most essential tools to understanding and navigating the sky is familiarity with the celestial coordinate system. The sky is divided into a coordinate system called the celestial sphere. There are north and south celestial poles and an imaginary circle equidistant from the poles called the celestial equator. The celestial sphere is divided into a reference grid similar to longitude and latitude but instead called right ascension and declination. The right ascension (analogous to longitude) is split into 24 parts (sidereal hours) and tells us an objects position in the east-west direction. It also provides information as to which time of the year the object is visible. Declination (analogous to latitude) tells us the objects location in the north/south orientation. Each deep sky objects has a fixed set of celestial coordinates, which enables the observer to find it. The organization of objects in this book is by order of right ascension.
The other essential element to navigating the sky is the organization of deep sky objects into various catalogs. There have been many catalogs of celestial objects throughout history but without question the most famous one is the Messier catalog. Named after the French astronomer Charles Messier it contains 110 of the brightest and most spectacular deep sky objects. Messier lived in the 18th century and was not aware of the true nature of the "diffuse glows" he cataloged. His primary purpose was the discovery of comets. The objects he cataloged were merely comet imposters to him. He made note of them so he wouldn't be fooled into mistaking them for his prized comets at some future time.
At about the same time a more purposeful endeavor to systematically catalog deep sky objects was made by first by William Herschel and later his son John. Using a large telescope together they cataloged many thousands of nonstellar deep sky objects. This data was later organized by J.L.E. Dreyer into the New General Catalog (NGC). The NGC was published in 1888. Two supplements to the NGC called the index catalogs were added in 1894 and 1907. All together these well known catalogs contain 13,226 deep sky objects, more than enough objects to keep one busy for a lifetime or even several lifetimes. All objects in this book have either a messier, NGC, or IC catalog number.