However, the Copernican revolution began a long process that changed completely our perception of the heavens and humanity's place in the Universe. As most everyone is aware, observations and increased understanding demonstrated that the Universe is enormous, that it has existed for periods that dwarf human lifetimes, and that we do not occupy the center of the Universe (for there is no center). Probably less appreciated is a change with antecedents in events observed hundreds of years ago, but that has accelerated at breathtaking pace over the last 30 years. As observational astronomy at wavelengths other than visible light (Radio-Frequency, X-Ray, Gamma-Ray, Ultraviolet, ...) has become more commonplace, we have begun to appreciate that the Universe is party to scenes of unimaginable violence. Far from an orderly stage for stately and gentle physical processes, the Universe at various times and various places undergoes violent cataclysms releasing energy on a scale to numb the mind of even the most analytic physical scientist.

The medieval natural philosopher would perhaps have had even greater difficulty accepting this insight than accepting the Copernican hypothesis that the earth was not the center of the Universe, for it would have destroyed the strongly held belief that the Universe existed as a nurturing cocoon for humanity. However, it is supremely ironic that these violent processes that on the surface seem hostile to the place of humanity in the Universe are in fact essential to the production of the present Universe. In particular, our modern understanding is that there would be no matter as we know it, no life as we know it, and no humanity to contemplate these questions, in the absence of violent processes that would, of themselves, destroy all life within countless light years. This lecture represents a brief introduction to the place of such violent events in the evolution of the Universe.

The Big Bang

The modern understanding of cosmology is that the Universe is some 10-20 billion years old (let's call it an even 15 billion years for our discussion), and that it came into being through an event called, rather prosaically, the Big Bang. We now give a brief overview of its main features; For a more extensive discussion of the present status of these issues, see the U. S. National Academy of Sciences document Cosmology, a Research Briefing.

Expansion of the Universe

The galaxies are all flying apart (on very large distance scales), with the velocity of recession proportional to the distance between them. The adjacent image, taken by the Wide-Field and Planetary Camera (WFPC2) of the Hubble Space Telescope, shows many galaxies billions of light years away. Most of the fuzzy patches are galaxies containing billions of stars. The galaxies in this image are receding from us at high velocities.

The details of this expansion are dictated by the value of the Hubble Constant. The objects furthest away from us appear to be receding at near the velocity of light. This expansion of the universe is a result of the original explosion that created the universe-the Big Bang. The Big Bang did not happen in space and in time; our modern understanding is that space and time as we presently experience them are themselves created in the Big Bang. It makes no more sense to ask what was before the Big Bang than to ask what is north of the north pole.

The Cosmic Background Radiation

In every direction, there is a very low energy and very uniform radiation that we see permeating the present universe. This is called the "3 Degree Kelvin Background Radiation". This radiation, which is detectable by sensitive radio frequency detectors, is the afterglow of the Big Bang, cooled to a faint whisper in the radio spectrum by the expansion of the Universe for 15 billion years. As shown in the adjacent intensity map of the background radiation in different directions taken by the Differential Microwave Radiometer on NASA's Cosmic Background Explorer (COBE) Satellite, it is not completely uniform (though it is very nearly so). In this image (click on it to get a larger version) red denotes hotter fluctuations and blue and black denote cooler fluctuations around the average. These fluctuations are extremely small, representing deviations from the average of only about 1/100,000 of the average temperature of the observed background radiation.

A small lack of uniformity in the background radiation is probably essential to the ultimate formation of the galaxies. The luminous matter in the universe that we observe on large scales is quite lumpy, as illustrated in the following figure.

FIGURE: Data from the survey of galaxies. The voids and "walls" that form the large-scale structure are mapped here by 11,000 galaxies. Our galaxy, the Milky Way, is at the center. The outer radius is at a distance of approximately 450 million light-years. Obscuration by the plane of the Milky Way is responsible for the missing pie-shaped sectors to the right and left.

The Growth of Large-Scale Structure

Thus, we require seed density fluctuations in the early universe that allow matter to clump into galaxies and groups of galaxies. Here is a link to a set of MPEG movies showing computer simulations of large-scale structural growth in the Universe. The adjacent image shows the end result of such a simulation (click on the image to get a larger version). In this simulation (Greg L. Bryan and Michael L. Norman, Grand Challenge Cosmology Consortium), filaments grow that eventually exceed 100,000 light years in length and surround gigantic bubbles of low-density gas. The intersections of the filaments produce regions of high density (signified by red) that are thought to generate bright X-ray clusters of galaxies.

Matter in the Universe

The matter in the universe is created by the Big Bang, but not in the form that we see today. First, there is very strong evidence that most of the matter in the Universe is in the form of unseen or Dark Matter matter that (at least so far) cannot be seen by standard astronomical methods, but whose presence can be inferred because it influences the Universe gravitationally. The nature of this dark matter is one of the major cosmological issues of our time.

Second, the Big Bang produces mostly the light elements hydrogen and helium. The heavier elements must be produced later, by stars. Furthermore,

1. Many of the heavier elements cannot be produced by stars in the stable periods of their lives, they must be produced in violent explosions associated with the death of stars.

2. The heavier elements produced either in the stable portion of stellar evolution, or in cataclysmic events, can only be distributed through the universe by such explosive events.

Thus, the existence of the heavy elements, and the biology built on them, depends crucially on violent processes taking place in stars and galaxies. Let us now turn to a consideration of such events.

The Stars

The stars are formed when great clouds of gas and dust called nebulae collapse because of gravitation. If these clouds are massive enough, the temperature and pressure in the central regions becomes high enough to ignite thermonuclear reactions fusing hydrogen to helium, forming a star.

The Birth of Stars

Here is a Hubble Space Telescope photograph of a region in the Eagle Nebula (M16) that is thought to be forming stars (click on the image to get a larger version (168 kB); here is an explanation of what you are seeing):

The vertical scale of this image is about a light year (!!) and the glow is associated with hot young stars that have formed recently in this nebula. Here is an expanded version of part of the preceding image (click on the image to get a larger version (242 kB); here is an explanation of what you are seeing):

Here is even a further blowup of this region. The "tiny" fingers sticking out from the main cloud in this image are regions larger than the solar system in which stars are presently forming.

An MPEG format movie (780 kB) of these star forming regions is available. (The original movie is available from the Hubble Space Telescope.)

There are many other nebulae in which there is evidence of star formation. For example, the Orion Nebula (M42) is a nebula visible to the naked eye where many hot young stars are forming.

The Death of Stars

The birth of stars is probably a rather orderly process, but their death can lead to extremely violent explosions. Let us discuss three such explosive processes, novae, x-ray bursters, and supernovae.

Novae

A nova (plural: novae) is believed to occur when matter from a large companion star accretes onto the surface of a white dwarf in a binary star system:

The white dwarf represents the death of an average star, and is not producing energy from thermonuclear reactions. The surface of the white dwarf is quite hot and dense (the hottest star known is thought to be a white dwarf with a surface temperature of approximately 200,000 degrees Celsius, which is 30 time hotter than the Sun's surface). The accumlated material on the surface can get sufficiently hot and dense to ignite a thermonuclear runaway that blows off the hot burning layer at the surface, leading to rapid increase of the luminosity of the binary system.

Here is a Hubble Space Telescope photograph of the blown-off shell responsible for a nova observed in 1992. Click on the image to get a larger version, and click here for an explanation of the figure.

Novae are probably responsible for producing many heavy elements that cannot be produced by other mechanisms.

X-Ray Bursters

An X-Ray Burster is believed to occur when matter from a large companion star accretes onto the surface of a neutron star in a binary star system:

Thus an X-Ray Burster is very similar to a nova in mechanism, but the energy associated with the burster is much higher because of the stronger gravitational field of the neutron star relative to the white dwarf. As a consequence, the light output is very strong in the X-Ray region of the spectrum for the burster, whereas the nova produces mostly visible light.

The following Hubble Space Telescope ultraviolet image represents the ultraviolet light from an X-Ray burster in a star cluster for which the companion star appears to be a white dwarf rather than a giant or supergiant star.

This binary star system has an orbital period of 11 minutes (the fastest known) and the average separation of the two stars is less than half that of the earth and moon. Click here for a more detailed description.

Supernovae

Late in the life a a very massive star (10 or more times the mass of the sun) the tiny core of the star develops a layered structure much like an onion, with heavier and heavier elements in deeper layers, culminating with iron in the center:

In very massive stars this situation eventually becomes unstable and the core of the star collapses catastropically in a time of only a few thousandths of a second. This collapse leads to an explosion called a (type II) Supernova that blows off the outer layers of the star and produces a prodigious light show that can rival the luminosity of an entire galaxy (billions of normal stars). As spectacular as this is, most of the energy of the supernova is actually contained in ghostly particles called neutrinos that are very difficult (but not impossible) to detect. There are other types of supernovae that involve a somewhat different mechanism associated with mass accretion by a white dwarf in a binary system, but the final result is similar: a gigantic explosion that destroys an entire star.

Finally we consider the strange case of the supermassive and violently unstable star Eta Carinae. The adjacent image shows a nebula larger than the Solar System that was ejected in a violent outburst in 1841. For a time, this outburst made Eta Carinae the second brightest star in the sky. Click on the image for a more detailed discussion of a star that seems destined to come to a hideous end in a few million years or less. Recent observations suggest that Eta Carinae may also power the first ultraviolet laser observed in the heavens.

The Galaxies

The galaxies are vast collections of stars, gas, and dust. Although many galaxies appear to be rather quiet, others show signs of violent activity. Two common examples of such activity are interacting galaxies, where two galaxies influence each other gravitationally and in extreme cases may even collide, and active galaxies exhibiting evidence of violent activity in the center or nucleus of the galaxy.

Interacting Galaxies

There are a variety of instances where galaxies appear to be interacting with each other enough to cause obvious distortions of the galaxies that interact. These interactions may have a significant connections with the manner in which galaxies evolve with time.

Interacting galaxies in the constellation Andromedae as photographed with the Hubble Space Telescope (click on the image for a larger version). This pair of interacting spiral galaxies was first described in the Catalogue of Peculiar Galaxies, compiled by Halton Arp in 1966. It is also pair number 64 in Igor Karachentsev's catalog of binary galaxies. The larger member is strongly tidally distorted, looking almost as though one side of the galaxy has been placed under a magnifying glass. The edge-on companion, however, retains a relatively undisturbed spiral disk, but has a luminous, heavily obscured but infrared-bright, star-burst nucleus. The nucleus of the large spiral, by way of a contrast, contains a low-ionization nuclear emission-line region (LINER), which is indicative of much less activity than the bright nuclear HII region of its companion.

We thus see an interesting example of the very different responses that different galaxies can have to interaction with their companions: the large galaxy has a shredded disk but essentially nothing in its nucleus, while the small galaxy has an undisturbed disk but a very active nucleus.

Location: 02 19.6 +39 14 (1970.0), constellation of Andromeda.
Distance: approximately 200 million light years.
Comments by e-mail to nsharp@noao.edu

Here is an example where there is evidence that two galaxies have actually collided with each other, with a violent wave of star-forming material ejected as a result. Click on the image to get a larger version (WARNING: 306 kB), and click here for an explanation of what you are seeing.

Other examples of galaxies where collisions appear to be taking place include the The Sleeping Beauty Galaxy (M64), where the center of the galaxy seems to be rotating in the opposite direction from the outer regions, and the galaxy NGC 6240, where the motion of objects in the center is extremely rapid and jumbled.

An extremely interesting example of possible galactic collisions can be seen in Stephen's Quintet, shown in the adjacent figure (click the image for a larger version). This is a close grouping of the visual image of 5 galaxies (the cores of the 5 galaxies are the brightest spots in the image). Four of these galaxies are probably physically close because they have similar redshifts; the 5th---in the lower left---is probably just accidentally in the line of sight since it has a very different redshift. In this image, the 3 galaxies in the upper left appear to be colliding with each other. Credit: Kitt Peak National Observatory.

The largest computers may be used to simulate the interaction of galaxies and the resulting distortions and star formation. Here are three MPEG movies and descriptive sound files showing computer simulations of colliding galaxies:

• Colliding Galaxies I (476 kB MPEG Movie)

• Colliding Galaxies II (493 kB MPEG Movie)

• Colliding Galaxies III (1.25 MB MPEG Movie)

• Sound File Describing First Two Movies (88 kB ulaw file).

• Sound File Describing the Third Movie (508 kB ulaw file).

Source: http://zebu.uoregon.edu/movie.html, where further details on these simulations may be obtained.

Active Galaxies

Many galaxies exhibit signs of violent activity, usually in their cores. Such galaxies are called active galaxies. The following Hubble Space Telescope photograph shows the center of the giant elliptical galaxy M87, which is the 87th entry in the famous Messier Catalog. Click on the image to get a larger version, and click here for a explanation of what you are seeing in this image.

This galaxy is believed to contain a supermassive black hole of several billion solar masses at its center. The observations indicate that approximately 3 billion solar masses are concentrated in a region at the galactic core that is only about the size of the solar system. The diagonal line across the right image is believed to be a jet of high- speed electrons approximately 6500 light years long that is being ejected from the galactic nucleus by the black hole located there.

The following figure illustrates schematically Doppler shift measurements made on the central region of M87 that suggest rapid rotation of the matter near the center. Click on the image to get a larger version, and click here for an explanation of the figure.

This is what would be expected for matter swirling around the supermassive black hole, with part of it falling forever into the black hole and part of it being ejected in the high-speed jet seen in the previous figure.

Here is another Hubble Space Telescope photograph of an active giant elliptical galaxy that is suspected of having a supermassive black hole at its center. The enormous radio frequency jets shooting from the center of the galaxy (left image) are thought to be associated with a black hole at the center of the gas and dust disk shown in the Hubble photograph on the right. The black hole is presumably at the center of the bright spot in the middle of the disk.

Click on the image for a larger version; click here for a more detailed description.

The image on the left is of the Sombrero Galaxy. This galaxy is a strong X-Ray emitter, and unusually high velocities are observed for stars near its center; this raises speculation that it may have a black hole of approximately 1 billion solar masses at its core.

Finally, recent observations of the center of dense globular clusters have suggested the possibility that massive black holes (thousands of solar masses) may exist there also.