LESSON 3: The Galaxy
The Milky Way
The Milky Way, the large, disk-shaped aggregation of stars, or galaxy, that includes the sun and its solar system. Its name is derived from its appearance as a faintly luminous band that stretches across earth's sky at night. This band is the disk in which the solar system lies. Its hazy appearance results from the combined light of stars too far away to be distinguished individually by the unaided eye. The individual stars that are distinct in the sky are those in the Milky Way galaxy that lie sufficiently close to the solar system to be discerned separately. From the middle northern latitudes, the Milky Way is best seen on clear, moonless, summer nights, when it appears as a luminous, irregular band circling the sky from the northeastern to the southeastern horizon. The Milky Way has been determined to be a large spiral galaxy, with several spiral arms coiling around a central bulge about 10,000 light-years thick. The stars in the central bulge are closer together than those in the arms, where more interstellar clouds of dust and gas are found. The diameter of the disk is about 100,000 light-years. It is surrounded by a larger cloud of hydrogen gas, warped and scalloped at its edges, and surrounding this in turn is a spheroidal or somewhat flattened halo that contains many separate, globular clusters of stars mainly lying above or below the disk. This halo may be more than twice as wide as the disk itself. In addition, studies of galactic movements suggest that the Milky Way system contains far more matter than is accounted for by the known disk and attendant clusters up to 2000 billion times more mass than the sun contains. Astronomers have therefore speculated that the known Milky Way system is in turn surrounded by a much larger corona of undetected matter. Another recent speculation is that the Milky Way is a barred spiral galaxy. The Milky Way contains both the so-called type I stars, brilliant, blue stars; and type II stars, giant red stars. The central Milky Way and the halo are composed of the type II population. Most of this region is obscured behind dust clouds, which prevent visual observation. Radiation from the central region has been recorded by use of such special devices as photoelectric cells, infrared filters, and radio telescopes. Such studies indicate compact objects near the galactic center, possibly starburst remnants or a massive black hole. Surrounding the central region is a fairly flat disk comprising stars of both type II and type I; the brightest members of the latter category are luminous, blue supergiants. Imbedded in the disk, and emerging from opposite sides of the central region, are the spiral arms, which contain a majority of the type I population together with much interstellar dust and gas. One arm passes in the vicinity of the sun and includes the great nebula in Orion. The Milky Way rotates around an axis joining the galactic poles. Viewed from the north galactic pole, the rotation of the Milky Way is clockwise, and the spiral arms trail in the same direction. The period of rotation decreases with the distance from the center of the galactic system. In the neighborhood of the solar system the period of rotation is more than 200 million years. The speed of the solar system due to the galactic rotation is about 270 km/sec (about 170 mi/sec).
Nebulae
A nebula, in astronomy, is a localized conglomerate of the gaseous and finely divided dust particles that are spread throughout interstellar space. Before the invention of the telescope, the term nebula (Latin, cloud ) was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were called nebulas. Nebulas exist within other galaxies as well as in our own Milky Way galaxy. They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of formation. Planetary nebulas, or planetaries, are so called because many of them superficially resemble planets through telescopes. They are actually shells of material that an old average star sheds during a late, red giant stage in its evolution, before becoming a white dwarf. The Ring nebula of the constellation Lyra, a typical planetary, has a rotational period of 132,900 years and a mass calculated to be about 14 times that of the earth's sun. Several thousand planetaries have been discovered in the Milky Way. More spectacular but fewer in number are nebulas that are the fragments of supernova explosions, perhaps the most famous of which is the Crab nebula in Taurus, now fading at the rate of about 0.4 percent per year. Nebulas of this kind are strong emitters of radio waves, as a result of the explosions that formed them and the probable pulsar remnants of the original star. Diffuse nebulas are extremely large structures, often many light-years wide, that have no definite outline and a tenuous, cloudlike appearance. They are either luminous or dark. The former shine as a result of the light of neighboring stars. They include some of the most striking objects in the sky, such as the Great nebula in Orion (the middle star in the sword). The tremendous streams of matter in the diffuse nebulas are intermingled in violent, chaotic currents. Many thousands of luminous nebulas are known. Spectral studies show that light emanating from them consists of reflected light from stars and also, in so-called emission nebulas, of stimulated radiation of ionized gases and dust from the nebulas themselves. Dark, diffuse nebulas are observed as nonluminous clouds or faintly luminous, obscuring portions of the Milky Way and too distant from the stimulation of neighboring stars to reflect or emit much light of their own. One of the most famous dark nebulas is the Horsehead nebula in Orion, so named for the silhouette of the dark mass in front of a more luminous nebular region. The longest dark rift observed on photographic plates of the star clouds of the Milky Way is a succession of dark nebulas. Both dark nebulas and luminous nebulas are considered likely sites for the processes of dust-cloud condensation and the formation of new stars.
Nova and Supernova
Nova and Supernova (Latin novus, new ), in astronomy, names of two kinds of explosive events that take place in some stars. A nova is a star that suddenly increases greatly in brightness and then slowly fades, but may continue to exist for some time. A supernova exhibits the same pattern of behavior, but the causative explosion destroys or profoundly alters the star. Supernovas are much rarer than novas, which are observed fairly frequently in photographs of the sky. Before the era of modern astronomy, a star that appeared suddenly where none had been seen before was called a nova, or new star. This is a misnomer, as the stars involved had existed long before they became visible to the naked eye. Astronomers estimate that perhaps about a dozen novas occur in the Milky Way, or the earth's galaxy, each year, but two or three of them are too distant to be seen or are obscured by interstellar matter. Indeed, novas are often more easily observed in other, nearby galaxies rather than in the earth's. Novas are named according to the year of their occurrence and the constellation in which they appear. Typically, a nova flares up to several thousand times its original brightness in a matter of days or hours. It next enters a transition stage, during which it may fade and grow bright again and then fade gradually to or near its original level of brightness. Novas may be considered variable stars in a late stage of evolution. They apparently behave as they do because their outer layers have built up an excess of helium through nuclear reactions and expand too rapidly to be contained. The star explosively emits a small fraction of its mass as a shell of gas the cause of the increase in brightness and then settles down. Such a star is typically a white dwarf and is commonly thought to be the smaller member of a binary (two-star) system, subject to a continuous infall of matter from the larger star. This is perhaps always the case with dwarf novas, which erupt repeatedly at regular intervals of a few to hundreds of days. Novas in general show a relationship between their maximum brightness and the time they take to fade a certain number of magnitudes. By means of measurements of nearer novas of known distance and magnitude, astronomers can use novas in other galaxies as indicators of the distance to those galaxies. A supernova explosion is far more spectacular and destructive than a nova and much rarer. Such events may occur no more than once every few years in the Galaxy; and despite their increase in brilliance by a factor of billions, only a few are ever observable to the naked eye. Until 1987, only three had been positively identified in recorded history, the best known of which is the one that occurred in ad 1054 and is now known as the Crab nebula. Supernovas, like novas, are more often seen in other galaxies. Thus, the most recent supernova, which appeared in the southern hemisphere on February 24, 1987, was found located in a companion galaxy, the Large Magellanic Cloud. This supernova, which exhibits some unusual traits, is now the object of intense astronomical scrutiny. The mechanisms that produce supernovas are less certain than those of novas, particularly in the case of stars approximately as massive as the earth's sun, an average star. Stars that are much more massive, however, sometimes explode in the late stages of their rapid evolution as a result of gravitational collapse, when the pressure created by nuclear processes within the star is no longer able to withstand the weight of the star's outlying layers. Little may remain after the explosion except the expanding shell of gases. The Crab nebula has left behind a pulsar, or rapidly rotating neutron star. Supernovas are significant contributors to the interstellar material that forms new stars.
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LESSON 4: The Formation of the Universe
Cosmology
Cosmology, the study of the universe as a whole, including theories about its origin, evolution, large-scale structure, and future. The more specific study of the origin of the universe and of astronomical systems, such as the solar system, is often called cosmogony. The earliest cosmological theories known from about 4000 bc are from the Mesopotamians, who believed that the earth is the center of the universe and that the other heavenly bodies move around it. The nightly motion of stars across the sky was explained by some ancients, such as Aristotle and the Greek astronomer Ptolemy, as the result of stars being fixed on rotating crystalline spheres. The Greek astronomer Aristarchus of Samos maintained, about 270 bc, that the earth revolves around the sun. Mainly because of Aristotle's authority, however, the concept of the earth as the center of the universe remained generally unchallenged until 1543, when the Polish astronomer Nicolaus Copernicus published his theories in De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres). Copernicus proposed a system in which the planets revolve in circular orbits around the sun, which he defined as the center of the universe. He attributed the rising and setting of the stars to the rotation of the earth on its axis. The German astronomer Johannes Kepler adopted the Copernican system and discovered that the planets move in elliptical orbits at varying speeds, according to three well-defined laws (since called Kepler's laws). Galileo, who first observed planets with a telescope, also rejected Aristotle's idea of the earth as the center of the universe and became a champion of the Copernican world view. The English mathematician and physicist Sir Isaac Newton showed that Kepler's laws of planetary motion could be derived from the general laws of motion and gravitation that Newton had discovered, thus indicating that these physical laws were valid in the heavens as well as on the earth.
The Big Bang Theory
In 1948 the Russian-American physicist George Gamow modified Lema”tre's theory of the primeval atom into the big bang theory of the origin of the universe. Gamow proposed that the universe was created in a gigantic explosion and that the various elements observed today were produced within the first few minutes after the big bang, when the extremely high temperature and density of the universe would fuse subatomic particles into the chemical elements. More recent calculations indicate that hydrogen and helium would have been the primary products of the big bang, with heavier elements being produced later within stars. This theory, however, provided a basis for understanding the earliest stages of the universe and its subsequent evolution. The extremely high density within the primeval atom would cause the universe to expand rapidly. As it expanded, the hydrogen and helium would cool and condense into stars and galaxies.
Black Hole
Stars, like any other thing, can shrink or grow in mass. At the same time the fundemental laws of physics state that there are forces which tend to keep matter apart. However, when a star grows large enough in mass so that it's gravitational attraction outweighs those forces which keep matter apart, an inexorable collapse occurs. The gravitational field may become so strong that even light cannot escape. The limits of such superdense objects, called BLACK HOLES, are defined by an EVENT HORIZON. This event horizon is not a physical surface but rather represents the point of no return for anything drawn towards it. Let's imagine that a poor unfortunate space traveler happens to enter an event horizon. As he or she teetered into oblivion, our space traveler would reach a point of singularity. We've already established that there is no space in said singularity, so it follows that time would also be as meaningless. So for our luckless space traveler who has reached the singularity, space and time, as well as existence, has come to an end. Now let's say that our space traveler is the luckiest person in the universe and happens to miss the singularity and instead goes through a small passage and comes out of a "white hole," which is the time reverse of a black hole. This white hole might be in another universe or at a different point in our universe. This point could be different in space or time (or both) from where the space traveler started out.
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