Astronomical Terminology
Total Page:16
File Type:pdf, Size:1020Kb
APPENDIX A Astronomical Terminology A: 1 Introduction When we disco ver a new type of astronomical entity on an optical image of the sky or in a radio-astronomical record, we refer to it as a new object. It need not be astar. It might be a galaxy, a planet, or perhaps a cloud of interstellar matter. The word "object" is convenient because it allows us to discuss the entity before its true character is established. Astronomy seeks to provide an accurate description of all natural objects beyond the Earth's atmosphere. From time to time the brightness of an object may change, or its color might become altered, or else it might go through some other kind oftransition. We then talk about the occurence of an event. Astrophysics attempts to explain the sequence of events that mark the evolution of astronomical objects. A great variety of different objects populate the Universe. Three ofthese concern us most immediately in everyday life: The Sun that lights our atmosphere during the day and establishes the moderate temperatures needed for the existence oflife, the Earth that forms our habitat, and the Moon that occasionally lights the night sky. Fainter, but far more numerous, are the stars that we can only see after the Sun has set. The objects we detect can be divided into two groups. Many ofthem are faint, and we would not be able to see them if they were not very close to the Sun; others are bright, but at much larger distances. The first group of objects, taken together with the Sun, comprise the Solar System. They form a gravitationally bound group orbiting a common center of mass. Within the Solar System the Sun itself is of greatest astronomical interest in many ways. It is the one star that we can study in great detail and at close range. Ultimately it may reveal precisely what nuclear processes take place in its center and just how a star derives its energy. Complementing such observations, the study of planets, comets, and meteorites 576 Appendix A may ultimately reveal the history ofthe Solar System and the origins oflife. Both of these are fascinating problems. A:2 The Sun The Sun is a star. Stars are luminous bodies whose masses range from about 1032 to 1035 g. Their luminosity in the visual part ofthe spectrum normally lies in the range between 10-4 and 104 times the Sun's energy outflow. The surface temperatures of these stars may range from no more than '" 1, 000 K to about 50,000 K. Just how we can determine the relative brightness of stars will be seen later in this Appendix. The determination oftemperatures is discussed in Chapter 4. The Sun, viewed as astar, has the following features: (a) Its radius is 6.96 x 10 10 cm. Although occasional prominences jut out from the solar surface, its basic shape is spherical. The equatorial radius is only a fractional amount larger than the polar radius: [(req - rpoI)/r] ~ 6 X 10-6 (Di86). (b) The Sun emits a total flux of3.9 x 1033 erg S-I. Nearly half ofthis radiation is visible, but an appreciable fraction of the power is emitted in the near ultraviolet and near infrared parts ofthe spectrum. Solar X-ray and radio emission make only slight contributions to the totalluminosity. (c) The Sun's mass is 1.99 x 1033 g. (d) Three principal layers make up the Sun's atmosphere. They are the photosphere, chromosphere, and corona. (i) The photosphere is the layer from which the Sun's visible light emanates. It has a temperature of ab out 6,000 K. (ii) The chromosphere is a layer some ten to fifteen thousand kilometers thick. It separates the relatively cool photosphere from the far hotter corona. (iii) The corona extends from 1.03 R0 , or about 20,000 km above the photo sphere, out to at least several solar radii. The outer boundary has not been defined. The corona is not a static structure; its outer edge merges continuously into the interplanetary gas that streams outward from the Sun at speeds of several hundred kilometers per second. This streaming, ionized gas, mainly protons and electrons, is called the solar wind. The temperature of the corona is '" 1.5 x 106 K. (e) Sunspots and Sunspot groups, cool regions on the solar surface, move with the Sun as it rotates, and allow us to determine a 27 -day rotation period. This period is only an apparent rotation rate as viewed from the Earth which itself orbits the Sun. The actual rotation period with respect to the fixed stars is only about 25! days at a latitude of 15 0 and varies slightly with latitude; the solar surface does not rotate as asolid shell. The Sun exhibits an II-year solar cycle during which time the number of sunspots increases to a maximum and then declines to a minimum. At minimum the number of spots on the Sun may be as low as zero. At maximum the number of individual sunspots or members of a sunspot group mayamount to 150. There are special ways of counting to arrive at this sunspot number and a continuous record is kept through the collaborative effort of a number of observatories. A:3 The Solar System 577 The ll-year cycle is actually only half of a longer 22-year cycle that takes into account the polarity and arrangement of magnetic fields in sunspot pairs. (f) A variety of different events can take place on the Sun. Each type has a name of its own. One of the most interesting is a jlare, a brief burst of light near a sunspot group. Associated with the visible flare is the emission of solar cosmic ray particles, X -rays, ultraviolet radiation, and radio waves. Flares are also associated with the emission of clouds of electrons and protons that constitute a large component added to the normal solar wind. After a day or two, required for the Sun-to-Earth transit at a speed of'" 10 3 km S-1 , these particles can impinge on the Earth's magnetosphere (magnetic field and ionosphere), giving rise to magnetic storms and aurorae. These disturbances tend to corrugate the ionosphere and make it difficult to reflect radio waves smoothly. Since radio communication depends on smooth, continuous ionospheric reflection, reliable radio communication is sometimes disrupted for as long as a day during such magnetic storms. A:3 The Solar System A variety of different objects orbit the Sun. Together they make up the Solar System (Fig. A.l). The Earth is representative ofplanetary objects. Planets are large bodies orbiting the Sun. They are seen primarily by reflected sunlight. The majority emit hardly any radiation by themselves. In order of increasing distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. All the planets orbit the Sun in one direction; this direction is called direct. Bodies moving in the opposite direction are said to have retrograde orbits. Table 1.3 gives same of the more important data ab out planets. It shows that the different planets are characterized by a wide range of size, surface temperature and chemistry, magnetic field strength, and other properties. One of the aims of astrophysics is to understand such differences, perhaps in terms of the history of the solar system. Besides the nine planets we have listed, there are many more minor planets, or asteroids, orbiting the Sun. Most ofthem travel along paths lying between the orbits of Mars and Jupiter, a region known as the asteroidal belt. The largest asteroid is Ceres. Its radius is 350 km. Its mass is about one ten-thousandth that ofEarth. A number ofobjects collectively known as Centaurs are intermediate in diameter between typical comets and small icy planets or planetary satellites. They have short-lived orbits intermingled with those ofthe outer planets. Their diameters are estimated at 30 to 200 km and they appear to be drawn from the Kuiper beft, a region beyond the outer planets inhabited by perhaps a hundred thousand objects with diameters greater than 100 km and orbits between 50 and 100 AU. Discovered as the first ofthis group in 1992, is 1992QB1, with a diameter of 180 km and a stable, nearly circular orbit about the Sun, some 14 AU beyond Neptune. More recently a number of comets have also been discovered at these distances, and 578 Appendix A 90· I FIGURE A.l. Cornparison of planetary, asteroidal, and short-period cornetary orbits. Al though the Earth, Mars, and Jupiter have nearly circular orbits, the orbits of asteroids, Icarus, Hermes, Eros, Apollo, Kepler, and Hidalgo are appreciably eccentric, as are those ofcornets Encke, Pons--Winnecke, Ternpel-Swift, Whippie, Tuttle--Giacobini-Kresak, and Biela. Cornets are narned after their discoverers. Many cornets and asteroids have aphelion distances near Jupiter's orbit, and Jupiter has a controlling infiuence on the shape of the orbits and rnay have "captured" cornets frorn parabolic orbits into short-period orbits. estimates suggest that the Kuiper belt may contain several hundred million to a billion smaller cometary bodies (St96). Many of the smaller known asteroids, whose orbits lie mainly between Mars and Jupiter, have diameters of the order of a kilometer. These objects number A:3 The Solar System 579 in the thousands and there must be many more orbiting masses that are too small to have been observed. Among these are bodies that might only be a few meters in diameter or smaller. From time to time, some of these approach the Earth and survive the journey through the atmosphere.