A Astronomical Terminology A:1 Introduction When we discover 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 a star. 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 of transition. We then talk about the occurrence 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 of these 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 of life, 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 nearest to us in space comprise the Solar System. They form a grav- itationally bound group orbiting a common center of mass. The Sun is the one star that we can study in great detail and at close range. Ultimately it may reveal pre- cisely 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 mete- orites may ultimately reveal the history of the Solar System and the origins of life. Both of these are fascinating problems. Beyond the Solar System lies the rest of the Universe, the grand structure of which we form a minuscule part. 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 of the spectrum normally lies in the range between 10−4 and 106 times the Sun’s energy outflow. The surface temperatures of these stars may range from no more than ∼1000 K to about 50,000 K. Later in 634 Appendix A this Appendix, we will see just how we can determine the relative brightness of stars, and the difference between stars and their lower-mass counterparts, the brown dwarfs. The determination of temperatures is discussed in Chapter 4. The Sun, viewed as a star, has the following features. (a) Its radius is 6.96 × 1010 cm. Although occasional prominences jut out from the solar surface, its basic shape is spherical. The equatorial radius is only a frac- −6 tional amount larger than the polar radius: [(req − rpol)/r] 6 × 10 (Di86). (b) The Sun’s radiant luminosity, the rate at which it emits electromagnetic en- ergy, is 3.85×1033 erg s−1. Nearly half of this radiation is visible, but an appreciable fraction of the power is emitted in the near ultraviolet and near infrared parts of the spectrum. Solar X-ray and radio emission make only slight contributions to the total luminosity. (c) The Sun’s mass is 1.99 × 1033 g. (d) Three principal layers make up the Sun’s atmosphere. They are the photo- sphere, chromosphere, and corona. (i) The photosphere is the layer from which the Sun’s visible light emanates. It has a temperature of about 6000 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. 6 (iii) The corona, whose temperature is ∼1.5 × 10 K, extends from 1.03R, or about 20,000 km above the photosphere, out to at least several solar radii. Its outer edge merges continuously into the solar wind — interplanetary gas, mainly protons and electrons — that streams out from the Sun at speeds of several hundred kilometers per second. (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 1 Sun. The actual rotation period with respect to the fixed stars is only about 25 2 days at a latitude of 15 ◦ and varies slightly with latitude; the solar surface does not rotate as a solid shell. The Sun exhibits an 11 year solar cycle during which the number of sunspots increases to a maximum and then declines to a minimum. There are special ways of counting sunspots, and a continuous record is kept through the collaborative effort of observatories. At maximum the sunspot number can range to 150. At minimum it can be zero. The 11 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 flare, 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 greatly amplify the solar wind. After a day or two, required for the Sun-to-Earth transit at a speed of ∼103 km s−1,these particles can impinge on the Earth’s magnetosphere (magnetic field and ionosphere), giving rise to magnetic storms and aurorae that corrugate the ionosphere, disrupting A:3 The Solar System 635 radio communication that depends on smooth ionospheric reflection, sometimes for as long as a day. A:3 The Solar System A variety of different objects orbit the Sun. Together they make up the Solar System. The Earth is representative of planetary objects. Planets are large bodies orbiting the Sun. They are seen primarily by reflected sunlight. The majority emit hardly any radiation 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 1.4 gives some of the more important data about 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. Many of these 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 of Earth. Comets are objects that, on approaching the Sun from large distances, disinte- grate through solar heating: gases that initially were in a frozen state are evaporated off and dust grains originally held in place by these volatile substances are released. The dust and gas, respectively, are seen in reflected and re-emitted sunlight. They make the comet appear diffuse (Fig. A.1). Comet tails are produced when freshly released dust and gas that becomes ionized are repelled from the Sun, respectively, by the pressure of sunlight and by the solar wind. The Solar System may contain as many as 1011 comets, most of them in a giant cloud stretching into interstellar space but still gravitationally bound to the Sun. This Oort cloud isnamedafterJanOort, the Dutch astronomer who originallyproposed its existence. Comets are named after their discoverers. Many comets and asteroids have aphelion distances near Jupiter’s orbit, and Jupiter has a controlling influence on the shape of the orbits and may have “captured” comets from parabolic orbits into short-period orbits. A number of objects collectively known as Centaurs are intermediate in diame- ter between typical comets and small icy planets or planetary satellites. They have short-period orbits intermingled with those of the outer planets. Their diameters are estimated at 30 to 200 km and they appear to be drawn from the Kuiper belt,are- gion 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 of this group in 1992, is 1992QB1, with a diameter of 180 km and a sta- ble, nearly circular orbit about the Sun, some 14 AU beyond Neptune. Pluto and its companion Charon may have originated in the Kuiper Belt. More recently a number of comets have also been discovered at Kuiper belt distances, and estimates sug- 636 Appendix A gest that the 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. They number in the thou- sands 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 di- ameter or smaller. From time to time, some of these approach the Earth and survive the journey through the atmosphere.