Each star is a self-luminous gaseous globe similar to the Sun. Some are larger, some smaller, some hotter, some cooler, some more brilliant, and some inherently dimmer. All are located at distances that are many orders of magnitude greater than the separation between the Earth and the Sun. Our knowledge of stars has been built up primarily from measurements of their brightnesses and changes in brightness, their positions and changes in position, and their colors and spectra, as well as by applying our knowledge of physics and chemistry to interpreting these observations.

BRIGHTNESS AND MAGNITUDES

The brightness of a star as seen in the sky is described by a quantity called apparent magnitude – a measure of the amount of light arriving here on Earth that owes its origins to the work of Hipparchus, the outstanding Greek observer of the second century B.C. Hipparchus divided stars into six classes, or magnitudes, with the brightest stars being designated first magnitude and the faintest visible stars, sixth magnitude.

The magnitude system was put on a firm mathematical footing in 1856 by the English astronomer N. R. Pogson. He defined the magnitude scale in such a way that a difference of five magnitudes (e.g., between magnitude 1 and magnitude 6 or between magnitude 6 and magnitude 11) corresponds to a difference in brightness of a factor of precisely one hundred, and a difference of one magnitude corresponds to a brightness factor equal to the fifth root of one hundred, which is 2.512.

Although all stars are born inside collapsing clouds of gas and dust, their masses, temperatures, luminosities, and final fates can be very different, and their lifespans can range from as little as a few million years to more than a thousand billion years.

STELLAR EVOLUTION AND THE H-R DIAGRAM

As described in Chapter 10, a collapsing gas cloud forms a protostar, which then continues slowly to contract while accreting more material from the surrounding nebula. As the protostar gains mass and continues to contract, its central temperature continues to rise until, when it reaches about 10 million K or so, nuclear fusion reactions commence in its core. Pressure exerted by the hot gas in the star's interior halts the contraction, and it then settles down to a long-lived stable phase of existence as a main-sequence star. The changing luminosity and surface temperature of a star, as it passes through its life cycle, can be plotted on a Hertzsprung-Russell (H-R) diagram, the resulting line of which is called an evolutionary track.

PRE–MAIN SEQUENCE

As the protostar continues to accrete more matter, its surface temperature increases and its luminosity rises for a time to a value much higher than its eventual main-sequence luminosity. Plotted on the H-R diagram, the newly forming protostar would appear above and to the right of the main sequence, moving, as it evolved, from right to left (in the direction of increasing temperature) and upward (in the direction of increasing luminosity).

In addition to the nine planets and their moons, the Solar System contains a wide variety of minor bodies – asteroids, comets, meteorites, and meteoroids. Although these wandering fragments of material are tiny compared with the planets, they hold many clues to the formation of the Solar System as a whole and sometimes give rise to spectacular phenomena.

ASTEROIDS

The asteroids, or minor planets, are small bodies, ranging in diameter from about 940 km down to less than 1 km, that revolve around the Sun in independent orbits. Although the first asteroid to be discovered was found by chance, a curious numerical relationship between the distances of the planets from the Sun had already led a number of astronomers to mount a search for what they thought might be a “missing planet” between the orbits of Mars and Jupiter.

In 1772, the German mathematician Johann Bode drew attention to the following relationship that had previously been noted by Johann Titius and that has generally come to be known as Bode's law or the Titius–Bode law: Take the sequence of numbers, 0, 3, 6, 12, 24, 48, 96, and so on, where each successive number after 3 is double the preceding one; add 4 to each (4, 7, 10, …), then divide by 10.

As conscious beings, we have a deep-seated desire to explore our own relationship with the cosmos, to answer questions such as: Does life exist elsewhere, and if so, is it widespread or rare? Does intelligent life exist “out there”? Are there advanced civilizations with whom we may be able to communicate, or are we unique and wholly alone in this vast universe? Is the universe itself unique, or is there a multiplicity of universes? Was the universe “designed” to provide the conditions in which sentient observers can arise and flourish, or is our existence the result merely of a fortunate combination of chance circumstances?

LIFE ELSEWHERE?

The only place in the universe where we know for certain that life does exist is on our own planet, Earth. Although it is conceivable that there may be lifeforms that are utterly different in all respects from life as we know it, we cannot conduct a rational discussion on the basis of out-and-out speculation. When trying to establish whether or not life exists elsewhere, therefore, we have to examine the conditions that are necessary for life based on the same kinds of chemistry and structures as we find here on Earth in order to originate, evolve, and flourish.

Life on Earth is based on the ability of carbon, in combination with such elements as hydrogen, nitrogen, and oxygen, to form the complex molecules, chains, and self-replicating structures that have enabled living organisms to grow and pass on the coded information (genetic information) that is needed to reproduce their own kind.

The Earth spins around from west to east. If you were hovering above the north pole, you would see the Earth turn beneath you in a counterclockwise direction, whereas, if you were above the south pole, it would be seen to spin in a clockwise fashion. An observer on the rotating Earth is rather like an observer on a roundabout. Looking outward from a spinning roundabout it appears as if the rest of the world is spinning around you; you know that it is not, but that is the way it looks and feels. Likewise, to an observer on the rotating Earth, it seems as if the Sun, Moon, stars, and planets are revolving round our planet from east to west. The ancient Greek astronomers believed that the stars were fixed to a huge sphere that rotated round the Earth once a day. Although we now know that the stars are all remote suns lying at vast and very different distances from us, when we try to describe their positions and apparent motions, it is convenient to imagine that they are indeed attached to the inside of a sphere – the celestial sphere – that rotates around our planet.

CELESTIAL POLES AND EQUATOR

By analogy with the Earth, we can define the equator and poles of the celestial sphere. The Earth's axis – extended into space – meets the imaginary sphere at two points, the north and south celestial poles.

THE EARTH – OUR HOME PLANET

The Earth is the largest and most massive of the four terrestrial planets. Although its diameter of 12,756 km is only a few hundred kilometers greater than that of Venus, it is marginally more massive than the other three terrestrial planets (Mercury, Venus, and Mars) put together. The Earth is unique among the known planets in having liquid water on its surface (just over 70 percent of its surface is covered by water), in having an atmosphere that contains substantial quantities of oxygen (about 21 percent by volume), and in supporting a diverse variety of lifeforms.

Composition and Structure

The fact that the densities of the surface rocks – about 2,500–3,000 kg/m3 – are significantly less than the mean density of the planet (5,520 kg/m3) indicates that the composition and density of the Earth's deep interior differs from that of its surface layer and implies the presence of a large central core consisting mainly of iron. Immediately after its formation (see Chapter 10), the Earth is believed to have been molten throughout. This enabled differentiation to take place – the heavier elements (e.g., iron) sank to the center, and the lighter rocks floated to the surface. Even today, 4,600 million years after the birth of our planet, energy released by the radioactive decay of elements such as uranium and thorium ensure that the central temperature of the Earth is maintained at about 5,000 K or more.

The nine planets that orbit the Sun can be divided into two principal groups – the terrestrial planets and the giant planets. Mercury, Venus, the Earth, and Mars are known as the terrestrial (Earth-like) planets. Like the Earth itself, they are all relatively small, dense bodies composed of rocks and metals such as iron and nickel; they all have solid surfaces on which a spacecraft could land. Jupiter, Saturn, Uranus, and Neptune are called the giant, or jovian (Jupiter-like), planets because they are much larger than the Earth and are similar in a number of respects to the planet Jupiter. There are significant differences, though, between the larger giants (Jupiter and Saturn), which, like the Sun, are composed mainly of hydrogen and helium, and the smaller giants (Uranus and Neptune), which contain a much higher proportion of “icy” materials. Pluto fits into neither category. It is a tiny world of rock and ice on the outer fringes of the Solar System.

THE TERRESTRIAL PLANETS

Mercury

Innermost member of the Sun's system of planets, Mercury, with a diameter of 4,878 km, is the smallest of the terrestrial planets. At its mean distance of 57.9 million km (0.387 AU), it travels around the Sun in just under 88 days. Its orbit is markedly elongated; the planet's distance from the Sun ranges from 45.9 million km (0.306 AU) at perihelion to 69.8 million km (0.467 AU) at aphelion.

Although in all respects an ordinary and unexceptional star, the Sun – because it is so much nearer to us than any other star – is the only star whose features and characteristics can be studied in depth and detail.

Located at a mean distance of 149,600,000 km – 1 AU – the Sun is an incandescent gaseous body with a radius of 696,000 km (109 times that of the Earth). Although its huge globe could contain about 1.3 million bodies the size of our planet, the Sun has a mass only about 330,000 Earth masses, and its mean density, therefore, is only about one quarter that of the Earth – about 1,400 kg/m3. This relatively low density, together with spectroscopic evidence, indicates that the Sun is composed predominantly of the lightest chemical elements. Its composition, by mass, is about 73.5 percent hydrogen, 25 percent helium, and about 1.5 percent heavier elements, with the most abundant of the heavier elements being carbon, oxygen, nitrogen, neon, and iron.

The Sun's effective surface temperature is about 5800 K and its luminosity – the total amount of energy radiated into space every second – is 3.86 × 1026 watts. Because solar radiation spreads out isotropically (equally in all directions), the solar flux – the amount of radiant energy per second passing perpendicularly through an area of 1 square meter – decreases with the square of distance.

The term active galaxy is used to describe a wide variety of galaxies that have unusual characteristics and that often have a peculiar, disturbed appearance. Whereas most of the energy radiated by a conventional galaxy like the Milky Way system is starlight – the combined output of the billions of stars and HII regions that it contains – an active galaxy radiates strongly over a wide range of wavelengths and is much brighter than an ordinary galaxy at radio, infrared, ultraviolet, and x-ray wavelengths. Their overall luminosities range from 1037 to more than 1040 watts (up to several thousand times that of the Milky Way Galaxy). Much of the energy radiated by an active galaxy is emitted by charged particles moving at high speeds in magnetic fields rather than by stars. An active galaxy contains a compact, highly luminous core that, in many cases, varies markedly and rapidly in brightness, and from which, in numerous cases, narrow jets of radiating material are being ejected. The seat of all this activity is known as an active galactic nucleus (AGN).

The principal types of active galaxies are radio galaxies, quasars, BL Lacertae objects (BL Lacs) or “blazars,” and Seyfert galaxies.

RADIO GALAXIES

Radio galaxies are so named because they are powerful sources of radio emission that radiate much more strongly at radio wavelengths than do conventional galaxies.

absolute magnitude The apparent magnitude that a star would have if it were located at a distance of 10 parsecs (32.6 light-years).

absolute zero The temperature at which all random molecular motion ceases, and hence the lowest possible temperature: OK (-273° C).

absorption line A dark spectral line corresponding to radiation of one particular wavelength that is absorbed when electrons jump up from a lower to a higher energy level within atoms or ions of a particular element.

absorption line spectrum Dark (absorption) lines superimposed on a continuous spectrum as a consequence of the absorption of those particular wavelengths by atoms, ions, or molecules.

acceleration The rate of change of velocity. Acceleration may take the form of a change in speed, a change in direction, or a change in both.

accretion The gradual accumulation of matter in a particular location under the action of gravity. The colliding and sticking together of small particles to make progressively larger ones.

accretion disc A disc of gas around a star or a compact massive object such as a white dwarf, neutron star, or black hole that has been accreted from a neighboring star or from its surroundings.

achromatic lens A lens consisting of two components, made of different types of glass, that reduces the effects of chromatic aberration.

active galactic nucleus (AGN) The compact core of an active galaxy. An AGN is normally highly luminous and in many cases varies markedly in brightness.

THE NATURE OF ASTRONOMY

Astronomy is the science of the universe. It deals with individual objects such as planets, moons, stars, and galaxies and with the large-scale structure of the universe as a whole. Astronomers are concerned not only with discovering what is “out there” but also with why celestial bodies are as they are and behave as they do, with the forces that govern the behavior of matter and radiation in the cosmos, and with the origin, evolution, future, and ultimate fate of the universe and all that it contains.

Astronomy is primarily an observational rather than an experimental, or laboratory, science. Whereas a physicist or chemist can set up an experiment in the laboratory under known conditions, change the conditions, and measure the outcome, an astronomer cannot, for example, compress a star to see what happens. Unlike in our immediate neighborhood, where spacecraft can investigate directly the properties of planets, moons, and interplanetary space, astronomers have to rely on receiving information in the form of radiation from distant objects – observing without influencing or touching what they observe. They use their observations to develop hypotheses and theories and then carry out further observations to test the validity of those theories.

Astronomers draw on many other sciences, notably physics, chemistry, and mathematics, but also geology, biology, and a variety of other subjects, in order to analyze, interpret, and understand their observations.