The Sun is a member of a system of stars known as the Galaxy. The system is popularly called the Milky Way, but this name is more properly restricted to the luminous band stretching across the sky. The name is the translation of the Latin Via Lactea; the Greek name was ‘Galaxies’.

The Milky Way has been known since very ancient times – on a dark, clear night it cannot be overlooked – and there are many legends about it. In Chinese tales there are two lovers, Chuc Nu and Nguu Lang, at the court of the Jade Emperor, who neglected their official duties and were banished to the sky as the stars Vega and Altair, on opposite sides of the Milky Way river. They are allowed to meet once a year, on the seventh day of the seventh month, when flocks of birds form a bridge across the river to enable the lovers to cross. In Finland, birds are again involved; it was thought that migratory birds used the Milky Way as a guideline to their home, Lintukoto – and there may be some truth in this! To the people of the Kalahari Desert the Milky Way was due to embers from a fire lit by a girl who wanted to find her way around on a starless night. The Aborigines of South Australia had a different idea; the Milky Way was Woodlipari, a celestial river where there are caves in which live dangerous creatures called yura (the caves themselves are yurakauwe, ‘homes of monsters’).

Patrick Moore has inspired generations of astronomers. He has done unparalleled service, through his handbooks, lectures and articles – not to mention his BBC programme The Sky at Night.

Over his prolific career, Patrick has witnessed, recorded and expounded a huge enlargement of our cosmic knowledge. To see this, one need only compare the present book with one of its precursors: the Guinness Book of Records in Astronomy published more than 50 years ago, at the dawn of the space age.

We owe this progress to sophisticated telescopes on the ground, and to a flotilla of instruments launched into space. The planets and moons of our Solar System are now better mapped that some parts of our Earth were before the twentieth century. An unsuspected population of trans-Neptunian objects has been revealed – telling us that the Solar System is more complex and extensive than thought hitherto. Even more important, planets have been detected around hundreds of other stars. The study of ‘extra-solar’ planets is proceeding apace: within a decade we will have discovered thousands of planetary systems, and will for the first time have evidence on just how unusual our Solar System is.

Novel technology has not only led to more powerful optical telescopes, but also to space telescopes that observe the cosmos in other wavebands out to distances exceeding 10 billion light years. We inhabit a much vaster Universe than was envisaged 50 years ago; we understand a surprising amount about how it evolved and what it contains.

This book is directed at the student undertaking a course in star formation for the first time. This may be in the later years of an undergraduate degree in physics, astrophysics, or physics with astronomy. Alternatively, it may be that the student only meets this subject for the first time during the first years of a masters degree. In either case we have assumed that the student already has a grounding in physics and mathematics, including, for example, Maxwell's equations, quantum mechanics and the laws of thermodynamics. Nevertheless, we find from teaching experience that brief reminders to students of things they learnt in other courses are generally welcomed as helpful. Hence, we remind the reader of some of the important points from other branches of physics where they are relevant.

We assume only a minimal knowledge of astronomy, and we derive the necessary astrophysical equations as we go along. We assume no prior knowledge of the subject of star formation itself and begin from first principles. Throughout the book we attempt to stay on ground that is firmly established, and try to avoid that which is trendy or the latest discovery. Experience has taught us that these matters often become outdated much more quickly than the solid foundations on which the subject is based. In cases where we stray onto less sure footing, we inform the reader that we are doing so.

The road to star formation

Thus far we have studied the places where stars form – molecular clouds. We have discussed the ways in which molecular clouds can be observed. We have explored the various constituents of molecular clouds – gas, dust, magnetic fields, cosmic rays and electromagnetic radiation. We have, so to speak, assembled the ingredients. In this chapter we discuss how these ingredients might come together to begin to form a star.

In the first half of the chapter we discuss theoretical considerations. We consider the collapse of an isothermal sphere of gas, ignoring the effects of rotation and magnetic fields, and we examine qualitatively what happens. We describe the method of solving the problem using similarity solutions.

We go on to discuss hierarchical fragmentation, as a means of breaking a large molecular cloud into an ensemble of stars. We also discuss the thermodynamics of protostellar gas, and explain how the minimum mass for star formation might be determined by the protostellar gas becoming optically thick to its own cooling radiation. We discuss the manner of the collapse to form a star and the possible effects of a magnetic field on this process.

In the second half of the chapter we examine some of the observational evidence. At the end we consider the initial mass function for stars. Note that in this chapter we concentrate mainly on relatively low-mass stars, i.e. stars of less than a few times the mass of the Sun. In Chapter 6 we continue to discuss relatively low-mass star formation.

When setting out to discuss the origin and evolution of the universe, we are at once confronted with three clear-cut alternatives. They are:

1. The universe began at a definite moment. This was also the beginning of time, so that there was no ‘before’. It will also cease to exist at a definite moment, so that there will be no ‘after’.

2. The universe has always existed, in which case we must accept a period of time which extends back for ever. It will always exist, so that there will be no ‘end’.

3. The universe began at a definite instant, which was also the beginning of time, so that there was no ‘before’. It will continue to exist for ever, so that there will be no ‘end’.

To explain any of these precepts in plain English is not easy, even for someone with the brain of a Newton or an Einstein!

TIMESCALE

The timescale of the universe was not appreciated until comparatively modern times. The age of the Earth itself was wildly underestimated by almost all scientists of the nineteenth century, and only gradually did the evidence provided by fossils and radiometric dating show that our world must be thousands of millions of years old.

James Ussher, Archbishop of Armagh (1581–1656) held very definite views. By basing his chronology upon key events recorded in the Bible, he found that the moment of the Creation was nine o'clock on Sunday, 23 October 4004 BC. Even today we find people who genuinely believe that everything in the Bible is literally true – and Creationism, re-named Intelligent Design, is so widespread in parts of the United States that some schools teach it as a serious alternative to Darwinian evolution!

Selecting a limited number of astronomers for short biographical notes may be somewhat invidious. However, the list given here includes most of the great pioneers and researchers. No astronomers still living at the time of writing are included. All dates are AD unless otherwise stated.

Abul Wafa, Mohammed. 959–88. Last of the famous Baghdad school of astronomers. He wrote a book called Almagest, a summary of Ptolemy's great work, also called the Almagest, in Arabic.

Adams, John Couch. 1819–92. English astronomer, born in Lidcot, Cornwall. He graduated brilliantly from Cambridge in 1843, but had already formulated a plan to search for a new planet by studying the perturbations of Uranus. By 1845, his results were ready, but no quick search was made, and the actual discovery was due to calculations by U. Le Verrier. Later he became Director of the Cambridge Observatory, and worked upon lunar acceleration, the orbit of the Leonid meteor shower, and upon various other investigations.

Airy, George Biddell. 1801–92. English astronomer. Born in Northumberland, he graduated from Cambridge 1823 and was Professor of Astronomy there (1826–35). On becoming Astronomer Royal (1835–81) he totally reorganised the Greenwich Observatory and raised it to its present eminence. He re-equipped the Observatory and ensured that the best use was made of its instruments; it is ironical that he is probably best remembered for his failure to instigate a prompt search for Neptune when receiving Adams' calculations.

Clusters and nebulæ are among the most striking of stellar objects. Several are easily visible with the naked eye. Few people can fail to recognize the lovely star cluster of the Pleiades or Seven Sisters, which has been known since prehistoric times and about which there are many old legends. The nebula in the Sword of Orion, the Sword-Handle in Perseus, Præsepe in Cancer and the Jewel Box cluster in Crux are other objects easily visible without optical aid. Keen-sighted people have little difficulty in locating the great Andromeda Spiral and the globular cluster in Hercules, while in the far south there are the two Clouds of Magellan which cannot possibly be overlooked, as well as the bright globular clusters ω Centauri and 47 Tucanæ.

The most famous of all catalogues of nebulous objects was compiled by the French astronomer Charles Messier, and published in 1781. Ironically, Messier was not interested in the objects he listed: he was a comet-hunter, and merely wanted a quick means of identifying misty patches which were non-cometary in nature. In 1888, J. L. E. Dreyer, Danish by birth (although he spent much of his life in Ireland, and finally in England) published his New General Catalogue (NGC), augmented in 1898 and again in 1908 by his Index Catalogue (I or IC).

Uranus, the seventh planet in order of distance from the Sun, was the first to be discovered in telescopic times, by William Herschel, in 1781. It is a giant world, but it and the outermost giant, Neptune, are very different from Jupiter and Saturn, both in size and in constitution. It is probably appropriate to refer to Jupiter and Saturn as gas giants and to Uranus and Neptune as ice giants.

Data for Uranus are given in Table 11.1.

MOVEMENTS

Since Uranus' synodic period is less than five days longer than our year, Uranus comes to opposition every year. Opposition dates for 2010–2020 are given in Table 11.2. The opposition magnitude does not vary a great deal; under good conditions the planet can just be seen with the naked eye. The most recent aphelion passage was that of 27 February 2007; Uranus was at its minimum distance from the Earth (21.09 a.u.) on 13 March of that year. The last perihelion passage was on 20 May 1966; Uranus was closest to the Earth (17.29 a.u.) on 9 March of that year. The next perihelion will be that of 13 August 2050.

In June 1989, Uranus reached its greatest southerly declination (–23.7°). Greatest northern declination had been reached in March 1950.

Close planetary conjunctions involving Uranus are listed in Table 11.3. It is interesting to note that between January and March 1610 Uranus was within 3° of Jupiter.

Variable star research is an important branch of modern astronomy – amateur observers make very valuable contributions. Variable stars are of many types; elaborate systems of classifying them have been proposed, and the data given here are not intended to be more than a general guide. Seven major categories are now recognised.

1. (1) Eclipsing stars (more properly eclipsing binaries, because they are not intrinsically variable).

2. (2) Pulsating variables: either radial or non-radial pulsations.

3. (3) Eruptive variables, where the changes are caused by flares or the ejection of shells of material.

4. (4) Cataclysmic variables, where the changes are due to explosions in the star or in an accretion disc round it. Novæ dwarf novæ and supernovæ come into this category.

5. (5) Rotating variables, where the changes are caused by star-spots, non-spherical shape or magnetic effects.

6. (6) X-ray variables, usually inherent in the neutron star or black hole companion of a binary.

7. (7) Unclassifiable stars, which do not fit into any accepted category.

We have already noted what are termed secular variables: stars which have permanently brightened or faded in historic times. Thus Ptolemy ranked β Leonis and θ Eridani as of the first magnitude, whereas today they are below magnitude 2 and 3 respectively: α Ophiuchi was ranked of magnitude 3, but is now 2.1. However, these changes must be regarded as highly suspect. It is unwise to trust the old observations too far.

OBSERVATORIES

Strictly speaking, an observatory is any place from which astronomical studies are carried out. It is even possible to claim that Stonehenge was an observatory, because there is little doubt that it is astronomically aligned. The oldest observatory building now standing seems to be that at Chomsong-dae in Kyingju, South Korea; it dates from AD 632. The name means ‘Star-gazing Tower’. Apparently, it was constructed under the reign of Queen Seondeok (632–647). It is 5.7 m wide at the base and 9.4 m high. Later, elaborate measuring instruments were built by the Arabs and the Indians; some of these still exist such as the great observatory at Delhi. In 1576, Tycho Brahe erected his elaborate observatory at Hven, in the Baltic, and used the equipment to draw up an amazingly accurate star catalogue. In the modern sense, observatories are of course associated with telescopes of some kind or another. A list of some great modern observatories is given in Table 30.4.

National observatories date back for centuries; the oldest seems to be that of Leiden in Holland (1632). The oldest truly national observatory was that at Copenhagen in Denmark, although unfortunately the original buildings were destroyed by fire.

The national British observatory, at Greenwich, was founded in 1675 by order of King Charles II, mainly so that a new star catalogue could be drawn up for the use of British seamen. The original buildings were designed by Wren and are now known as Flamsteed House (after the first Astronomer Royal).

The Earth is the largest and most massive of the inner group of planets. Data are given in Table 6.1. In the Solar System, only the Earth is suited for advanced life of our kind; it lies in the middle of the ‘ecosphere’, the region round the Sun where temperatures are neither too high nor too low. Venus lies at the extreme inner edge of the ecosphere, and Mars at the extreme outer edge.

The Earth–Moon system is often regarded as a double planet rather than as a planet and a satellite. The effect of tidal friction increases the Earth's axial rotation period by an average of 1.7 m s–1 per century.

STRUCTURE OF THE EARTH

The rigid outer crust and the upper mantle of the Earth's globe make up what is termed the lithosphere; below this comes the æsthenosphere, where rock is partially melted. Details of the Earth's structure are given in Table 6.2. The crust has an average depth of 10 km below the oceans, but down to around 50 km below the continents. The base of the crust is marked by the Mohorovičić discontinuity (the Moho) named after the Jugoslav scientist Andrija Mohorovičić, who discovered that the velocity of seismic waves changes abruptly at this depth, indicating a sudden change in density. Between 50 and 100 km below the surface the lithospheric rocks become hot and structurally weak.

The Solar System is made up of one star (the Sun), the eight planets with their satellites (Table 1.1) and various minor members such as asteroids, comets and meteoroids, plus a vast amount of thinly spread interplanetary matter. The Sun contains 99.86% of the total mass of the System, while Jupiter and Saturn account for 90% of what is left. Jupiter is the largest member of the planetary family, and is in fact more massive than all the other planets combined. Mainly because of Jupiter, the centre of gravity of the Solar System lies just outside the surface of the Sun.

The Solar System is divided into two parts. There are four comparatively small, rocky planets (Mercury, Venus, the Earth and Mars), beyond which comes the zone of the Main-Belt asteroids, of which only one (Ceres) is over 900 km in diameter. Next come the four giants (Jupiter, Saturn, Uranus and Neptune), plus a swarm of trans-Neptunian objects, of which the largest known are Eris and Pluto. For many years after its discovery, in 1930, Pluto was regarded as a true planet, but in August 2006 the International Astronomical Union, the controlling body of world astronomy, introduced a new scheme of classification, as follows:

A planet is any body in orbit round the Sun which is massive enough to assume a spherical shape, and has cleared its immediate neighbourhood of all smaller objects. All these criteria are met by the eight familiar planets, from Mercury to Neptune.

Venus, the second planet in order of distance from the Sun, is almost a twin of the Earth in size and mass; it is only very slightly smaller and less dense. However, in all other respects it is quite unlike the Earth. Only during the past 40 years have we been able to find out what Venus is really like; its surface is permanently hidden by its thick, cloudy atmosphere, and before the Space Age Venus was often referred to as ‘the planet of mystery’. Data are given in Table 5.1.

Venus is the brightest object in the sky apart from the Sun and the Moon. At its best it can even cast shadows – as was noted by the Greek astronomer Simplicius, in his Commentary on the Heavens of Aristotle, and by the Roman writer Pliny around 60 AD. Venus must have been known since prehistoric times. The most ancient observations which have come down to us are Babylonian, and are recorded on the Venus Tablet found by Sir Henry Layard at Konyunjik, now to be seen in the British Museum. Homer (Iliad, XXII, 318) refers to Venus as ‘the most beautiful star set in the sky’ and the name is, of course, that of the Goddess of Love and Beauty.

It may have been Pythagoras, in the sixth century BC, who first realised that the evening and morning apparitions of Venus relate to the same body – though like almost everyone else at that period, he believed Earth to be the centre of the universe.

Introduction

In this chapter we discuss some of the phenomena observed as a consequence of star formation. We describe some of the phenomena surrounding star formation, such as discs, outflows, and binary and multiple stars, and we discuss the difference between hydrogen-burning stars and brown dwarf stars.

We then go on to detail some of the larger-scale consequences, such as how star formation affects the host galaxy in which it occurs. In this context we also discuss starburst galaxies and galaxy mergers. Finally, we outline current understanding on when the major epoch of star formation occurred in the Universe.

Circumstellar discs

In Chapter 6 we discussed accretion onto protostars. In particular, we discussed spherically symmetric accretion. However, if the material accreting onto a protostar has angular momentum (and in general it does), the infall is not spherically symmetric, nor is it direct. Instead, the material accumulates in a circumstellar disc, and then spirals inwards onto the equator of the star on a time-scale determined by the efficiency of the processes which redistribute or remove the angular momentum in the disc. Such a disc is often termed an accretion disc. We also mentioned this in Chapter 7 as a method for increasing the accretion onto a high-mass protostar in the context of significant radiation pressure potentially halting the accretion.

Saturn is often regarded as the most beautiful object in the entire sky. Jupiter, Uranus and Neptune also have rings, but these systems are dark and obscure; Saturn's glorious, icy rings are unrivalled.

Saturn was the outermost planet known in pre-telescopic times, and is sixth in order of distance from the Sun. It is named in honour of the second ruler of Olympus, who succeeded his father Uranus and was himself succeeded by his son Jupiter (Zeus). It moves in the sky more slowly than the other bright planets, and has been associated with the passage of time. Data are given in Table 10.1.

MOVEMENTS

Saturn reaches opposition about 13 days later every year. Opposition dates for the period 2010–2020 are given in Table 10.2.

The opposition magnitude is affected both by Saturn's varying distance and by the angle of presentation of the rings. At its best, Saturn may outshine any star apart from Sirius and Canopus, but at the least favourable oppositions from this point of view – when the rings are edgewise-on, as in 2009 – the maximum magnitude may be little brighter than Aldebaran. A list of edgewise presentations is given in Table 10.3.

The intervals between successive edgewise presentations are 13 years 9 months and 15 years 9 months. During the shorter interval, the south pole is sunward; the southern ring-face is seen, and Saturn passes through perihelion. Perihelion fell in 1944 and 1974; the next will be in 2032. The aphelion dates are 1959, 1988 and 2018.

Meteors are cometary débris, too small and too friable to reach the surface of the Earth intact. Larger bodies, however, can survive the dash through the atmosphere, and land without being destroyed, though they may be fragmented. It may be helpful to give some definitions.

A meteoroid is defined by the IAU as ‘a solid object moving in interplanetary space, smaller than an asteroid and considerably larger than an atom’. This is all very well, but where exactly is the boundary between a meteoroid and an asteroid? The Royal Astronomical Society gives it as 10 m, but consider then the asteroid 2008 TN3, which impacted Earth on 7 October 2008. Its diameter was just about 10 metres, so that it could be classed either as a large meteoroid or a small asteroid; it was given an asteroid designation because it was followed telescopically well before it entered the atmosphere, exploded and broke into fragments. However, all the definitions could well be tightened up.

A meteorite is a body which has reached the Earth, or other planet, in recognisable form. If sufficiently large and dense, it may produce an impact crater. Note that the famous structure in Arizona is generally known as Meteor Crater; it really should be Meteorite Crater.

Meteorites and shooting-star meteors are very different. Most meteorites come from the asteroid belt, though some are believed to come from the Moon (see p. x) and others (the SNC meteorites) from Mars (see p. x).

About this book

It can be argued that astronomy is the oldest science. Since pre-historic times humans have gazed at the night sky and wondered about the nature and origin of stars. We now believe we understand a great deal about the nature of stars, but many aspects of the origin of stars remain the subject of intense study to this day.

In this book we aim to introduce the reader to the fundamentals of the subject of star formation. We describe the background physics underlying theories of star formation, and take the reader to the frontiers of current knowledge of this subject. However, we will make clear as we go along the points where we reach material that is less well established.

One of the most fundamental observations in astronomy is the fact that the night sky appears to be full of stars. Yet the processes which lead to the formation of those stars have taken astronomers many years to work out. Unlocking the mysteries of star formation has required the use of new techniques and the opening of new wavelength regimes to astronomy. We describe the chief physical processes which are believed to be important for star formation, and point out the role which each branch of observational astronomy has played in solving the various problems associated with star formation.