M Abbreviation for the ➤ Messier catalog of galaxies, nebulae and star clusters.

Mab A small inner satellite of Uranus, discovered in 2003. It is about 25 km (16 miles) across. Mab orbits Uranus at the same distance as one of Uranus's rings and may be the source of the material for that ring.

Magellan A US spacecraft placed in orbit around ➤ Venus to map the surface by means of ➤ synthetic aperture radar. It was launched from the Space Shuttle Atlantis on May 4, 1989. The use of radar was essential because Venus is perpetually covered by opaque cloud. Magellan arrived at Venus on August 10, 1990, and completed its first phase of operations in May 1991, having mapped 84 percent of the surface. The next phase of observation involved filling in gaps and making more detailed observations. Magellan burned up in the venusian atmosphere in 1994.

Magellanic Clouds Two small, irregular galaxies, which are satellites of our own ➤ Galaxy. They are visible as hazy patches in the southern sky. The Large Magellanic Cloud (LMC) is in the constellation Dorado and is about 170 000 light years away. The Small Magellanic Cloud (SMC), in Tucana, is about 210 000 light years distant.

Magellanic Stream A long streamer of neutral hydrogen gas apparently spanning the 200 000 light years between the ➤ Magellanic Clouds and our own ➤ Galaxy. It forms an arc 150° long in the southern sky.

There is always something new in astronomy. Exciting discoveries follow one after another at a dizzying pace, thanks to the batteries of giant telescopes perched on mountain tops and equipped with the latest technological innovations, observatories orbiting high above the troublesome atmosphere, and spacecraft exploring the worlds of the solar system from close quarters. Keeping abreast of it all can be a challenge!

For this illustrated A-to-Z, I have made an up-to-date selection of 1300 entries covering hundreds of named astronomical objects as well as the terms and abbreviations most commonly encountered in astronomy. I have also included biographical entries on 70 people who have made significant contributions to the development of astronomy. Three hundred entries are illustrated, nearly all in color.

The idea for an illustrated dictionary grew from the dictionary I originally compiled in 1988–90, the most recent edition of which was published by Cambridge University Press in 2001. But this is a new book with a fresh style, which I hope will appeal to a wide range of readers young and old – not just as a reference source in which to look things up, but also as a book full of fascinating facts and beautiful pictures to dip into anytime.

Using the book

The alphabetical order takes no account of word breaks or hyphens. Entries beginning with a Greek letter are treated as if the letter were spelled out.

Gravitation theory and relativistic astrophysics have gone through extensive developments in recent decades, following the discovery of quasars in the 1960s, and other very high energy phenomena in the universe such as gamma ray bursts. Compact objects such as neutron stars and pulsars also display intriguing physical properties, where the effects of strong gravity fields are seen to play a fundamental role. When the masses and energy densities involved in the physical phenomena are sufficiently high, as is the case in the situations above, it has become increasingly clear that the strong gravitational fields, as governed by the general theory of relativity, play an important and much more dominant role. This gravitational dynamics must be taken into account for any meaningful description of these observed ultra-high energy objects.

A similar situation involving very strong gravitational fields, and which may be connected to some of the above phenomena, is that of a massive star undergoing a continual gravitational collapse at the end of its life cycle. This happens when the star has exhausted its nuclear fuel that provided a balance against the internal pull of gravity. This phenomenon, dominated essentially by the force of gravity, is fundamental to basic theory and astrophysical applications in blackhole physics that have received increasing attention in past decades, and also in cosmology. In the past two decades, there have been extensive investigations of gravitational collapse models within the framework of Einstein's theory of gravity, and these have provided useful insights into the final fate of a massive star.

A collapsing matter cloud, such as a massive star undergoing a continual gravitational collapse at the end of its life cycle, is modeled in general relativity as a dynamical spacetime geometry with a suitable energy–momentum tensor. The time evolution here is governed by the Einstein equations. The cloud has a boundary, with its interior collapsing continually as time evolves, and, at the boundary the interior spacetime is matched to a suitable exterior geometry so as to complete the full model of gravitational collapse. The physical situation considered here is that of the force of gravity being so overwhelming that no final, stable configuration, such as a neutron star or white dwarf, is possible as a collapse endstate, and a continual collapse inevitably proceeds. If the initial mass of the collapsing star is sufficiently high, then such a situation is realized.

In such a scenario, the classical theory leads the collapse to the formation of a spacetime singularity, as predicted by the singularity theorems of general relativity. The spacetime singularity is a region close to where the densities, spacetime curvatures, and all other physical quantities grow without bounds. At the singularity itself these are infinite, and hence, strictly speaking, the singularity is not part of the spacetime and is regarded as the boundary of the spacetime manifold. Eventually, as one moves closer to the singularity, the quantum gravity effects may dominate, which could resolve the classical singularity.

As indicated here, the cosmic censorship hypothesis is fundamental to the basic theory and applications in blackhole physics. It follows from the previous considerations that the cosmic censorship, if it does hold, is not valid in any obvious and plain manner in general relativity, but it has to be carefully designed and formulated. This is important for any clear basis and foundation for blackhole physics. As yet, such a mathematical formulation is not available, and much work is needed to achieve one. In such a case, the existence and formation of blackholes as the final state of a gravitational collapse, whenever a massive star collapses in the universe on exhausting its nuclear fuel, cannot be taken as automatic.

Basically, cosmic censorship is a statement about the causal structure of spacetime, as related to the dynamical collapse scenarios that are fundamental processes in astrophysics and cosmology. Therefore, in this chapter, which aims to discuss various aspects related to the censorship conjecture and its possible formulations, are considered in Section 4.1 some basic aspects of the causal structure of spacetime. The physics related to gravity becomes much more important in situations such as gravitational collapse and those of the early universe phases in cosmology. The general theory of relativity predicts the occurrence of spacetime singularity in such situations, which are the extreme gravity regions where densities, spacetime curvatures, and other physical quantities take extreme values. The quantum gravity effects would become much more prominent in such regions.

Here, the essential fundamentals of general relativity and related mathematical aspects are described. For further details, see texts such as Weinberg (1972), Misner, Thorne, and Wheeler (1973), and Wald (1984). Other necessary techniques are developed in later chapters as necessary. While defining vectors, tensors, and other quantities, we use both a local and a coordinate free global approach, and indicate how to make a transition from one to the other representation, which is useful in several situations.

In Section 2.1 the manifold model for spacetime is introduced. Basic definitions of a differentiable manifold, and various topological and orientability properties are discussed. The metric tensor and related aspects are considered in Section 2.2, and the connection on a spacetime is considered in Section 2.3. Timelike and null geodesics play a basic role in the considerations here on gravitational collapse. These are a special set of non-spacelike trajectories that represent the motion of freely falling material particles and light rays, and they clarify many properties of a spacetime. These are discussed in Section 2.4. The spacetime curvature is considered in Section 2.5, and the Einstein equations governing the dynamics of matter in the spacetime are discussed in Section 2.6. Many exact solutions have been found to the Einstein equations so far; however, the Schwarzschild and Vaidya geometries are particularly relevant to gravitational collapse scenarios, and Section 2.7 discusses these.

The physical phenomena in astrophysics and cosmology involve gravitational collapse in a fundamental way. The final fate of a massive star, when it collapses under its own gravity at the end of its life cycle, is one of the most important questions in gravitation theory and relativistic astrophysics today. The applications and basic theory of blackholes vigorously developed over the past decades crucially depend on this outcome.

A sufficiently massive star many times the size of the Sun would undergo a continual gravitational collapse on exhausting its nuclear fuel, without achieving an equilibrium state such as a neutron star or white dwarf. The singularity theorems in general relativity then predict that the collapse gives rise to a spacetime singularity, either hidden within an event horizon of gravity or visible to the external universe. The densities and spacetime curvatures get arbitrarily high and diverge at these ultra-strong gravity regions. Their visibility to outside observers is determined by the causal structure within the dynamically developing collapsing cloud, as governed by the Einstein field equations. When the internal dynamics of the collapse delays the horizon formation, these become visible, and may communicate physical effects to the external universe. These issues are investigated here, and the treatment is aimed at showing how such visible ultra-dense regions arise naturally and generically as the outcome of a dynamical gravitational collapse in Einstein gravity.