3. Stars

Most of what we know about the Universe has been gleaned from the study of stars, and a major achievement of 20th-century science was to understand how stars work and their lifecycles from birth to death. ‘Stars’ describes this lifecycle beginning with star formation when a cloud of interstellar gas suffers a runaway of its central density. It then considers nuclear fusion, key stellar masses, and life after the main sequence when the star burns its core helium. The surfaces of stars are described along with stellar coronae and exploding stars—both core-collapse and deflagration supernovae. Finally, globular star clusters, solar neutrinos, stellar seismology, and binary stars are discussed.

4. Accretion

The principle of angular momentum conservation was crucial for the creation of the disc of the Milky Way and plays a large role in many of the most exotic and luminous objects in the Universe. ‘Accretion’ describes accretion discs including basic disc dynamics, accretion onto stellar-mass objects, and quasars, and goes on to consider jets, which carry material away from the accreting object extremely supersonically. Much can be learnt about a disc and the system in which it is located by monitoring the system’s light curve—the luminosity as a function of time. Further information can be extracted if spectra taken at different times are available. This is time-domain astronomy.

8. The big picture

By far the biggest contribution of general relativity to astrophysics was to make it possible to discuss the geometry and dynamics of the entire universe—it made cosmology a branch of physics. ‘The big picture’ outlines our current understanding of how stars and galaxies emerged from the big bang, providing some context for the physical processes already described. Much of the physics involved is extremely complex and we are far from understanding how the various processes played out. The universe is a huge canvas, and nature has wrought on it with very many techniques. Our knowledge of the universe is growing rapidly, but we have much, much more to learn.

7. Galaxies

‘Galaxies’ describes galaxy morphology, the chemical evolution of galaxies, and stellar dynamics. A galaxy consists of a huge number of point masses—stars and dark matter—that move freely in the gravitational field that they jointly generate. The nature of a galaxy is largely determined by its luminosity, its bulge-to-disc ratio, and the ratio of its mass of cold gas to the mass in stars. Galaxies are also shaped by their environments. Dense environments are rich in elliptical and lenticular galaxies, while abnormally under-dense environments are rich in dwarf irregular galaxies. Spiral galaxies like our own tend to inhabit regions of intermediate density.

6. Relativistic astrophysics

‘Relativistic astrophysics’ outlines the relativity theory and lists some situations where this theory is needed: radio galaxies, micro-quasars, gamma rays, cosmic rays, neutron stars, x-ray sources, the solar system, and the universe. It goes on to explain muon lifetimes, rest-mass energy, plasma jets, shocks and particle acceleration, and synchrotron radiation. Einstein’s theory of general relativity is dependent on non-linear equations and the only exact solutions of Einstein’s equations that we have are ones with special symmetries. The phenomena of gravitational redshift, gravitational lensing, gravitational microlensing, deflection of light by the Sun, Shapiro delays, and pulsars and gravitational waves are then discussed.

1. Big ideas

Isaac Newton laid the foundations of astrophysics when he showed that it is possible to obtain precise quantitative predictions from appropriately defined physical laws. To do this, he had to invent new mathematics—the infinitesimal calculus—and use its language to encapsulate physical laws. Astrophysics is the application of the laws of physics to everything that lies outside our planet. ‘Big ideas’ outlines the differential equations that make up these physical laws and explains that they are valid in every part of the universe and at all time. The universal and eternal nature of the laws of physics gives rise to three important conserved quantities: momentum, angular momentum, and energy.

5. Planetary systems

Since 1995, roughly a thousand extra-solar planetary systems have been discovered, and the process of understanding how these systems formed and evolved to their present states is having a profound impact on how we think about our own planetary system. ‘Planetary systems’ explains the dynamics of planetary systems, including perturbation theory, and how planets form by the collision of asteroids to form bigger and bigger asteroids. The evolution of planetary systems is also discussed with details of the young solar system. It concludes that were it not for Einstein’s absolutely tiny corrections to Newton’s equations, planet Earth would almost certainly not be in a position to offer us shelter.

2. Gas between the stars

The space between stars is taken up by gas—mostly hydrogen and helium—which manifests itself in many ways, the most important being the absorption of starlight. ‘Gas between the stars’ describes interstellar absorption and the reddening of stars—when dust grains in the gas absorb blue and ultraviolet light, but let red light from the Sun pass through. These dust grains play a crucial role in regulating the temperature, density, and chemical composition of the gas. This composition of interstellar gas hinges on the balance between the destructive power of ultraviolet photons and the catalytic action of dust grains.