To test which of these models applies to our universe, one needs to extend redshift measurements to large distances, out to several Giga-light years. The most successful approach has been to use white dwarf supernovae (SN type Ia) as very luminous standard candles. One of the greatest surprises of modern astronomy is that the expansion of the universe must be accelerating! This implies there must be a positive, repulsive force that pushes galaxies apart, in opposition to gravity. We dub this force "dark energy."

Earth’s moon is quite distinct from other moons in the solar system, in being a comparable size to Earth. We explore the theory that a giant impact in the chaotic early solar system led to the Moon’s formation, and bombardment by ice-laden asteroids provided the abundant water we find on our planet. Further we find that Earth’s magnetic field shields us from solar wind protons, that protect our atmosphere from being stripped away. The icy moons of Jupiter and Saturn are the best targets for exploring if life exists elsewhere in the solar system.

The close proximity of the Sun, and its extreme apparent brightness, makes it by far the most important star for lives here on Earth. In modern times we have access to powerful telescopes, both on the ground and in space, that observe and monitor the Sun over a wide range of wavelength bands. These vividly demonstrate that the Sun is in fact highly structured and variable over a wide range of spatial and temporal scales.

As a basis for interpreting observations of binary systems in terms of the orbital velocity of the component stars, we review the astrometric and spectrometric techniques used to measure the motion of stars through space. Nearby stars generally exhibit some systematic motion relative to the Sun, generally with components both transverse (i.e., perpendicular) to and along (parallel to) the observed line of sight.

Our initial introduction of surface brightness characterized it as a flux confined within an observed solid angle. But actually the surface brightness is directly related to a more general and fundamental quantity known as the "specific intensity." The light we see from a star is the result of competition between thermal emission and absorption by material within the star.

We now consider why stars shine with such extreme brightness. Over the long-term (i.e., millions of years), the enormous energy emitted comes from the energy generated (by nuclear fusion) in the stellar core, as discussed further in Chapter 18. But the more immediate reason stars shine is more direct, namely because their surfaces are so very hot. We explore the key physical laws governing such thermal radiation and how it depends on temperature.

As a star ages, more and more of the hydrogen in its core becomes consumed by fusion into helium. Once this core hydrogen is used up, how does the star react and adjust? Stars at this post-main-sequence stage of life actually start to expand, eventually becoming much brighter giant or supergiant stars, shining with a luminosity that can be thousands or even tens of thousands that of their core-H-burning main sequence. We discuss how such stars reach their stellar end points as planetary nebulae or white dwarfs.

To understand ways we might infer stellar distances, we first consider how we intuitively estimate distance in our everyday world, through apparent angular size, and/or using our stereoscopic vision. We explain a practical, quite direct way to infer distances to relatively nearby stars, namely through the method of trigonometric parallax. This leads to the definition of the astronomical unit and parsec, and the concept of solid angles on the sky, measured in steradians or square degrees.

What are the key physical properties we can aspire to know about a star? In this chapter we consider the properties of stars, identifying first what we can directly observe about a given star: position on the sky, apparent brightness, color/spectrum. When these observations are combined with a clear understanding of some basic physical principles, we can infer many of the key physical properties of stars. We also make a brief aside to discuss ways to get our heads around the enormous distances and timescales we encounter in astrophysics.