This chapter gives a brief overview of observational astronomy, using optical instruments and other wavelengths. We present a general formula for the increase in the limiting magnitude resulting from an increased telescope aperture. For light of particular wavelength, the diffraction from a telescope with a specific diameter sets a fundamental limit to the smallest possible angular separation that can be resolved.

The tendency for conservation of angular momentum of a gravitationally collapsing cloud to form a disk gives rise to the disk in our own galaxy, the Milky Way. We explore the main components, including the disk, bulge and halo. Studies of galaxy rotation curves lead us to the existence of "dark matter," the nature of which is unknown but is detectable through its gravitational interactions with normal, baryonic matter. We finish by exploring the super-massive black hole at the Milky Way’s center.

In reality stars are not perfect blackbodies, and so their emitted spectra don’t depend solely on temperature, but instead contain detailed signatures of key physical properties like elemental composition. For atoms in a gas, the ability to absorb, scatter and emit light can likewise depend on the wavelength, sometimes quite sharply. We find that the discrete energies levels associated with atoms of different elements are quite distinct. We introduce the stellar spectral classes (OBAFGKM).

This chapter explores what is known as the Cosmic Microwave Background (CMB), what it is, how it was discovered and our recent efforts to measure and map it. In general, the analysis finds remarkably good overall agreement with predictions of the now-standard "Lambda CDM" model of a universe, in which there is both cold dark matter (CDM) to spur structure formation, as well as dark energy acceleration that is well-represented by a cosmological constant, Lambda. From this we can infer 13.8 Gyr for the age of the universe

Stars generally form in clusters from the gravitational contraction of a dense, cold giant molecular cloud. We explore the critical requirement for such a contraction, known as the Jeans criterion, and the factors that affect the star formation rates and the initial mass function in star clusters and galaxies. We finish by looking at how the conservation of angular momentum can lead to proto-stellar disks, with important implications for forming planets.

The disk formation process of the previous chapter forms the basis for the "Nebular Model" for the formation of planetary systems, including our own solar system. As a proto-stellar cloud collapses under the pull of its own gravity, conservation of its initial angular momentum leads naturally to formation of an orbiting disk, which surrounds the central core mass that forms the developing star. We then explore the "ice line" between inner rocky dwarf planets and outer gas giants.

This chapter explores observations and properties of quasars, which were first observed in the 1960s as point-like sources that emit over a wide range of energies from the radio through the IR, visible, UV and even extending to the X-ray and gamma-rays. They are now known to be a type of active galactic nucleus thought to be the result of matter accreting onto a super-massive black hole (SMBH) at the center of the host galaxy.

It turns out that stellar binary (and even triple and quadruple) systems are quite common. In Chapter 10 we show how we can infer the masses of stars through the study of stellar binary systems. For some systems, where the inclination of orbits can be determined unambiguously, we can infer the masses of the stellar components, as well as the distance to the system. Together with the observed apparent magnitudes, this also gives the associated luminosities of their component stars.

Following directly the from the previous chapter, we see that in addition to a shift toward shorter peak wavelength, a higher temperature also increases the overall brightness of blackbody emission at all wavelengths. This suggests that the total energy emitted over all wavelengths should increase quite sharply with temperature. We introduce the Stefan-Boltzmann law, one of the linchpins of stellar astronomy.

Mass is clearly a physically important parameter for a star, as it will determine the strength of the gravity that tries to pull the star’s matter together. We discuss one basic way we can determine mass, from orbits of stars in stellar binaries, and see the range of stellar masses. This leads us to the virial theorem, which describes a stably bound gravitational system.

We conclude our discussion of stellar properties by considering ways to infer the rotation of stars. All stars rotate, but in cool, low-mass stars like the Sun the rotation is quite slow. In hotter, more-massive stars, the rotation can be more rapid, with some cases (e.g., the Berillium stars) near the "critical" rotation speed at the star’s surface.

This chapter considers stellar ages. Just how old are stars like the Sun? What provides the energy that keeps them shining? And what will happen to them as they exhaust various available energy sources? We show that the ages and lifetimes of stars like the Sun are set by long nuclear burning timescales and the implications that high-mass stars should have much shorter lifetimes than low-mass stars.

The timescale analyses in Chapter 8 show that nuclear fusion provides a long-lasting energy source that we can associate with main sequence stars in the H-R diagram. This chapter addresses the following questions: What are the requirements for H to He fusion to occur in the stellar core? And how is this to be related to the luminosity vs. surface temperature scaling for main sequence stars? In particular, how might this determine the relation between mass and radius? What does it imply about the lower mass limit for stars to undergo hydrogen fusion?