This is a review chapter. Important concepts treated include the nuclear model of the atom, types of nuclear reactions, wave properties, the electromagnetic spectrum of light, Doppler shifts, blackbody radiation, and galaxy classifications. Read through this material to obtain background for later chapters. Some topics are of particular importance to understanding cosmology, including:

• Blackbody radiation is a very specific type of spectrum that corresponds to photons in equilibrium. This radiation is completely characterized by one parameter, the temperature of the emitter. The cosmic background radiation is an example of blackbody radiation, where the emitter is the universe itself. The temperature of the radiation has dropped throughout the history of the cosmos; currently it is a chilly 2.73 Kelvins, i.e. 2.73 degrees above absolute zero.

• Nuclear Fusion is the phrase for nuclear reactions that combine light elements into heavier. Stars are powered by fusion. This process also explains how the elements in the universe are built up from the original protons and neutrons in the process of nucleosynthesis . Nuclear reactions that took place in the early universe created helium, whereas nearly all other elements and isotopes are manufactured in stars at various stages of their lifetimes, with the heaviest elements created during supernovae.

• Redshifts and Blueshifts are the result of several processes that produce shifts in an observed wave spectrum. One of the most important is the Doppler effect, the shifting of the frequency (and thus the wavelength) due to relative motion between emitter and receiver. The Doppler effect is essentially classical, but must be modified slightly in order to take special relativity (see Chapter 7) into account. Doppler shifts are very familiar when they affect sound waves; this is the reason that an approaching siren seems to rise in pitch until it reaches the observer, after which it drops in pitch as the vehicle recedes.

  Another important effect is the gravitational shift due to light moving in a gravitational field; light climbing from a point of stronger to a point of weaker gravity is redshifted, whereas light falling in a gravitational field is blueshifted.

  Finally, the cosmological redshift due to the expansion of space is one of the most important overall shift effects in cosmology. The cosmological redshift causes the temperature of the microwave background to drop as time passes, and affects the light from distant sources.

• Luminosity distance is a distance to a source as determined by observing the attenuation of the source's light intensity. Finding the distances to astronomical objects is one great challenges of cosmology; The luminosity distance is an important part of this task. The luminosity distance exploits the simple fact that light intensity is attenuated as the light travels through space, because the light wave front spreads out over an ever-increasing area. Thus a comparison of the observed amount of light received from a given source with some estimate of the intrinsic brightness of the source should enable us to compute the distance to the object. In practice, the determination of luminosity distances is fraught with many potential sources of error, including absorption by intervening matter of unknown type and density, but the principle is very simple. Despite the difficulties, luminosity distance is the best means of measuring the distances to very distant objects.

The chapter concludes with a brief description of the types of galaxies seen in the universe. Galaxies are assigned to one of three categories according to their overall shapes and other properties. Elliptical galaxies are roughly spheroidal or ellipsoidal, contain mostly old stars, have little dust or gas, and do not contain well-defined nuclei. Spiral galaxies are flattened disks threaded by pronounced bright spirals, contain much gas and dust, are usually sites of young stars and active star formation, and feature reasonably well defined nuclear bulges. The Milky Way Galaxy is a spiral. The third category, irregular galaxies, is a catchall for galaxies that do not fit one of the other two groups. Many irregular galaxies are interacting with other galaxies in one way or another, either as satellites or in collisions or near encounters with other galaxies.

The giant elliptical galaxy M87 is a prime example of its type. This huge galaxy lies at the core of the great cluster of galaxies in Virgo.

The Elliptical Galaxy M87

M100, a spiral galaxy

The Large Magellanic Cloud, an irregular galaxy, is a satellite of the Milky Way.

Questions and Answers related to Chapter 4.

For more information on the periodic table and the elements, try the web elements link.