In order to make this course self-contained, you need to understand some of the basic laws of physics introduced in this unit - in particular the law of gravity and the physics of light (production, interaction with matter). In a conceptual physics class, this might take several weeks to understand. You will get a quick dose, so that can make the material difficult. Students do get through this stuff, though, and so can you. The reason for introduction of this material early in the course is clear - gravity is THE organizing force for material in the universe, and light detection and interpretation is our dominant method for observing things in the universe.

During the 100's of years of history discussed, the methods of modern science evolved. All of the famous individuals described had ideas and beliefs that by today’s standards would look both bizarre and non-scientific. Collectively, the ideas that turned out to be scientifically valid survived. Newton was able to elucidate his three laws of motion and derive the law of gravity from those laws. The tension between scientific study and religion also peaked during this era, and one can trace the evolution of that tension to modern times (viz the fight between creationists and evolutionists, which it appears will continue for at least several more decades). The history presented in the book is one of the best I have seen in any astronomy text. The text also mentions that this 100 year period falls roughly at the end of the Renaissance period, which is also well known for the changes in art and music. The earliest written music we have records for is Renaissance music.

While the text gives you some insight into the character of Copernicus, Kepler, and Galileo, it omits any character development for Isaac Newton. Those of you interested in history might enjoy a little library work to peer into Newton’s character and how he spent the latter part of his life. He had a reputation for being cold, calculating, and vengeful (what we would call a jerk). Newton’s work on the laws of motion form the basis for the industrial revolution, and modern mechanical engineering is still based on Newtonian principles. The relationship you will need to remember and understand for the remainder of the course is Newton’s law of gravity.

In words, the law of gravity states that, given two objects (for example, the Sun and the Earth) with mass, there exists a force between these two objects that is proportional to the PRODUCT of the two masses and INVERSELY proportional to the square of the distance between the two mass centers. Newton was able to show that the proportionality constant in the equation, G, is the same throughout the (then) known solar system. This law describes how the Earth pulls on you (and vice versa), how the Earth manages to hold the Moon in its orbit, and how the Sun holds all the planets (with their moons) in orbit. Note that ANY two objects with mass will attract one another, AND that the attraction is never exactly zero because gravity never goes to zero - it just gets smaller as the separation gets larger. Fortunately for us, the solar system is fairly simple in spite of all the possible force pairs between all the planets and the Sun. The reason for this simplicity is that the Sun has 99% of the mass of the entire solar system, so the force on any planet is mostly just that due to the Sun’s attraction, and only a small amount of the total force any planet feels is due to attraction from other planets or moons. As far as we can tell, Newton’s law of gravity is as valid anywhere in the universe as it is for our solar system; physicists therefore refer to the constant G as a universal constant. In a later chapter, you will see that Newton’s theory of gravity survived unchallenged until the time of Einstein (early 1900’s) and the introduction of the general theory of relativity, which we will eventually need in order to understand weird things like neutron stars and black holes. Except for these weird situations and for problems dealing with the large-scale structure of the universe, Newton’s law of gravity is generally an excellent approximation.

Those of us who live on the coast and/or do some kayaking in Puget Sound experience tides and currents - the consequence of our planet’s having a liquid covering over 2/3 of its surface and having a moon in orbit around us. This subject usually appears in a text in the chapter dealing with gravity, because this is a differential gravitational force effect. That is, the gravitational force of Moon on the near side of Earth’s water is greater than the force on the center of Earth is greater than the force on the far side of Earth’s water. The result is that bulges point roughly toward and away from the Moon. As Earth rotates (beneath this bulging water blanket) in a 24-hour period on its axis, any point on it will then pass through 2 maxima (high tides) and 2 minima (low tides). Things get much more complicated in Puget Sound, which has one inlet into a large body of water that has many islands. Each day this body of water fills twice and drains twice, and the islands cause very complicated, constantly changing current patterns. Navigators in unpowered (or low-powered) boats must pay careful attention to these currents or end up where they don’t want to be. Fortunately, we can purchase books containing highly visual depictions of these currents that even I can understand. Note that Moon is the dominant feature causing these tides, but that Sun also has an influence (about 1/4 that of Moon).

It is VERY important that you remember the distinction between OBSERVATION and a MODEL or THEORY. Astronomy is an OBSERVATIONAL science - we must just watch as stars and planets do what they do, and from that watching deduce the science of astronomy. This would have resulted in not much science if it were not for the science of physics (such as Newton’s law of gravity), which involves doing human-directed experiments here on Earth. So astronomers ASSUME that the laws of physics we deduce in Earth experiments apply throughout the universe, and the astronomical observations yield clues that eventually result in MODELS or THEORIES governed by physics. Once a model or theory evolves to describe a particular situation, use of the laws of physics can generally tell astronomers other things to look for to further test the model/theory. This cycle never ends in principle, and all models and theories may be subject to change or rejection if future observations so dictate. Because of this veto power of observations, scientists are careful to assure that the observations are valid - they require confirmation by independent observers, and peer review of published results. Occasionally, scientists get caught up in media frenzy (and you thought this was the exclusive province of politicians?) and get out in front of this confirmation/peer review process and make fools of themselves. The latest major example that made lots of news was the cold fusion fiasco. More on that later when we talk about the power source for stars, which is HOT fusion. Cold fusion was bad science and a huge embarrassment for all involved.  Question for discussion:  so if observations continue to get better (more precise), and theories continue to change, what is "truth" in science?

There are several things to keep in mind from the perspective of astronomy as you study this material.
1. Almost everything we know about astronomy depends on detecting and interpreting light from distant objects.  Some exceptions:  Earth science, exploration of Mars and our Moon, certain measurements by spacecraft such as the Galileo mission to Jupiter, measurements of cosmic rays (charged particles, not light) coming from beyond the solar system, and detection of neutrinos from the Sun and beyond.
2. Almost all objects in astronomy give off light of some kind, and lots of astronomical objects (viz, planets) also reflect light. For any given object, we see the total light - that emitted by the object PLUS that reflected by the object.
3. Only a very small part of all the "light" in the universe penetrates our atmosphere and reaches us, so astronomers use rockets and orbiting satellites to observe the "light" blocked by Earth’s atmosphere.
4. All stars exhibit a kind of light spectrum that includes two components (involving two explanations in physics terms) - a "continuous" spectrum, plus dark (aka 'absorption') lines (where relatively little light exists) superimposed on continuous spectrum.
5. If a source of light (like a star, a planet, or a gas cloud) is moving either toward us or away from us (or has a component of motion toward us or away from us), every photon of light we receive from the source will have its wavelength shifted to longer wavelength (for a receding source) or to shorter wavelength (for an approaching source). Furthermore, the amount of shift, called Doppler shift, is proportional to the recession or approach speed.
    DEFINITION:  Red shift = shift to longer wavelength (receding source)
                              Blue shift = shift to shorter wavelength (approaching source)

The modern understanding of continuous spectra and line spectra evolved from 1900 to about 1930, and its explanation lies in the province of the quantum theory of light and matter. This is heavy stuff when taken in short doses, so tighten your seatbelt and keep focused on what it is you need to understand (continuous spectrum, plus dark lines - also bright emission lines when looking at a gas cloud near a star).

While the understanding of stellar spectra took until 1930 to achieve, astronomers realized in the 19th century that the spectra contained information about the elements making up stars - just not why. Earth-based laboratory experiments showed that hot gases emitted light in the form of lines (emission lines) characteristic of the element - allowing kind of a fingerprint analysis. Because all stars are made mostly of hydrogen, you might think the spectra would be easy to interpret - as hot hydrogen gas emission line spectra. But life ain’t so simple! In any case, long before the time we understood the physics causes of the spectra, astronomers had (from chemists) an astonishing encyclopedia of stellar spectrum information, nicely organized, including deductions of elemental composition of the various stars (major exception - the element helium).

Definition of "spectrum" - a graphical display of light intensity (relative amount of light) versus wavelength (or frequency or color)
The three observational laws of light production by matter are fairly simple, and were summarized about 150 years ago:

a. A hot solid (or dense gas, as in the case of a star) produces a continuous light spectrum - see Figure 6-6. White light really consists of light of various wavelengths, as documented  first by Isaac Newton, who passed sunlight through a prism to separate it into its various colors (or spectrum).  Further, the peak of light intensity shifts to shorter wavelength (higher frequency) as the surface temperature increases.

b. A hot, low-density gas will emit lines of light that are unique to the gas composition. Further, the gas temperature will influence the relative brightness (intensity) of the various spectral lines.

c. A continuous spectrum of light, when passed through a low-density gas, will result in an "absorption" spectrum, or (this is a definition) continuous spectrum with dark lines superimposed. Furthermore, the wavelengths of the dark lines exactly correspond (for a given gas element) to the wavelengths of the bright emission lines of the same hot gas.

In order to succeed in the remainder of the course, it is VERY important (hard for me to overstate this) for you to remember these rules. Your text does a good job explaining, in terms of quantum theory, why hydrogen gas emits characteristic lines and why the same gas will produce dark absorption lines when a continuous spectrum shines through the gas, so I will not elaborate here and shall restrict myself to answering YOUR QUESTIONS about the explanation.

The second EXTREMELY important thing to remember is that our own atmosphere absorbs most of the light incident on it, as mentioned before. Some objects in the universe emit radiation throughout the electromagnetic spectrum (WHAT IS THAT?), and in order to see it all astronomers must place different kinds of detectors above our atmosphere.

EXPLANATION FOR ELECTROMAGNETIC SPECTRUM: Whenever a charged particle changes its motion (or accelerates) it emits some electromagnetic radiation, or "light". The greater the acceleration, the higher the energy of the light, and vice versa. Electromagnetic radiation consists of electric and magnetic fields (analogous to forces) that oscillate so that the net result is a complex wave in which the electric and magnetic fields oscillate together to produce what we call light. Note the definition of wavelength. About 150 years ago, a man named Maxwell developed the theory for light waves, and he predicted that E/M waves of all wavelengths should have the same speed - in particular what we now call light-speed. Many 20th century experiments have proven that ALL light, regardless of source or wavelength, travels in a vacuum at light-speed (designated with the symbol "c" in all physics and astronomy books - the actual value is about 300,000 km/sec). The mathematical relation between wavelength and speed for any wave, including light, is

speed = wavelength x frequency,

where the wave frequency corresponds to the number of waves passing an observer each second. If the speed is the same for all light, while the wavelength varies for different "colors," it follows that the frequency must also vary. It turns out that higher frequency (shorter wavelength) corresponds to higher energy light. Furthermore, as you probably recall from reading chapter 6, light is "quantized," that is, light is made up of wave-like particles physicists call PHOTONS, and the energy of a photon is proportional to the frequency of the light. If this is not confusing, you have had quite a bit of physics before. Note that the "photon" is a quantum of light, and has zero mass and zero charge. The "proton" is a nuclear particle and has both mass and charge. Keeping these words distinct in your mind is essential.

Note that the E/M spectrum spans many orders of magnitude in wavelength, from very long wavelength radio waves (on the right side) to very short wavelength (and high energy) gamma rays (on the left side). That little bitty piece of the spectrum that we have evolved to be able to "see" with our eyes is the so-called "visible" or "optical" part of the spectrum. Because our atmosphere lets in "visible" light, it should be no surprise that critters’ eyes evolved to sense that part of the spectrum. Also note that part of the radio wave range penetrates through our atmosphere. Just about everything else does not, and that is good. Modern evolution theory (biological and geophysical) posits that life on land did not - could not - evolve until Earth’s atmosphere contained oxygen (which was not part of our original atmosphere). The eventual presence of oxygen in the lower atmosphere led naturally to the presence of ozone (3 oxygen atoms in a molecule) in the upper atmosphere, and ozone blocks high-energy ultraviolet (UV) light that can cause cell damage that leads to skin cancer. Other atmospheric constituents collectively block all higher energy light. In modern times, release of certain refrigerants (CFCs by acronym name) causes destruction of ozone in Earth's upper atmosphere, leading to spring-time ozone holes over the two polar regions of Earth, which in turn leads to higher fluxes of UV radiation in those areas, which in turn leads to higher incidence of skin cancer in sensitive folks like us. An international agreement led to a ban on CFCs, but the ozone holes will be with you the rest of your lives because of the long lifetime of the chlorine in the upper atmosphere.

At the other end of the visible part of the spectrum, not much infrared (IR) radiation penetrates to the ground. The water vapor and carbon dioxide in our lower atmosphere block that radiation. Those two gases also intercept IR radiation leaving the warmed up Earth, resulting in a higher surface temperature than we would otherwise have. Mankind’s carbon dioxide emissions are causing higher concentrations of that gas in our atmosphere, and that in turn is leading to a slow global warming trend. So far, we are not doing much about that problem, but as changing climate leads inexorably to new (and expensive) problems, you may see global warming become one of the hottest political topics of the twenty-first century.