In Chapter 2 we discussed the continuous spectra of stars and saw that they could be closely described by blackbody spectra. In this chapter, we will discuss the situations in which the spectrum shows an increase or decrease in intensity over a very narrow wavelength range.

Spectral lines

We know that if we pass white light through a prism, light of different colors (wavelengths) will emerge at different angles with respect to the initial beam of light. If we pass white light through a slit before it strikes the prism (Fig. 3.1), and then let the spread-out light fall on the screen, at each position on the screen we get the image of the slit at a particular wavelength.

Both William Hyde Wollaston (1804) and Josef von Fraunhofer (1811) used this method to examine sunlight. They found that the normal spectrum was crossed by dark lines. These lines represent wavelengths where there is less radiation than at nearby wavelengths. (The lines are only dark in comparison with the nearby bright regions.) The linelike appearance comes from the fact that, at each wavelength, we are seeing the image of the slit. It is this linelike appearance that leads us to call these features spectral lines. If we were to make a graph of intensity vs. wavelength, we would find narrow dips superimposed on the continuum. The solar spectrum with dark lines is sometimes referred to as the Fraunhofer spectrum. Fraunhofer gave the strongest lines letter designations that we still use today.

If we are to understand the workings of stars, it is important to know their masses. The best way to measure the mass of an object is to measure its gravitational influence on another object. (When you stand on a bathroom scale, you are measuring the Earth's gravitational effect on your mass.) For stars, we are fortunate to be able to measure the gravitational effects from pairs of stars, called binary stars.

Binary stars

Many stars we can observe appear to have companions, the two stars orbiting their common center of mass. It appears that approximately half of all stars in our galaxy are in binary systems. By studying the orbits of binary stars, we can measure the gravitational forces that the two stars exert on each other. This allows us to determine the masses of the stars.

We classify binaries according to how the companion star manifests its presence:

1. Optical double. This is not really a binary star. Two stars just happen to appear along almost the same line of sight. The two stars can be at very different distances.

2. Visual binary. These stars are in orbit about each other and we can see both stars directly.

3. Composite spectrum binary. When we take a spectrum of the star, we see the lines of two different spectral type stars. From this we infer the presence of two stars.

4. […]

To this point we have been studying the stellar life cycle and how stars and other material are arranged in the Milky Way Galaxy. We will now turn to studies on a much larger scale. We will first look at other galaxies, and see that some of them tell us more about our own galaxy, which is so hard to observe. When we talk about how the universe is put together, each galaxy has only as much importance as a single molecule of oxygen has in describing the gas in your room.

As we go to larger scales, we will look at how galaxies are distributed on the sky, and how they move relative to one another. We will also see how the problem of dark matter becomes more important as we go to larger and larger scales.

As we go to larger scales, increasing the number of galaxies that we observe, we also find a variety of interesting phenomena associated with galaxies. In Chapter 19 we will discuss aspects of galactic activity, particularly as evidenced by radio galaxies and quasars.

In Chapter 20 and 21 we will turn to cosmology, the study of the universe on the largest scales. This also includes the past and future evolution of the universe. It is in the study of the past that we encounter one of the most fascinating aspects of modern astrophysics research, the merging of physics on the smallest (elementary particles) and largest (structure of the universe) scales.

An understandable universe

Our curiosity about the world around us is most naturally manifested when we look up at the night sky. We don't need any special instruments to tell us something interesting is going on. However, only with the scrutiny afforded by a variety of instruments can these patches of light, and the dark regions between them, offer clues about their nature. We have to be clever to collect those clues, and just as clever to interpret them. It is the total of these studies that we call astronomy.

We are fortunate to live in an era of extraordinary astronomical discovery. Some have even called this the ‘Golden Era of Astronomy’. For centuries astronomers were restricted to making visual observations from the surface of the Earth. We can now detect virtually any type of radiation given off by an astronomical object, from radio waves to gamma rays. Where necessary, we can put observatories in space. For the Solar System, we can even visit the objects we are studying.

For all of these capabilities, there is a major drawback. We cannot do traditional experiments on remote astronomical objects. We cannot change their environment and see how they respond. We must passively study the radiation that they give off. For this reason, we refer to astronomy as an observational science rather than an experimental one. It is because of this difference that we must be clever in using the information that we do receive.

The Solar System naturally divides into two groups of planets, separated by the asteroid belt. The four inner planets have many things in common with the Earth, whereas the next four planets present worlds of an entirely different type. (Pluto is an additional enigma.) In this chapter, we look at Mercury, Venus and Mars, comparing their properties with each other, and with the Earth.

Basic features

Mercury

Mercury is the closest planet to the Sun, and is not much larger than our Moon. There is an interesting story concerning its rotation period. Since Mercury is so close to the Sun, we never have a really good view of it, and surface features are hard to recognize. By noting the positions of large surface features, it appeared that the rotation period was 88 days, the same as the planet's orbital period. This would have meant that Mercury always keeps the same face towards the Sun (just as the Moon keeps the same face towards the Earth). Since Mercury is so close to the Sun, it seemed plausible that some tidal effect could keep its rotation period synchronized with its orbital period.

However, the situation was corrected following radar observations. Radio waves were bounced off Mercury and then detected back on Earth. The planet's rotation causes a spread in the Doppler shifts of the reflected waves. From the amount of spread, we can tell how fast the planet is rotating.

When we look at the spatial distribution of stars in our galaxy, we find that most of the light is concentrated in a thin disk. We are inside this disk, so we see it as a band of light on the sky, called the Milky Way. We will discuss this farther in Part III, but we will see in this chapter that location of stars in the galaxy can tell us something about those stars. In particular, some stars are confined to the thin disk of the Milky Way, while others form a more spherical distribution. In this chapter, we will discuss groupings of stars, called clusters, and see how they vary in size, content and galactic distribution.

Types of clusters

We distinguish between two types of star clusters – galactic clusters and globular clusters.

Galactic clusters are named for their confinement to the galactic disk. A selection of images of galactic clusters is in Fig. 13.1. A familiar galactic cluster, the Pleiades, is shown in Fig. 13.1(a). Note the open appearance in which individual stars can be seen. Because of this appearance, galactic clusters are also called open clusters. Galactic clusters typically contain <103 stars, and are less than ~10 pc across. Recent sensitive near IR surveys are showing more members than we had previously thought in many clusters. In the photograph, we see some starlight reflected from interstellar dust. Galactic clusters are sometimes associated with interstellar gas and dust.

Our study of the Milky Way has been aided greatly by studies of other galaxies. However, for a long time it wasn't clear that the spiral nebulae we see in the sky are really other galaxies. From their appearance, it might just be assumed that these nebulae are small nearby objects, just as HII regions are part of our galaxy.

The issues were crystallized in 1920 in a debate between Harlow Shapley and Heber D. Curtis. Curtis argued that spiral nebulae were really other galaxies. His argument was based on some erroneous assumptions. First, he confused novae in our galaxy with supernovae in other galaxies. Shapley thought the spiral nebulae were part of our own galaxy, partly based on an erroneous report of a measurable proper motion for some nebulae.

The issue was settled in 1924 by the observational astronomer Edwin Hubble (after whom the Space Telescope is named). Hubble studied Cepheids in three spiral nebulae (including the Andromeda Galaxy), and clearly established their distance as being large compared with the size of the Milky Way. There is some problem with Hubble's analysis, involving type I vs. type II Cepheids. However, even this factor of 2 error in the distance was not enough to alter the basic conclusion that spiral nebulae are not part of our own galaxy. Following this work, Hubble made a number of pioneering studies of other galaxies, essentially opening up the field of extragalactic astronomy.

In this chapter we look at galaxies with unusual activity within and around them. For many years astronomers thought of these various types of activity as being distinct. We now realize that many of them have similar origins, but differ in the specific conditions within the galaxy or its environment. We realize that all of this activity takes place in the nucleus of the galaxy, or is driven by activity in the nucleus. We say that these phenomena are associated with active galactic nuclei (AGN).

Starburst galaxies

Some galaxies appear to be giving out excessive amounts of radiation in the infrared. When we studied star formation in Chapter 15, we saw that regions with recent star formation give off a lot of infrared radiation. The energy comes from the newly formed stars, and heats the dust (from the parent cloud) surrounding the stars. The dust then glows in the infrared. The more energy the young stars put into the cloud, the more infrared radiation is released. The excess infrared radiation from some galaxies suggests that those galaxies have very high rates of star formation. The rate is so high that it cannot be sustained for very long, or it would use up all of the interstellar material. This leads to the idea that this excessive star formation is a short-lived phenomenon. We therefore call such galaxies starburst galaxies.

There are a vast number of smaller objects in our Solar System, not as substantial as our Moon, but which provide important clues on the history of the Solar System. These are asteroids, comets and meteoroids. We have also included the ninth planet, Pluto, in this chapter. As we will see below, recent determinations of Pluto's mass make it by far the least massive planet, and it has more properties in common with the other less massive objects in the Solar System.

Pluto

Pluto was discovered in 1930, following an extensive search, by Clyde Tombaugh. The search was initiated by Percival Lowell after it was thought that a planet beyond Neptune might be perturbing Neptune's orbit. Calculations narrowed the range of possible locations on the sky, and a search was carried out. As Fig. 26.1 shows, Pluto doesn't stand out very well against the background of stars. It is detectable as a planet only by its very slow motion with respect to the stars.

For Pluto to have a perturbing effect on other planets, its mass must be greater than that of the Earth. For this reason, since its discovery, Pluto's mass has been overestimated. We now know that its mass is much less than previously thought, and that it has no measurable effect on other planets. In a sense, Pluto's discovery was accidental. It was a result of an extensive search of a particular region in the sky.

Overview

When we look at photographs of the Milky Way (see Fig. 16.1), we note large regions where no light is seen. We think that these are due to dust blocking the light between us and the stars. We can see the same effect on a smaller scale (Fig. 14.1). Note that there is a high density of stars near the edges of the image. As one moves close to the center, the density of stars declines sharply. Near the center, no stars can be seen. This apparent hole in the distribution of stars is really caused by a small dust cloud, called a globule. The more dust there is in the globule, the fewer background stars we can see through the globule. We can use images like this to trace out the interstellar dust. We find that it is not uniformly distributed. Rather, it is mostly confined to concentrations or interstellar clouds.

We detect the presence of the gas by observing absorption or emission lines from the gas. By tracing these lines, we find that the gas also has an irregular distribution. Often the gas appears along the same lines of sight as the dust clouds. From this apparent coincidence we form the idea that the gas and dust are generally well mixed, with the gas having about 99% of the mass in a given cloud. In this chapter, we will see how the masses of different types of clouds are determined.

The past decades have seen dramatic improvements in our observing capabilities. There have been improvements in our ability to detect visible radiation, and there have also been exciting extensions to other parts of the spectrum. These improved observing capabilities have had a major impact on astronomy and astrophysics. In this chapter we will first discuss the basic concepts behind optical observations. We will then discuss observations in other parts of the spectrum.

What a telescope does

An optical telescope provides two important capabilities:

1. (1) It provides us with light-gathering power. This means that we can see fainter objects with a telescope than we can see with our naked eye.

2. (2) It provides us with angular resolution. This means that we can see greater detail with a telescope than without.

For ground-based optical telescopes, light-gathering power is usually the most important feature.

Light gathering

We can think of light from a star as a steady stream of photons striking the ground with a certain number of photons per unit area per second. If we look straight at a star, we will see only the photons that directly strike our eyes. If we can somehow collect photons over an area much larger than our eye, and concentrate them on the eye, then the eye will receive more photons per second than the unaided eye. A telescope provides us with a large collecting area to intercept as much of the beam of incoming photons as possible, and then has the optics to focus those photons on the eye, or a camera, or onto some detector.

Overview

Throughout this book we have discussed the components of our galaxy: stars, clusters of stars, interstellar gas and dust. We now look at how these components are arranged in the galaxy. The study of the large scale structure of our galaxy is difficult from our particular viewing point. We are in the plane of the galaxy, so all we see is a band of light (Fig. 16.1). The interstellar dust prevents us from seeing very far into the galaxy. We see a distorted view.

The first evidence on our true position in the galaxy came from the work of Harlow Shapley, who studied the distribution of globular clusters (Fig. 16.2). He found the distances to the clusters from observations of Cepheids and RR Lyrae stars. Shapley found that the globular clusters form a spherical distribution. The center of this distribution is some 10 kpc from the Sun. Presumably, the center of the globular cluster distribution is the center of the galaxy. This means that we are about 10 kpc from the galactic center.

In Chapter 13, when we studied HR diagrams for clusters, we introduced the concept of stellar populations I and II. The distribution of these populations in the galaxy can help us understand how the galaxy has evolved. Population I material is loosely thought of as being the young material in the galaxy.

Now that we know the basic properties of stars, we look at how the laws of physics determine those properties, and then how stars change with time – how they evolve. Stars go through a recurring full life cycle. They are born, they live through middle age, and they die. In their death, they distribute material into interstellar space to be incorporated into the next generation of stars.

In describing the life cycle, we can start anywhere in the process. In Chapter 9, we discuss the most stable part of their life cycle, life on the main sequence – stellar middle age. In Chapters 10, 11 and 12 we will look at the deaths of different types of stars. After discussing the interstellar medium in Chapter 14, we will look at star formation in Chapter 15.

Brightness of starlight

When we look at the sky, we note that some stars appear brighter than others. At this point we are not concerned with what causes these brightness differences. (They may result from stars actually having different power outputs, or from stars being at different distances.) All we know at first glance is that stars appear to have different brightnesses.

We would like to have some way of quantifying the observed brightnesses of stars. When we speak loosely of brightness, we are really talking about the energy flux, f, which is the energy per unit area per unit time received from the star. This can be measured with current instruments (as we will discuss in Chapter 4). However, the study of stellar brightness started long before such instruments, or even telescopes, were available. Ancient astronomers made naked eye estimates of brightness. Hipparchus, the Greek astronomer, and later Ptolemy, a Greek living in Alexandria, Egypt, around 150 BC, divided stars into six classes of brightness. These classes were called magnitudes. This was an ordinal arrangement, with first-magnitude stars being the brightest and sixth-magnitude stars being the faintest.

When quantitative measurements were made, it was found that each jump of one magnitude corresponded to a fixed flux ratio, not a flux difference. Because of this, the magnitude scale is essentially a logarithmic one. This is not too surprising, since the eye is approximately logarithmic in its response to light.

Aristotle, observing the Greeks in the fourth century b.c., thought that man's natural proclivities were toward discourse and political activity. Adam Smith, observing the Scots in the eighteenth century a.d., saw instead a propensity to engage in economic exchange. From the observations of these two intellectual giants, two separate fields in the social sciences have developed: the science of politics and the science of economics.

Traditionally, these two fields have been separated by the types of questions they ask, the assumptions they make about individual motivation, and the methodologies they employ. Political science has studied man's behavior in the public arena; economics has studied man in the marketplace. Political science has often assumed that political man pursues the public interest.