The extension of astronomical study beyond the solar system is the observation of the stars, which constitute the major population of our Milky Way galaxy and the other galaxies. Ancient civilizations sometimes thought of the stars as small lights hanging from a celestial dome or as holes in the dome through which the fires of hell could be seen. The Greek astronomers alluded to the stars as suns, and the Arabic astronomers were aware of this possibility as well, but it was not until the time of Copernicus that stars were definitely established as bright, distant objects. In the following years it was determined that stars are balls of hot, glowing gases. However, men did not learn the source of stellar energy until recent decades.
Distance and Brightness Measurement
The ability to measure immense distances is essential to the study of stars. Astronomers have had to develop new techniques and adopt new units of measurement. Thus stellar distances are usually expressed in light-years or parsecs. A light-year is the distance that light travels in one year, moving at approximately 186,000 miles, or 300,000 kilometers, per second. One parsec is equivalent to 3.26 light-years.
Measurement of distances to nearby stars is relatively straightforward, the method employed being similar to that of the surveyor. The radius of the earth’s orbit is used as a baseline and also as a distance unit called the astronomical unit. The position of a nearby star is observed in relation to the background of more distant stars. The star is again observed from the opposite point of the earth’s orbit, and its relative position is seen to have changed. This is called parallax, and the angle subtended by the astronomical unit at the distance of the star is called the parallactic angle.
If the star’s apparent brightness is measured, the astronomer can then compute the actual brightness of the star. For convenience, 10 parsecs is used as the standard distance for expressing actual brightness. That is, the brightness of the star as it would appear at a distance of 10 parsecs from the earth is considered to be its actual brightness, or absolute magnitude.
Magnitude. In expressing stellar brightness, astronomers reduce their measurements to a convenient power relation that has its origins in the response of the eye to light. This response is measured in magnitudes and is related to the intensity of the stimulus on a logarithmic scale. The term magnitude has been used to express stellar brightness since the days of the ancient Greek astronomers, who thought that a star’s brightness depends simply on its size or magnitude. (It is now known that other factors are involved as well.)
The Hertzsprung-Russell Diagram. If the absolute magnitudes of a large number of stars are determined and plotted against the temperatures of these stars, a diagram known as the Hertzsprung-Russell (H-R) diagram results. This type of diagram is fundamental to astronomy because it reveals that there is a systematic organization to the physical properties of the stars.
The H-R diagram is of immediate interest here, however, because it is also of importance in the measurement of stellar distances. The astrometric technique used for measuring distances to nearby stars cannot be used for greater distances because the parallactic angle simply becomes too small to measure. However, if an astronomer can identify where a distant star would lie on an H-R diagram, he then knows its absolute brightness and can determine its distance. Since an astronomer can measure very feeble light sources, this photometric method is a powerful method indeed.
The Period-Luminosity Relation and Hubble’s Law. Nature has provided two other methods of establishing stellar distances. One method involves true variable stars—pulsating stars with a periodic change in brightness that is related to their changes in size. The period of pulsation of any gaseous sphere is related in a simple way to the density of the sphere. Basically, the more dense a star is, the shorter is its period of pulsation. Also, the more dense a star is, the smaller it is, and hence the smaller is its surface area. Other things being equal, therefore, the denser a star is, the fainter is its absolute magnitude.
Astronomers have established a period-brightness, or period-luminosity (P-L relation), for pulsating stars. All that they need to do to use the P-L relation is to measure the period of pulsation of a star. They then know the star’s absolute magnitude and can compute its distance. Pulsating stars are very easily recognized even in other nearby galaxies, and the distances to these galaxies can therefore be measured.
The other method of measuring distances pertains to the galaxies themselves. When galaxies are far enough away, they all appear to be moving away from us. The speed of their recession seems to follow a simple law called Hubble’s law (after Edwin Hubble), which states that the speed is proportional to the distances of the galaxies. Using this law, astronomers can measure the great distances of the universe.
Stellar Masses and Composition
Distances do not tell astronomers how stars are born, evolve, and die, however. To obtain such information they must also be able to measure the mass and determine the composition of stars.
Mass is a difficult property to measure, because it is necessary to weigh a star against something. That “something” can only be another star, and for this reason astronomers study binaries (double-star systems in which two stars revolve around one another). From such studies it has been deduced that there is a fundamental relation between a star’s mass and its luminosity—the so-called mass-luminosity relation. This relation holds true for most stars but there are exceptions, notably those stars known as white dwarfs. To increase knowledge about stellar masses a considerable amount of work must be done over the coming years.
The problem of composition is even more perplexing. A spectrogram of a star reveals only those elements contained in the star’s atmosphere, mainly the lower chromosphere. From such information an astronomer has to infer the composition of the main body of the star. All of the available evidence leads to the conclusion that, in general, stars are composed of 70 percent hydrogen, 28 percent helium, and 2 percent heavier elements by weight. Different stellar models can be set up and studied on a computer by varying these percentages. In this way an astronomer can look at stellar evolution in theory and then test the theory by observation.
Classification of Stars
Spectrographs are also used in the classification of stars. That is, the spectral classification of a star is determined by the presence or absence of certain lines in its spectrum and by the strength and shape of those lines. Since the appearance of the lines depends primarily upon the brightness and temperature of the star, it is possible to simplify the identification of stars to the point where an astronomer need only look at the spectrum of a star to do this.
The basic classification system in general use today is the Morgan-Keenan system, referred to as the MK Classification. It is essentially an excellent extension and refinement of an earlier Harvard Classification. In the MK system, stars are assigned the letters O, B, A, F, G, K, or M, in order of decreasing temperature. Each letter type is further subdivided into 10 classes, 0 through 9, again in order of decreasing temperature. There are also certain types of stars that do not fit into the general system and are instead assigned special letters. Thus carbon, or C, stars are similar to the cool M stars in temperature but have quite different spectra.
Stars of the same spectral type and lying at about the same distance from the earth are often quite different in brightness. Since the stars must have the same surface temperature, this can only mean that they have different surface areas—that is, they are different in size. This in turn leads to the concept of luminosity classes and the fact that there are dwarf stars, giant stars, and even supergiant stars. In the MK Classification these types are indicated by additional numerals, ranging from 0 for the most luminous supergiants to VI for subdwarfs. For example, the sun is a type G2V star; a G2III star would be a giant with the same temperature as the sun.
Giant stars are objects of great interest in modern astronomy. These huge stars are very tenuous. In fact, their density is so low that it is possible for matter to escape from them, and this has been observed to occur. Intensive studies of this mechanism of loss of mass, with subsequent enrichment of the interstellar medium, are currently under way.
Another area of interest to modern astronomers is the study of stellar evolution. It is thought that there is an evolutionary order to the types of stars, and that the life history of a star like the sun can be described in the following way.
The star begins as a cloud of dust and gas in the interstellar medium. Somehow, perhaps because of magnetic fields, the cloud exceeds a certain density and begins to collapse inwards. Pressure in the protostar’s center causes it to glow. In order to reach a stable configuration, convective currents carry heat to the surface of the protostar, which rapidly contracts until it is one of the main sequence of stars on the H-R diagram and is generating its energy by nuclear fusion. This process takes about a million years. The star then remains stable for billions of years until a large portion of its core is converted to helium by nuclear reactions. At this point the star expands because of increased internal temperatures and becomes a giant, until continuing reactions cause the core to collapse slowly again. The star continues to contract until it becomes a white dwarf. The dying star takes billions of years to cool off and become a black dwarf, a burned-out sun.
For stars a little less massive than the sun the evolutionary process may take much longer, whereas stars much less massive may proceed directly towards becoming black dwarfs in only a few million years. On the other hand, events in stars much more massive than the sun are more drastic and proceed at a much faster rate. In such stars it is thought that the collapse of the core after the giant stage occurs in a catastrophic way, producing what is known as a nova—an exploding star. If the star is very massive, it may become a supernova. After the explosion a white dwarf remains, with a great amount of ejected material being returned to the interstellar medium. In this process the content of the interstellar medium is altered, and the next stars formed from it will be quite different.
Studies of such evolutionary concepts have only begun. Testing the results of these studies requires a combination of observational efforts covering the entire electromagnetic spectrum. One area of study that may be very useful is that of binary systems in which the stars are quite close to one another. Spectroscopic evidence indicates that such stars are constantly exchanging mass—that is, one star is gaining mass at the expense of the other. In some close binaries the stars are both losing mass, which is streaming away from the system in a great pinwheel pattern. In all such systems the stars are evolving faster than they ordinarily would and provide interesting subjects for evolutionary studies.