The Milky Way and Beyond (9 page)

To say that stars are balls of gas that shine through the workings of their internal energy does not do justice to their full nature and complexity. Not all stars are like our Sun. Some stars are massive giants doomed to burn away in merely millions of years. Others are dim brown dwarfs that are in some ways like stars and in others like the even smaller giant planets.

The Sun seems like an impressive star. It casts aside the gloom of night and bathes the entire planet in its life-giving
rays. However, when the Sun is considered among stars, it is merely average in its size and the activity of its winds.

With regard to mass, size, and intrinsic brightness, the Sun is a typical star. Its approximate mass is 2 × 10
³⁰
kg (about 330,000 Earth masses), its approximate radius 700,000 km (430,000 miles), and its approximate luminosity 4 × 10
³³
ergs per second (or equivalently 4 × 10
²³
kilowatts of power). Other stars often have their respective quantities measured in terms of those of the Sun.

The table lists data pertaining to the 20 brightest stars, or, more precisely, stellar systems, since some of them are double (binary stars) or even triple stars. Successive columns give the name of the star, its brightness expressed in units called visual magnitudes and the spectral type or types to which the star or its components belong, the distance in light-years (a light-year being the distance that light waves travel in one Earth year: 9.46 trillion km, or 5.88 trillion miles), and the visual luminosity in terms of that of the Sun. All the primary stars (designated as the A component) are intrinsically as bright as or brighter than the Sun. Some of the companion stars are fainter.

Many stars vary in the amount of light they radiate. Stars such as Altair, Alpha Centauri A and B, and Procyon A are called dwarf stars. Their dimensions are roughly comparable to those of the Sun. Sirius A and Vega, though much brighter, also are dwarf stars; their higher temperatures yield a larger rate of emission per unit area. Aldebaran A, Arcturus, and Capella A are examples of giant stars, whose dimensions are much larger than those of the Sun. Observations with an interferometer (an instrument that measures the angle subtended by the diameter of a star at the observer's position), combined with parallax measurements, which yield a star's distance, give sizes of 12 and 22 solar radii for Arcturus and Aldebaran A. Betelgeuse and Antares A are examples of supergiant stars. The latter has a radius some 300 times that of the Sun, whereas the variable star Betelgeuse oscillates between roughly 300 and 600 solar radii.

Several of the stellar class of white dwarf stars, which have low luminosities and high densities, also are listed. Sirius B is a prime example, having a radius one-thousandth that of the Sun, which is comparable to the size of Earth. Among other notable stars, Rigel A is a young supergiant in the constellation Orion, and Canopus is a bright beacon in the Southern Hemisphere often used for spacecraft navigation.

The Sun's activity is apparently not unique. It has been found that stars of many types are active and have stellar winds analogous to the solar wind. The importance and ubiquity of strong stellar winds became apparent only through advances in spaceborne ultraviolet and X-ray astronomy, as well as in radio and infrared surface-based astronomy.

X-ray observations that were made during the early 1980s yielded some rather unexpected findings. They revealed that nearly all types of stars are surrounded by coronas having temperatures of one million kelvins (K) or more. Furthermore, all stars seemingly display active regions, including spots, flares, and prominences much like those of the Sun. Some stars exhibit starspots so large that an entire face of the star is relatively dark, while others display flare activity thousands of times more intense than that on the Sun.

The highly luminous hot, blue stars have by far the strongest stellar winds. Observations of their ultraviolet spectra with telescopes on sounding rockets and spacecraft have shown that their wind speeds often reach 3,000 km (roughly 2,000 miles) per second, while losing mass at rates up to a billion times that of the solar wind. The corresponding mass-loss rates approach and sometimes exceed one hundred-thousandth of a solar mass per year, which means that one entire solar mass (perhaps a tenth of the total mass of the star) is carried away into space in a relatively short span of 100,000 years. Accordingly, the most luminous stars are thought to lose substantial fractions of their mass during their lifetimes, which are calculated to be only a few million years.

Ultraviolet observations have proved that to produce such great winds the pressure of hot gases in a corona, which drives the solar wind, is not enough. Instead, the winds of the hot stars must be driven directly by the pressure of the energetic ultraviolet radiation emitted by these stars. Aside from the simple realization that copious quantities of ultraviolet radiation flow from such hot stars, the details of the process are not well understood. Whatever is going on, it is surely complex, for the ultraviolet spectra of the stars tend to vary with time, implying that the wind is not steady. In an effort to understand better the variations in the rate of flow, theorists are investigating possible kinds of instabilities that might be peculiar to luminous hot stars.

Observations made with radio and infrared telescopes, as well as with optical instruments, prove that luminous cool stars also have winds whose total mass-flow rates are comparable to those of the luminous hot stars, though their velocities are much lower—about 30 km (20 miles) per second. Because luminous red stars are inherently cool objects (having a surface temperature of about 3,000 K, or half that of the Sun), they emit very little detectable ultraviolet or X-ray radiation. Thus, the mechanism driving the winds must differ from that in luminous hot stars.

Winds from luminous cool stars, unlike those from hot stars, are rich in dust grains and molecules. Since nearly all stars more massive than the Sun eventually evolve into such cool stars, their winds, pouring into space from vast numbers of stars, provide a major source of
new gas and dust in interstellar space, thereby furnishing a vital link in the cycle of star formation and galactic evolution. As in the case of the hot stars, the specific mechanism that drives the winds of the cool stars is not understood. At this time, investigators can only surmise that gas turbulence, magnetic fields, or both in the atmospheres of these stars are somehow responsible.

Strong winds also are found to be associated with objects called protostars, which are huge gas balls that have not yet become full-fledged stars in which energy is provided by nuclear reactions. Radio and infrared observations of deuterium (heavy hydrogen) and carbon monoxide (CO) molecules in the Orion Nebula have revealed clouds of gas expanding outward at velocities approaching 100 km (60 miles) per second. Furthermore, high-resolution, very-long-baseline interferometry observations have disclosed expanding knots of natural maser (coherent microwave) emission of water vapour near the star-forming regions in Orion, thus linking the strong winds to the protostars themselves. The specific causes of these winds remain unknown, but if they generally accompany star formation, astronomers will have to consider the implications for the early solar system. After all, the Sun was presumably once a protostar too.

For thousands of years humanity has wondered about how far it was to the stars. Among the ancient Greeks, the fact that the stars did not seem to move was evidence that Earth did not move around the Sun. The real answer was that the stars were very far away. How far away was not known until astronomical technology had advanced far enough for parallax techniques to be used in the 19th century.

Distances to stars were first determined by the technique of trigonometric parallax, a method still used for nearby stars. When the position of a nearby star is measured from two points on opposite sides of Earth's orbit (i.e., six months apart), a small angular (artificial) displacement is observed relative to a background of very remote (essentially fixed) stars. Using the radius of Earth's orbit as the baseline, the distance of the star can be found from the parallactic angle, p. If p = 1” (one second of arc), the distance of the star is 206,265 times Earth's distance from the Sun—namely, 3.26 light-years. This unit of distance is termed the parsec, defined as the distance of an object whose parallax equals one arc second. Therefore, one parsec equals 3.26 light-years. Since parallax is inversely proportional to distance, a star at 10 parsecs would have a parallax of 0.1”. The nearest star to Earth, Proxima Centauri (a member of the triple system of Alpha Centauri), has a parallax of 0.7723”, meaning that its distance is 1/0.7723, or 1.295, parsecs, which equals 4.22 light-years.

The parallax of Barnard's star, the next closest after the Alpha Centauri system, is 0.549”, so that its distance is nearly 6 light-years. Errors of such parallaxes are now typically 0.005”, meaning that there is a 50 percent probability that a star whose parallax is 0.065” lies between 14.3 and 16.7 parsecs (corresponding to parallaxes of 0.070” and 0.060”, respectively) and an equal chance that it lies outside that range. Thus, measurements of trigonometric parallaxes are useful for only the nearby stars within a few hundred light-years. In fact, of the billions of stars in the Milky Way Galaxy, only about 700 are close enough to have their parallaxes measured with useful accuracy. For more distant stars indirect methods are used. Most of them depend on comparing the intrinsic brightness of a star (found, for example, from its spectrum or other observable property) with its apparent brightness.

The table lists information about the 20 nearest known stars. Only three stars, Alpha Centauri, Procyon, and Sirius, are among the 20 brightest and the 20 nearest stars. Ironically, most of the relatively nearby stars are dimmer than the Sun and are invisible without the aid of a telescope. By contrast, some of the well-known bright stars outlining the constellations have parallaxes as small as the limiting value of 0.001” and are therefore well beyond several hundred light-years distance from the Sun. The most luminous stars can be seen at great distances, whereas the intrinsically faint stars can be observed only if they are relatively close to Earth.

The brightest and nearest stars fall roughly into three categories: (1) giant stars and supergiant stars that are tens or even hundreds of solar radii and extremely low average densities—in fact, several orders of magnitude less than that of water (one gram per cubic centimetre [1 cubic centimetre = .06 cubic inch]); (2) dwarf stars ranging from 0.1 to 5 solar radii and with masses from 0.1 to about 10 solar masses; and (3) white dwarf stars, with masses comparable to that of the Sun but dimensions appropriate to planets, meaning that their average densities are hundreds of thousands of times greater than that of water.

These rough groupings of stars correspond to stages in their life histories. The second category is identified with what is called the main sequence and includes stars that emit energy mainly by converting hydrogen into helium in their cores. The first category comprises stars that have exhausted the hydrogen in their cores and are burning hydrogen within a shell surrounding the core. The white dwarfs represent the final stage in the life of a typical star, when most available sources of energy have been exhausted and the star has become relatively dim.

The large number of binary stars and even multiple systems is notable. These star systems exhibit scales comparable in size to that of the solar system. Some, and perhaps many, of the nearby single stars have invisible (or very dim) companions detectable by their gravitational effects on the primary star; this orbital motion of the unseen member causes the visible star to “wobble” in its motion through space. Some of the invisible companions have been found to have masses on the order of 0.001 solar mass or less, which is in the range of planetary rather than stellar dimensions. Current observations suggest that they are genuine planets, though some are merely extremely dim stars (sometimes called brown dwarfs). Nonetheless, a reasonable inference that can be drawn from these data is that double stars and planetary systems are formed by similar evolutionary processes.