The Milky Way and Beyond (24 page)

The fifth-magnitude star 51 Pegasi is located 48 light-years away from Earth in the constellation Pegasus and was the first sunlike star confirmed to possess a planet. 51 Pegasi, which has physical properties (luminosity and temperature, for example) very similar to those of the Sun, became the focus of attention in 1995 when astronomers announced the detection of a planet orbiting it. The extrasolar planet is not visible from Earth, but its presence was deduced from the wobble that its gravity induces in the parent star's motion in a 4.23-day cycle. It has a mass 47 percent that of Jupiter and orbits surprisingly close (7.8 million km [4.8 million miles]) to the star—much closer than Mercury, which orbits the Sun at a distance of 57.9 million km (35.9 million miles).

Alpha Centauri is a triple star, the faintest component of which, Proxima Centauri, is the closest star to the Sun, at about 4.2 light-years' distance. The two brighter components, about 1/5 light-year farther from the Sun, revolve around each other with a period of about 80 years, while Proxima may be circling them with a period probably of 500,000 years. The brightest component star resembles the Sun in spectral type, diameter, and absolute magnitude. Its apparent visual magnitude is 0.0. The second brightest component, of visual magnitude 1.4, is a redder star. The third component, of 11th magnitude, is a red dwarf star.

As seen from Earth, the system is the fourth brightest star (after Sirius, Canopus, and Arcturus); the red dwarf Proxima is invisible to the unaided eye. Alpha Centauri lies in the southern constellation Centaurus and can be seen only from south of about 40° north latitude.

Arcturus (also called Alpha Boötis,  ) is one of the five brightest stars in the night sky and the brightest star in the northern constellation Boötes, with an apparent visual magnitude of −0.05. It is an orangecoloured giant star 36.7 light-years from Earth. It lies in an almost direct line with the tail of Ursa Major (the Great Bear); hence its name, derived from the Greek words for “bear guard.”

Betelgeuse, or Alpha Orionis, is the brightest star in the constellation Orion, marking the eastern shoulder of the hunter. Its name is derived from the Arabic word
bat al-dshauzâ
, which means “the giant's shoulder.” Betelgeuse has a variable apparent magnitude of about 0.6 and is one of the most luminous stars in the night sky. It is easily discernible to even the casual observer, not only because of its brightness and position in the brilliant Orion but also because of its deep-reddish colour. The star is approximately 640 light-years from Earth.

Betelgeuse is a red supergiant star roughly 950 times as large as the Sun, making it one of the largest stars known. For comparison, the diameter of Mars's orbit around the Sun is 328 times the Sun's diameter. Infrared studies from spacecraft have revealed that Betelgeuse is surrounded by immense shells of material evidently shed by the star during episodes of mass loss over the past 100,000 years. The largest of these shells has a radius of nearly 7.5 light-years.

Deneb, which is also called Alpha Cygni, is one of the brightest stars, with an apparent magnitude of 1.25. Its name is Arabic for “tail” (of the Swan, Cygnus). This star, at about 1,500 light-years' distance, is the most remote (and brightest intrinsically) of the 20 apparently brightest stars. It lies in the northern constellation Cygnus and, with Vega and Altair, forms the prominent “Summer Triangle.”

Fomalhaut, or Alpha Piscis Austrini, is the 17th star (in order of apparent brightness). It is used in navigation because of its conspicuous place in a sky region otherwise lacking in bright stars. It lies in the southern constellation Piscis Austrinus, 25 light-years from Earth. A white star, it has an apparent magnitude of 1.16. A sixth-magnitude companion star, HR 8721, is yellow and orbits at a distance of about 0.9 light-year. A belt of dust orbits between 19.9 and 23.6 billion km (12.4 and 14.7 billion miles) from the star. Images taken with the Hubble Space Telescope in 2004 and 2006 showed a planet, Fomalhaut b, orbiting inside the dust belt at a distance of 17.8 billion km (11.1 billion miles) from the star. These were the first confirmed images of an extrasolar planet. The planet has a mass three times that of Jupiter and an orbital period of 872 years.

Fomalhaut was associated with the Roman goddess Ceres (associated with the analogous Sicilian and Greek goddess Demeter) and was worshipped; in astrology it is one of four royal stars.

The Pleiades (catalog number M45) is an open cluster of young stars in the zodiacal constellation Taurus, about 430 light-years from the solar system. It contains a large amount of bright nebulous material and more than 1,000 stars, of which six or seven can be seen by the unaided eye and have figured prominently in the myths and literature of many cultures. In Greek mythology the Seven Sisters (Alcyone, Maia, Electra, Merope, Taygete, Celaeno, and Sterope, names now assigned to individual stars), daughters of Atlas and Pleione, were changed into the stars. The heliacal (near dawn) rising of the Pleiades in spring of the Northern Hemisphere has marked from ancient times the opening of seafaring and farming seasons, as the morning setting of the group in autumn signified the seasons' ends. Some South American Indians use the same word for “Pleiades” and “year.”

The cluster was first examined telescopically by Galileo, who found more than 40 members; it was first photographed by Paul and Prosper Henry in 1885.

Polaris, which is also called Alpha Ursae Minoris, is Earth's present northern polestar, or North Star. It can be found at the end of the “handle” of the so-called Little Dipper in the constellation Ursa Minor. Polaris is actually a triple star, the brighter of two visual components being a spectroscopic binary with a period of about 30 years and a Cepheid variable with a period of about 4 days. Its changes in brightness are too slight to be detected with the unaided eye. Apparent visual magnitude of the Polaris system is 2.00.

Sirius—which is also called Alpha Canis Majoris, or the Dog Star—is the brightest star in the night sky, with apparent visual magnitude of −1.44. It is a binary star in the constellation Canis Major. The bright component of the binary is a blue-white star 23 times as luminous as the Sun and somewhat larger and considerably hotter than the Sun. Its distance from the solar system is about 8.6 light-years, only twice the distance of the nearest known star beyond the Sun. Its name probably comes from a Greek word meaning “sparkling,” or “scorching.”

Sirius was known as Sothis to the ancient Egyptians, who were aware that it made its first heliacal rising (i.e., rose just before sunrise) of the year at about the time the annual floods were beginning in the Nile River delta. They long believed that Sothis caused the Nile floods; and they discovered that the heliacal rising of the star occurred at intervals of 365.25 days rather than the 365 days of their calendar year, a correction in the length of the year that was later incorporated in the Julian calendar. Among the ancient Romans, the hottest part of the year was associated with the heliacal rising of the Dog Star, a connection that survives in the expression “dog days.”

That Sirius is a binary star was first reported by the German astronomer Friedrich Wilhelm Bessel in 1844. He had observed that the bright star was pursuing a slightly wavy course among its neighbours in the sky and concluded that it had a companion star, with which it revolved in a period of about 50 years. The companion was first seen in 1862 by Alvan Clark, an American astronomer and telescope maker.

Sirius and its companion revolve together in orbits of considerable eccentricity and with average separation of the stars of about 20 times the Earth's distance from the Sun. Despite the glare of the bright star, the seventh-magnitude companion is readily seen with a large telescope. This companion star, known as Sirius B, is about as massive as the Sun, though much more condensed, and was the first white dwarf star to be discovered.

Vega, or Alpha Lyrae, is the brightest star in the northern constellation Lyra and the fifth brightest in the night sky, with a visual magnitude of 0.03. It is also one of the Sun's closer neighbours, at a distance of about 25 light-years. Vega's spectral type is A (white) and its luminosity class V (main sequence). It will become the northern polestar by about 14,000 CE because of the precession of the equinoxes. Vega is surrounded by a disk of circumstellar dust that may be similar to the solar system's Kuiper Belt.

S
cattered between the stars in interstellar space are various tenuous clouds of gas and dust. These clouds are called nebulae (Latin: “mist” or “cloud”). The term was formerly applied to any object outside the solar system that had a diffuse appearance rather than a pointlike image, as in the case of a star. This definition, adopted at a time when very distant objects could not be resolved into great detail, unfortunately includes two unrelated classes of objects: the extragalactic nebulae, the enormous collections of stars and gas now called galaxies, and the galactic nebulae, which are composed of the interstellar medium (the gas between the stars, with its accompanying small solid particles) within a single galaxy. Today, the term
nebula
generally refers exclusively to the interstellar medium.

In a spiral galaxy the interstellar medium makes up 3 percent to 5 percent of the galaxy's mass, but within a spiral arm its mass fraction increases to about 20 percent. About 1 percent of the mass of the interstellar medium is in the form of “dust”—small solid particles that are efficient in absorbing and scattering radiation. Much of the rest of the mass within a galaxy is concentrated in visible stars, but there is also some form of dark matter that accounts for a substantial fraction of the mass in the outer regions.

The most conspicuous property of interstellar gas is its clumpy distribution on all size scales observed, from the size of the entire Milky Way Galaxy (about 10
²⁰
metres, or hundreds of thousands of light-years) down to the distance from Earth to the Sun (about 10
¹¹
metres, or a few light-minutes). The large-scale variations are seen by direct observation; the small-scale variations are observed by fluctuations in the intensity of radio waves, similar to the “twinkling” of starlight caused by unsteadiness in the Earth's atmosphere. Various regions exhibit an enormous range of densities and temperatures. Within the Galaxy's spiral arms about half the mass of the interstellar medium is concentrated in molecular clouds, in which hydrogen occurs in molecular form (H
₂
) and temperatures are as low as 10 kelvins (K). These clouds are inconspicuous optically and are detected principally by their carbon monoxide (CO) emissions in the millimetre wavelength range. Their densities in the regions studied by CO emissions are typically 1,000 H
₂
molecules per cubic cm (1 cubic cm = .06 cubic in). At the other extreme is the gas between the clouds, with a temperature of 10 million K and a density of only 0.001 H
⁺
ion per cubic cm. Such gas is produced by supernovae, the violent explosions of unstable stars.