The Milky Way and Beyond by Britannica Educational Publishing Page B
Authors: Britannica Educational Publishing
Ads: Link
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.
D ISTANCES TO THE S TARS
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.
D ETERMINING S TELLAR D ISTANCES
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 Earthâs 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 Earthâs orbit as the baseline, the distance of the star can be found from the parallactic angle, p. If p = 1â (one second of arc), the distance of the star is 206,265 times Earthâs distance from the Sunânamely, 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 0.1â. The nearest star to Earth, Proxima Centauri (a member of the triple system of Alpha Centauri), has a parallax of 0.7723â, meaning that its distance is 1/0.7723, or 1.295, parsecs, which equals 4.22 light-years.
The parallax of Barnardâs star, the next closest after the Alpha Centauri system, is 0.549â, so that its distance is nearly 6 light-years. Errors of such parallaxes are now typically 0.005â, meaning that there is a 50 percent probability that a star whose parallax is 0.065â lies between 14.3 and 16.7 parsecs (corresponding to parallaxes of 0.070â and 0.060â, 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.
N EAREST S TARS
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 0.001â and are therefore well beyond several hundred light-years distance from the Sun. The
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.
D ISTANCES TO THE S TARS
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.
D ETERMINING S TELLAR D ISTANCES
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 Earthâs 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 Earthâs orbit as the baseline, the distance of the star can be found from the parallactic angle, p. If p = 1â (one second of arc), the distance of the star is 206,265 times Earthâs distance from the Sunânamely, 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 0.1â. The nearest star to Earth, Proxima Centauri (a member of the triple system of Alpha Centauri), has a parallax of 0.7723â, meaning that its distance is 1/0.7723, or 1.295, parsecs, which equals 4.22 light-years.
The parallax of Barnardâs star, the next closest after the Alpha Centauri system, is 0.549â, so that its distance is nearly 6 light-years. Errors of such parallaxes are now typically 0.005â, meaning that there is a 50 percent probability that a star whose parallax is 0.065â lies between 14.3 and 16.7 parsecs (corresponding to parallaxes of 0.070â and 0.060â, 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.
N EAREST S TARS
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 0.001â and are therefore well beyond several hundred light-years distance from the Sun. The
Similar Books
