Stars Essays - Observational Astronomy, Constellations,

Stars A star is a large ball of hot gas, thousands to millions of kilometers in diameter, emitting large amounts of radiant energy from nuclear reactions in its interior. Stars differ fundamentally from planets in that they are self-luminous, whereas planets shine by reflected sunlight. Except for the SUN, which is the nearest star, stars appear only as points of light, even in the largest telescopes, because of their distance. The brightest stars have long been given names. Most of the familiar names originated with the ancient Greeks or with later Arab astronomers; an entirely different system was used by the Chinese, starting hundreds of years earlier, about 1000 BC. Polaris, the North Star, has a Greek name; Betelgeuse, a bright red star, has an Arabic name. Modern astronomers designate the bright stars according to the CONSTELLATIONS they are in. Thus, the brightest star in the Big Dipper (part of the constellation Ursa Major) is called alpha Ursa Majoris. Polaris, in the Little Dipper (Ursa Minor), is gamma (designated by the Greek lower-case letter gamma) Ursa Minoris, and Betelgeuse, in Orion, is gamma Orionis. VARIABLE STARS (those which periodically change in brightness) have lettered names, such as RR Lyrae in the constellation Lyra. Fainter stars are known by their numbers in a catalog; HD 12938 is the 12,938th star in the Henry Draper Catalogue. CHARACTERISTICS OF STARS Each star in the universe has its own position, motion, size, mass, chemical composition, and temperature. Some stars are grouped into clusters, and stars and star clusters are collected in the larger groupings called galaxies. Our GALAXY, the Milky Way, contains more than 100 billion stars. Because tens of millions of other galaxies are known to exist, the total number of stars in the universe exceeds a billion billion. Positions, Motions, and Distances Stars are seen in the same relative positions, night after night, year after year. They provided early astronomers with a reference system for measuring the motions of planets ("wandering stars"), the Moon, and the Sun. The westward rotation of the celestial sphere simply reflects the daily eastward rotation of the Earth, and the Sun's apparent motion among the stars reflects the Earth's annual orbit around the Sun. As the construction of larger telescopes during the 19th century improved the accuracy of determining stellar positions, it was found that some stars are not precisely "fixed." They move at various speeds, measured as changes of direction in fractions of a second of arc per year, where one second of arc is the angular size of a pinhead 183 m (200 yd) away. Most of the faint stars are truly fixed as viewed from Earth and are used as a reference frame for the minute motions of nearby stars, known as PROPER MOTION. PARALLAX is another apparent motion of nearby stars. It is caused by the Earth's orbit around the Sun: the star seems to shift, first one way, then the other, as the Earth moves from 150 million km (93 million mi) on one side of the Sun to 150 million km on the other side. Stellar parallax can be used to determine astronomical DISTANCE. If the shift is 1 second of arc each way, the star is about 32 million million km (20 million million mi) from an observer. This distance is called the parsec and is equal to 3.26 light-years. The parallaxes of several thousand stars have been measured during the past several decades. The nearest star is Proxima Centauri, at about 1 parsec (3.3 light-years). Most of the measured distances are greater than 20 parsecs (65 light-years), which shows why the average star in the sky is so much fainter than the nearby Sun. Brightness and Luminosity Star brightness was first estimated by eye, and the brightest stars in the sky were described as "stars of the first magnitude." Later, the magnitude scale was defined more accurately: 6th magnitude stars are just 1/100 as bright as 1st magnitude stars; 11th magnitude stars are 1/100 as bright as 6th magnitude, and so on. The magnitude scale is logarithmic; that is, each magnitude corresponds to a factor of 1/2.54, because (1/2.54) to the power of 5 =1/100 (see MAGNITUDE). Photographs are also used to measure star brightness from the size and blackness of images on a photographic plate exposed in a telescope-camera. With the photographic emulsions available in the early 1900s, a blue star that appeared to the eye to have the same brightness as a red star photographed much brighter. This discrepancy occurred because emulsions at that time were much