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 more sensitive to blue light than to red.Because of this variation, two magnitude scales came into use: visual magnitude and photographic magnitude. The difference for any one star, photographic magnitude minus visual magnitude, measures the color of that star–positive for red stars, negative for blue (see COLOR INDEX).
By using filters and special emulsions, astronomers soon had several other magnitude scales, including ultraviolet and infrared. When photoelectric detectors were introduced, the brightnesses of stars were measured with a photoelectric photometer at the focus of a telescope. Standard colors (wavelengths) of light were adopted, and the symbols were changed to V and B, with U for the ultraviolet scale, and several other letters for infrared scales. Measuring the brightness of a star on any of these scales is complicated by factors related to the Earth’s atmosphere, which absorbs more light when a star is near the horizon than when it is overhead.The atmosphere also absorbs different amounts of the different colors and can change during the night because of changing dust or moisture in the air. Nevertheless, by comparing a star with a standard at the same height above the horizon, astronomers using photoelectric photometers can measure U, B, and V magnitudes with an accuracy of 0.01 magnitude (see PHOTOMETRY, ASTRONOMICAL).
Such photometry has provided a great deal of information regarding the temperatures and energy output of stars, but it does not give the total energy output. Each measurement (U, B, V) gives only a fraction of the star’s light reaching the Earth; even if the measurements are combined, they give only the part that is not absorbed as it passes through the Earth’s atmosphere. The atmosphere absorbs all light of short wavelengths below ultraviolet and many of the long wavelengths above red. A theoretical correction can be made, based on the star’s temperature, to give a “bolometric” magnitude, m(b), adding the energy absorbed by the atmosphere.True bolometric magnitudes, however, are measured only from rockets and spacecraft outside the Earth’s atmosphere. From parallax-distance measurements it is possible to calculate the absolute bolometric magnitude, or luminosity, of a star, which is a measure of its brightness relative to the Sun if it were at the Sun’s distance from an observer on Earth. During the 1920s it was found that some stars (giants) are 100,000 times as luminous as the Sun; others (white dwarfs) are 1,000 times less luminous. Composition During ancient times and the Middle Ages stars were thought to be made of an ethereal element different from matter on Earth.
Their actual composition did not become known until the invention of the SPECTROSCOPE in the 19th century.Through the refraction of light by a prism (see PRISM, physics) or through its diffraction by a DIFFRACTION GRATING, the light from a source is spread out into its different visual wavelengths, from red to blue; this is known as its SPECTRUM. The spectra of the Sun and stars exhibited bright and dark lines, which were shown to be caused by elements emitting or absorbing light at specific wavelengths. Because each element emits or absorbs light only at specific wavelengths, the chemical composition of stars can be determined. In this way the spectroscope demonstrated that the gases in the Sun and stars are those of common elements such as hydrogen, helium, iron, and calcium at temperatures of several thousand degrees. It was found that the average star’s atmosphere consists mostly of hydrogen (87%) and helium (10%), an element discovered from spectra of the Sun, with all other elements making up about 3%.Helium actually was first discovered in the Sun’s spectrum. At first, visual estimates of the strengths of spectral lines were used to estimate the amounts of the elements present in the Sun and a few stars, based on an analysis of the lines produced by a laboratory light source.
When photographic emulsions came into use, the spectroscope became the spectrograph, with a photographic film or plate replacing the human eye. During the first half of the 20th century, spectrographs were used on telescopes to observe thousands of stars. On the spectrogram, the intensities of the lines are measured from the blackness of the film or plate.Most recently, photoelectric detectors are used to scan the spectrum in a spectrophotometer. Stellar spectra can also be measured by interferometer techniques.
Although the ultraviolet, visual, and infrared parts of a star’s spectrum can be measured in this way, other techniques must be used, above the atmosphere, to measure the shorter wavelength spectra of X-ray stars and gamma-ray stars. Instead of gratings and prisms, various combinations of filters and detectors are used to measure portions of the X-ray and gamma-ray spectra. At the other extreme (long wavelengths), radio spectra of stars and other radio sources are measured by “tuning” a radio telescope to different frequencies.
A radio telescope–the largest is more than 305 m (1,000 ft) across–is like a giant optical reflector with a radio amplifier at the focus. Radio spectra are much more accurate than optical spectra.Multiple radio telescopes, placed thousands of kilometers apart, can determine the position of a radio-emitting star as accurately as an optical telescope can, to better than 0.1 second of arc (see RADIO ASTRONOMY). Spectral Type and Surface Temperature During the early decades of the 20th century, Annie J. Cannon at Harvard University examined thousands of stellar spectra. Without concern for the actual atmospheric gases or temperatures, Cannon classified each spectrum as A, B, C, . .
.S, depending on the number of absorption lines. Class A has few strong lines, class F has more, and classes M to S have bands, which are many lines close together, produced by molecules (see HARVARD CLASSIFICATION OF STARS).Later studies showed that Cannon’s classes are a measure of surface temperature in the sequence O, B, A, F, G, K, M, R, N, S. This measurement is based partly on physicist Max Planck’s formula, which gives the relative emissions of various colors from a hot body.
A cool star emits most of its light in the red; a hot star emits most of its light in the blue. A measurement of the ratio of blue to red light coming from a star (its color index) determines its temperature. O stars are hot (surface temperature =30,000 K); A stars have surface temperature = 10,000 K; G stars, such as the Sun, have surface temperature =6,000 K; and M stars have surface temperature =3,000 K.Other spectrographic measurements of absorption lines and emission lines help to confirm or modify this so-called color temperature. From 1911 to 1913, Einar Hertzsprung and H. N. Russell first plotted the luminosity (L) versus the surface temperature (Ts) of stars, using as a measure of temperature the spectral types determined by Cannon. The HERTZSPRUNG-RUSSELL DIAGRAM first showed that highly luminous stars are mostly of classes O and B, with helium lines and surface temperature =25,000 K, whereas low-luminosity stars are mostly of class M and surface temperature =3,000 K.
Size Once the temperature and the bolometric luminosity of a star are known, its size can easily be calculated. Planck’s formula gives the total emission of radiant energy per unit area of a hot body’s surface at each temperature. From the bolometric luminosity, the total energy emitted is known; from the temperature, the radiant energy emitted per square centimeter is known. The ratio gives the number of square centimeters, from which the radius of the star can be calculated. This rough calculation shows that the radii of stars vary from 1/100 of that of the Sun for WHITE DWARFS to 400 times that of the Sun for SUPERGIANTS.
The radius of a nearby star can also be measured directly with an interferometer on a telescope. Astronomers theorize that ob …