Stellar positions

Basic measurements

Accurate observations of stellar positions are essential to many problems of astronomy. Positions of the brighter stars can be measured very accurately in the equatorial system (the coordinates of which are called right ascension [α, or RA] and declination [δ, or DEC] and are given for some epoch—for example, 1950.0 or, currently, 2000.0). Positions of fainter stars are measured by using electronic imaging devices (e.g., a charge-coupled device, or CCD) with respect to the brighter stars, and, finally, the entire group is referred to the positions of known external galaxies. These distant galaxies are far enough away to define an essentially fixed, or immovable, system, whereas in the Milky Way the positions of both bright and faint stars are affected over relatively short periods of time by galactic rotation and by their own motions through the Galaxy.

Stellar motions

Accurate measurements of position make it possible to determine the movement of a star across the line of sight (i.e., perpendicular to the observer)—its proper motion. The amount of proper motion, denoted by μ (in arc seconds per year), divided by the parallax of the star and multiplied by a factor of 4.74 equals the tangential velocity, V_T, in kilometres per second in the plane of the celestial sphere.

The motion along the line of sight (i.e., toward the observer), called radial velocity, is obtained directly from spectroscopic observations. If λ is the wavelength of a characteristic spectral line of some atom or ion present in the star and λ_L is the wavelength of the same line measured in the laboratory, then the difference Δλ, or λ − λ_L, divided by λ_L equals the radial velocity, V_R, divided by the velocity of light, c—namely, Δλ/λ_L = V_R/c. Shifts of a spectral line toward the red end of the electromagnetic spectrum (i.e., positive V_R) indicate recession, and those toward the blue end (negative V_R) indicate approach (see Doppler effect; redshift). If the parallax is known, measurements of μ and V_R enable a determination of the space motion of the star. Normally, radial velocities are corrected for Earth’s rotation and for its motion around the Sun, so that they refer to the line-of-sight motion of the star with respect to the Sun.

Consider a pertinent example. The proper motion of Alpha Centauri is about 3.5 arc seconds, which, at a distance of 4.4 light-years, means that this star moves 0.00007 light-year in one year. It thus has a projected velocity in the plane of the sky of 22 km per second. (One kilometre is about 0.62 mile.) As for motion along the line of sight, Alpha Centauri’s spectral lines are slightly blueshifted, implying a velocity of approach of about 20 km per second. The true space motion, equal to (22² + 20²)^1/2 or about 30 km per second, suggests that this star will make its closest approach to the Sun (at three light-years’ distance) some 280 centuries from now.

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Measuring starlight intensity

Stellar brightnesses are usually expressed by means of their magnitudes, a usage inherited from classical times. A star of the first magnitude is about 2.5 times as bright as one of the second magnitude, which in turn is some 2.5 times as bright as one of the third magnitude, and so on. A star of the first magnitude is therefore 2.5⁵ or 100 times as bright as one of the sixth magnitude. The magnitude of Sirius, which appears to an observer on Earth as the brightest star in the sky (save the Sun), is −1.4. Canopus, the second brightest, has a magnitude of −0.7, while the faintest star normally seen without the aid of a telescope is of the sixth magnitude. Stars as faint as the 30th magnitude have been measured with modern telescopes, meaning that these instruments can detect stars about four billion times fainter than can the human eye alone.

The scale of magnitudes comprises a geometric progression of brightness. Magnitudes can be converted to light ratios by letting l_n and l_m be the brightnesses of stars of magnitudes n and m; the logarithm of the ratio of the two brightnesses then equals 0.4 times the difference between them—i.e., log(l_m/l_n) = 0.4(n − m). Magnitudes are actually defined in terms of observed brightness, a quantity that depends on the light-detecting device employed. Visual magnitudes were originally measured with the eye, which is most sensitive to yellow-green light, while photographic magnitudes were obtained from images on old photographic plates, which were most sensitive to blue light. Today, magnitudes are measured electronically, using detectors such as CCDs equipped with yellow-green or blue filters to create conditions that roughly correspond to those under which the original visual and photographic magnitudes were measured. Yellow-green magnitudes are still often designated V magnitudes, but blue magnitudes are now designated B. The scheme has been extended to other magnitudes, such as ultraviolet (U), red (R), and near-infrared (I). Other systems vary the details of this scheme. All magnitude systems must have a reference, or zero, point. In practice, this is fixed arbitrarily by agreed-upon magnitudes measured for a variety of standard stars.

The actually measured brightnesses of stars give apparent magnitudes. These cannot be converted to intrinsic brightnesses until the distances of the objects concerned are known. The absolute magnitude of a star is defined as the magnitude it would have if it were viewed at a standard distance of 10 parsecs (32.6 light-years). Since the apparent visual magnitude of the Sun is −26.75, its absolute magnitude corresponds to a diminution in brightness by a factor of (2,062,650)² and is, using logarithms, −26.75 + 2.5 × log(2,062,650)², or −26.75 + 31.57 = 4.82. This is the magnitude that the Sun would have if it were at a distance of 10 parsecs—an object still visible to the naked eye, though not a very conspicuous one and certainly not the brightest in the sky. Very luminous stars, such as Deneb, Rigel, and Betelgeuse, have absolute magnitudes of −7 to −9, while one of the faintest known stars, the companion to the star with the catalog name BD + 4°4048, has an absolute visual magnitude of +19, which is about a million times fainter than the Sun. Many astronomers suspect that large numbers of such faint stars exist, but most of these objects have so far eluded detection.