An Etymological Dictionary of Astronomy and Astrophysics

Same as → rapidly oscillating Ap star

See also: → rapidly oscillating Ap star

Same as → rapidly oscillating Ap star

See also: → rapidly oscillating Ap star

The mathematical description of the interval (→ space-time separation) between → events (“points” in space-time) in a → homogeneous and → isotropic → Universe. It results from an exact solution of → Einstein’s field equations of → general relativity.
Under the assumptions, the Robertson-Walker interval is expressed by:

ds² = c²dt² - R²(t) [dr²/(1 - kr²) + r²dθ² +
r²sin²θ dθ²)].

Same as Friedmann-Lemaître-Robertson-Walker metric. Compare → Minkowski metric.

See also: Named after Howard Percy Robertson (1903-1961), American mathematician and physicist, and Arthur Geoffrey Walker (1909-2001), British mathematician and physicist, for their contributions to physics and physical cosmology; → metric.

The mathematical description of the interval (→ space-time separation) between → events (“points” in space-time) in a → homogeneous and → isotropic → Universe. It results from an exact solution of → Einstein’s field equations of → general relativity.
Under the assumptions, the Robertson-Walker interval is expressed by:

ds² = c²dt² - R²(t) [dr²/(1 - kr²) + r²dθ² +
r²sin²θ dθ²)].

Same as Friedmann-Lemaître-Robertson-Walker metric. Compare → Minkowski metric.

See also: Named after Howard Percy Robertson (1903-1961), American mathematician and physicist, and Arthur Geoffrey Walker (1909-2001), British mathematician and physicist, for their contributions to physics and physical cosmology; → metric.

A machine that does mechanical, routine tasks on command.

Etymology (EN): From Czech, coined by Karel Čapek in the play R.U.R. (1920), from the base robot-, as in robota “compulsory labor,” robotník “peasant owing such labor,” from robotiti “to work, drudge.”

Etymology (PE): Robot, loan from E., as above.

A machine that does mechanical, routine tasks on command.

Etymology (EN): From Czech, coined by Karel Čapek in the play R.U.R. (1920), from the base robot-, as in robota “compulsory labor,” robotník “peasant owing such labor,” from robotiti “to work, drudge.”

Etymology (PE): Robot, loan from E., as above.

The quality of a model when it is insensitive to small discrepancies in assumptions.

Etymology (EN): From L. robustus “strong and hardy,” literally “as strong as oak,”
from robur, robus “hard timber, strength,” also “a special kind of oak,” named for its reddish heartwood, from L. ruber, → red.

Etymology (PE): Tanâvar “robust, stout, corpulent,” from tan “corpus, body,” → if and only if + âvar contraction of âvarandé agent noun of âvardan “to bring; to cause, produce,” → collect.

The quality of a model when it is insensitive to small discrepancies in assumptions.

Etymology (EN): From L. robustus “strong and hardy,” literally “as strong as oak,”
from robur, robus “hard timber, strength,” also “a special kind of oak,” named for its reddish heartwood, from L. ruber, → red.

Etymology (PE): Tanâvar “robust, stout, corpulent,” from tan “corpus, body,” → if and only if + âvar contraction of âvarandé agent noun of âvardan “to bring; to cause, produce,” → collect.

The smallest distance at which a → satellite under the influence of its own → gravitation and that of a central mass about which it is describing a → Keplerian orbit can be in equilibrium. This does not, however, apply to a body held together by the stronger forces between atoms and molecules. At a lesser distance the → tidal forces of the → primary body would break up the → secondary body. The Roche limit is given by the formula d = 1.26 R_M (ρ_M/ρ_m)^1/3, where R_M is the radius of the → primary body, ρ_M is the → density of the primary, and ρ_m is the density of the secondary body. This formula can also be expressed as:

d = 1.26 R_m (M_M/M_m)^1/3, where R_m is the radius of the secondary. As an example, for the Earth-Moon system, where R_M = 6,378 km, ρ_M = 5.5 g cm^-3, and ρ_m = 2.5 g cm^-3 is 1.68 Earth radii.

See also: Named after Edouard Albert Roche (1820-1883), the French astronomer who first calculated this theoretical limit in 1848; → limit.

The smallest distance at which a → satellite under the influence of its own → gravitation and that of a central mass about which it is describing a → Keplerian orbit can be in equilibrium. This does not, however, apply to a body held together by the stronger forces between atoms and molecules. At a lesser distance the → tidal forces of the → primary body would break up the → secondary body. The Roche limit is given by the formula d = 1.26 R_M (ρ_M/ρ_m)^1/3, where R_M is the radius of the → primary body, ρ_M is the → density of the primary, and ρ_m is the density of the secondary body. This formula can also be expressed as:

d = 1.26 R_m (M_M/M_m)^1/3, where R_m is the radius of the secondary. As an example, for the Earth-Moon system, where R_M = 6,378 km, ρ_M = 5.5 g cm^-3, and ρ_m = 2.5 g cm^-3 is 1.68 Earth radii.

See also: Named after Edouard Albert Roche (1820-1883), the French astronomer who first calculated this theoretical limit in 1848; → limit.

The region around a star in a → binary system within which orbiting material is gravitationally bound to that star. The point at which the Roche lobes of the two stars touch is called the → inner Lagrangian point.
→ equipotential surface.

See also: → Roche limit; → lobe.

The region around a star in a → binary system within which orbiting material is gravitationally bound to that star. The point at which the Roche lobes of the two stars touch is called the → inner Lagrangian point.
→ equipotential surface.

See also: → Roche limit; → lobe.

A process in a → binary system when a star fills its → Roche lobe, often by becoming a → giant or → supergiant during the later stages of → stellar evolution. When the star expands, any material that passes beyond the Roche lobe will flow onto the binary → companion, often by way of an → accretion disk. This occurs through the → inner Lagrangian point where the gravity of the two stars cancels. The RLOF is responsible for a number of phenomena including → cataclysmic variables, → Type Ia supernovae, and many → X-ray binary systems.

See also: → Roche lobe; → overflow.

A process in a → binary system when a star fills its → Roche lobe, often by becoming a → giant or → supergiant during the later stages of → stellar evolution. When the star expands, any material that passes beyond the Roche lobe will flow onto the binary → companion, often by way of an → accretion disk. This occurs through the → inner Lagrangian point where the gravity of the two stars cancels. The RLOF is responsible for a number of phenomena including → cataclysmic variables, → Type Ia supernovae, and many → X-ray binary systems.

See also: → Roche lobe; → overflow.

1. A large mass of → stone forming a hill, cliff, promontory, or the like.
   

2. Geology: Mineral matter of variable composition, consolidated or unconsolidated, assembled in masses or considerable quantities in nature, as by the action of heat or water (Dictionary.com).

Etymology (EN): M.E. rokk(e), from O.Fr. ro(c)que, roche (cf. Sp., Provençal roca, It. rocca, M.L. rocca,
V.L. *rocca, of uncertain origin.

Etymology (PE): Bard (Dehxodâ) “rock, stone,” used in a large part of Western Iran, specifically
in Lori and Kurd., related to Kurd. pal “rock, stone;” cf. Gk. poros “rock.”
Sang, → stone.

1. A large mass of → stone forming a hill, cliff, promontory, or the like.
   

2. Geology: Mineral matter of variable composition, consolidated or unconsolidated, assembled in masses or considerable quantities in nature, as by the action of heat or water (Dictionary.com).

Etymology (EN): M.E. rokk(e), from O.Fr. ro(c)que, roche (cf. Sp., Provençal roca, It. rocca, M.L. rocca,
V.L. *rocca, of uncertain origin.

Etymology (PE): Bard (Dehxodâ) “rock, stone,” used in a large part of Western Iran, specifically
in Lori and Kurd., related to Kurd. pal “rock, stone;” cf. Gk. poros “rock.”
Sang, → stone.

Pure natural crystalline form of → silica, SiO₂, which is transparent and colorless.

See also: → rock; → crystal.

Pure natural crystalline form of → silica, SiO₂, which is transparent and colorless.

See also: → rock; → crystal.

A projectile driven by reaction propulsion that carries its own propellants. → missile = mušak (موشک).

See also: From It. rocchetto “a rocket,” literally “a bobbin,” diminutive of rocca “a distaff,” with reference to its shape.

A projectile driven by reaction propulsion that carries its own propellants. → missile = mušak (موشک).

See also: From It. rocchetto “a rocket,” literally “a bobbin,” diminutive of rocca “a distaff,” with reference to its shape.

The study of celestial bodies in the wavelengths that are almost completely absorbed by the atmosphere,
by using a rocket to carry instruments above 250 km to measure the searched for phenomena.

See also: → rocket; → astronomy.

The study of celestial bodies in the wavelengths that are almost completely absorbed by the atmosphere,
by using a rocket to carry instruments above 250 km to measure the searched for phenomena.

See also: → rocket; → astronomy.

The science of rocket design, development, and flight.

Etymology (EN): → rocket + -ry a noun suffix.

Etymology (PE): Roket šenâsi, from roket, → rocket, + šenâsi, → -logy; roketgari with suffix -gari, from -gar, → -or.

The science of rocket design, development, and flight.

Etymology (EN): → rocket + -ry a noun suffix.

Etymology (PE): Roket šenâsi, from roket, → rocket, + šenâsi, → -logy; roketgari with suffix -gari, from -gar, → -or.

A rocket launched from a balloon at a pre-determined height and fired by a ground-controlled radio relay when some particular event, e.g. a solar flare, occurs.

Etymology (EN): From rocket + balloon.

Etymology (PE): From roket + bâlon, → ballon astronomy.

A rocket launched from a balloon at a pre-determined height and fired by a ground-controlled radio relay when some particular event, e.g. a solar flare, occurs.

Etymology (EN): From rocket + balloon.

Etymology (PE): From roket + bâlon, → ballon astronomy.

A time delay caused by the light travel across a → dynamical system. The finite → speed of light causes a delay, for example, between the → primary eclipse and the → secondary eclipse in → binary systems.

See also: Named after Ole Rømer (1664-1710), who discovered the finite speed of light, → Roemer’s measurement; → delay.

A time delay caused by the light travel across a → dynamical system. The finite → speed of light causes a delay, for example, between the → primary eclipse and the → secondary eclipse in → binary systems.

See also: Named after Ole Rømer (1664-1710), who discovered the finite speed of light, → Roemer’s measurement; → delay.

The first successful measurement of the → speed of light carried out by the Danish astronomer Ole Rømer in 1675 at Paris Observatory. Astronomers knew that the mean period of revolution for Jupiter’s innermost satellite → Io (Jupiter I) was 42.5 hours. During this period Io was sometimes eclipsed by Jupiter. Astronomers expected that if Io was visible at some time it must be visible 42.5 hours later. But Ole Rømer discovered that there were many irregularities in Io’s orbital period. Sometimes Io appeared too early and other times too late in relation to the expected times. The irregularities repeated themselves precisely at a one-year interval, which meant that they must be connected to the Earth’s rotation around the Sun. Rømer attributed this difference in time to the additional distance the light from Io had to travel at different times, and used this information to calculate the speed of light. He found that it takes light 22 minutes to traverse the Earth’s orbital diameter; the correct figure was later determined to be 16 minutes and 40 seconds. Rømer was able to measure the speed of light to be 230,000 km s^-1. Although this figure was very close to the currently accepted value of 300,000 km s^-1, it was rejected by the scientific community of the time, who assumed it to be much too high a figure.

See also: Ole Rømer (1664-1710); → measurement.

The first successful measurement of the → speed of light carried out by the Danish astronomer Ole Rømer in 1675 at Paris Observatory. Astronomers knew that the mean period of revolution for Jupiter’s innermost satellite → Io (Jupiter I) was 42.5 hours. During this period Io was sometimes eclipsed by Jupiter. Astronomers expected that if Io was visible at some time it must be visible 42.5 hours later. But Ole Rømer discovered that there were many irregularities in Io’s orbital period. Sometimes Io appeared too early and other times too late in relation to the expected times. The irregularities repeated themselves precisely at a one-year interval, which meant that they must be connected to the Earth’s rotation around the Sun. Rømer attributed this difference in time to the additional distance the light from Io had to travel at different times, and used this information to calculate the speed of light. He found that it takes light 22 minutes to traverse the Earth’s orbital diameter; the correct figure was later determined to be 16 minutes and 40 seconds. Rømer was able to measure the speed of light to be 230,000 km s^-1. Although this figure was very close to the currently accepted value of 300,000 km s^-1, it was rejected by the scientific community of the time, who assumed it to be much too high a figure.

See also: Ole Rømer (1664-1710); → measurement.

A unit of radiation exposure defined as a charge release rate of 258 micro-coulombs per kilogram of air.

See also: Named after the German physicist Wilhelm Konrad Röntgen (1845-1923), one of the early investigators of radioactivity.

A unit of radiation exposure defined as a charge release rate of 258 micro-coulombs per kilogram of air.

See also: Named after the German physicist Wilhelm Konrad Röntgen (1845-1923), one of the early investigators of radioactivity.

An artificially produced radioactive chemical element; symbol Rg. Atomic number 111; mass number of most stable isotope 272; melting point, boiling point, specific gravity,
and valence unknown.

See also: Named after the German physicist Wilhelm Konrad Röntgen (1845-1923), one of the early investigators of radioactivity.

An artificially produced radioactive chemical element; symbol Rg. Atomic number 111; mass number of most stable isotope 272; melting point, boiling point, specific gravity,
and valence unknown.

See also: Named after the German physicist Wilhelm Konrad Röntgen (1845-1923), one of the early investigators of radioactivity.

A vagabond or tramp. A dishonest or unprincipled person.

Etymology (EN): Perhaps short for obsolete roger “begging vagabond.”

Etymology (PE): Velgard “vagabond, roamer, tramp.”

A vagabond or tramp. A dishonest or unprincipled person.

Etymology (EN): Perhaps short for obsolete roger “begging vagabond.”

Etymology (PE): Velgard “vagabond, roamer, tramp.”

Same as → free-floating object.

See also: → rogue; → planet.

Same as → free-floating object.

See also: → rogue; → planet.

A low, horizontal, tube-shaped, and relatively rare type of → arcus cloud.

Etymology (EN): M.E. scroll, inscribed scroll, register, cylindrical object < OF ro(u)lle

M.E. rolle, from O.Fr. roule, rolle, from M.L. rotulus “a roll of paper,” from L. rotula “small wheel,” diminutive of rota “wheel;” → cloud.

Etymology (PE): Abr, → cloud; lule-vâr “tube like,” from lulé “tube, pipe,” related to lulidan “to roll, rotate; to stir, vibrate” + -vâr suffix of resemblance.

A low, horizontal, tube-shaped, and relatively rare type of → arcus cloud.

Etymology (EN): M.E. scroll, inscribed scroll, register, cylindrical object < OF ro(u)lle

M.E. rolle, from O.Fr. roule, rolle, from M.L. rotulus “a roll of paper,” from L. rotula “small wheel,” diminutive of rota “wheel;” → cloud.

Etymology (PE): Abr, → cloud; lule-vâr “tube like,” from lulé “tube, pipe,” related to lulidan “to roll, rotate; to stir, vibrate” + -vâr suffix of resemblance.

If a function f(x) is → continuous on an interval [a,b] and is → differentiable at all points within this interval, and vanishes at the end points x = a and x = b, that is f(a) = f(b) = 0, then inside [a,b] there exists at least one point x = c, a < c < b, at which the derivative f’(x) vanishes.

See also: Named after Michel Rolle (1652-1719), a French mathematician; → theorem.

If a function f(x) is → continuous on an interval [a,b] and is → differentiable at all points within this interval, and vanishes at the end points x = a and x = b, that is f(a) = f(b) = 0, then inside [a,b] there exists at least one point x = c, a < c < b, at which the derivative f’(x) vanishes.

See also: Named after Michel Rolle (1652-1719), a French mathematician; → theorem.

Any of several → lunar calendars used by Romans before the advent of the → Julian calendar in 46 B.C. The original Roman calendar, which had 10 months and 304 days, went back to the Greek calendar, although Romulas, the ruler of Rome, is given credit for starting the Roman calendar. Originally, the Roman calendar started the year in March with the → vernal equinox. The Roman calendar went through several changes from 800 B.C. to the Julian calendar. The 800 B.C. calendar had 10 months and a winter period, with a year of
304 days. In this calendar, the first month, March, was followed by Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, December, and Winter. The months starting with and following Quintilis all used the Latin numbers for names. Finally, for political reasons, the Romans made a change around 150 B.C. when they started using January as the beginning of their calendar year. Around 700 B.C. the 304 day calendar was expanded to 355 days by adding the months of February and January to the end of the year. Later in 450 B.C., January was moved in front of February. Finally, in 150 B.C. the Romans began to use January as the beginning of the calendar year. This calendar was replaced by the Julian calendar in 46 B.C.

See also: From L. Romanus “of Rome, Roman,” from Roma “Rome,” of uncertain origin.

Any of several → lunar calendars used by Romans before the advent of the → Julian calendar in 46 B.C. The original Roman calendar, which had 10 months and 304 days, went back to the Greek calendar, although Romulas, the ruler of Rome, is given credit for starting the Roman calendar. Originally, the Roman calendar started the year in March with the → vernal equinox. The Roman calendar went through several changes from 800 B.C. to the Julian calendar. The 800 B.C. calendar had 10 months and a winter period, with a year of
304 days. In this calendar, the first month, March, was followed by Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, December, and Winter. The months starting with and following Quintilis all used the Latin numbers for names. Finally, for political reasons, the Romans made a change around 150 B.C. when they started using January as the beginning of their calendar year. Around 700 B.C. the 304 day calendar was expanded to 355 days by adding the months of February and January to the end of the year. Later in 450 B.C., January was moved in front of February. Finally, in 150 B.C. the Romans began to use January as the beginning of the calendar year. This calendar was replaced by the Julian calendar in 46 B.C.

See also: From L. Romanus “of Rome, Roman,” from Roma “Rome,” of uncertain origin.

A → number system in which letters represent numbers, still used occasionally today. The cardinal numbers are expressed by the following seven letters: I (1), V (5), X (10), L (50), C (100), D (500), and M (1,000).

If a numeral with smaller value is written on right of greater value then smaller value is added to the greater one. If it is preceded by one of lower value, the smaller numeral is subtracted from the greater. Thus VI = 6 (V + I), but IV = 4 (V - I). Other examples are XC (90), CL (150), XXII (22), XCVII (97), CCCXCV (395). If symbol is repeated then its value is added. The symbols I, X, C and M can be repeated maximum 3 times. A dash line over a numeral multiplies the value by 1,000. For example
V^- = 5000, X^- = 10,000, C^- = 100,000, and DLIX^- = 559,000.

See also: → numeral; → system.

A → number system in which letters represent numbers, still used occasionally today. The cardinal numbers are expressed by the following seven letters: I (1), V (5), X (10), L (50), C (100), D (500), and M (1,000).

If a numeral with smaller value is written on right of greater value then smaller value is added to the greater one. If it is preceded by one of lower value, the smaller numeral is subtracted from the greater. Thus VI = 6 (V + I), but IV = 4 (V - I). Other examples are XC (90), CL (150), XXII (22), XCVII (97), CCCXCV (395). If symbol is repeated then its value is added. The symbols I, X, C and M can be repeated maximum 3 times. A dash line over a numeral multiplies the value by 1,000. For example
V^- = 5000, X^- = 10,000, C^- = 100,000, and DLIX^- = 559,000.

See also: → numeral; → system.

A common Old World gregarious crow (Corvus frugilegus).

Etymology (EN): M.E., from O.E. hrôc; akin to O.H.G. hruoch “crow.”

Etymology (PE): Zâq, from Mid.Pers. zâγ “crow.”

A common Old World gregarious crow (Corvus frugilegus).

Etymology (EN): M.E., from O.E. hrôc; akin to O.H.G. hruoch “crow.”

Etymology (PE): Zâq, from Mid.Pers. zâγ “crow.”

1. Math.: A quantity that, when multiplied by itself a certain number of times, produces a given quantity. For example, since 3 × 3 × 3 × 3 = 81, 3 is a fourth root of 81.
   

2. Math.: A solution to an equation f(x) = 0. We say that x₀ is a root or zero of a → polynomial if f(x₀) = 0. For example, the roots of the equation x² - 9 = 0 are +3 and -3.
   

3. In → graph theory, a special → vertex that turns a → tree into a → rooted tree or a graph into a → rooted graph.

Etymology (EN): From M.E., from O.E. rot, from O.N. rot “root;” cf. O.H.G. wurz “plant, herb;” Ger. Wurz; cognate with L. radix, radius “staff;” Gk. rhiza “root;” Albanian rrânzë “root;” PIE base *u(e)rad- “twig, root.”

Etymology (PE): Rišé “root” (dialectal Tabari rexa; Kurd. regez, riše), from Mid.Pers. rêšak “root,” maybe ultimately related to PIE *u(e)rad-, as above, although the Skt. offshoot is absent.

1. Math.: A quantity that, when multiplied by itself a certain number of times, produces a given quantity. For example, since 3 × 3 × 3 × 3 = 81, 3 is a fourth root of 81.
   

2. Math.: A solution to an equation f(x) = 0. We say that x₀ is a root or zero of a → polynomial if f(x₀) = 0. For example, the roots of the equation x² - 9 = 0 are +3 and -3.
   

3. In → graph theory, a special → vertex that turns a → tree into a → rooted tree or a graph into a → rooted graph.

Etymology (EN): From M.E., from O.E. rot, from O.N. rot “root;” cf. O.H.G. wurz “plant, herb;” Ger. Wurz; cognate with L. radix, radius “staff;” Gk. rhiza “root;” Albanian rrânzë “root;” PIE base *u(e)rad- “twig, root.”

Etymology (PE): Rišé “root” (dialectal Tabari rexa; Kurd. regez, riše), from Mid.Pers. rêšak “root,” maybe ultimately related to PIE *u(e)rad-, as above, although the Skt. offshoot is absent.

The square root of the arithmetic mean of the squares of the numbers in a given set.

See also: → root; → mean; → square.

The square root of the arithmetic mean of the squares of the numbers in a given set.

See also: → root; → mean; → square.

The square root of the second moment corresponding to the frequency function of a random variable.

See also: → root; → mean; → square; → error.

The square root of the second moment corresponding to the frequency function of a random variable.

See also: → root; → mean; → square; → error.

Statistics: The square root of the arithmetic mean of the squares of the deviation of observed values from their arithmetic mean.

See also: → root; → mean; → square;
→ deviation.

Statistics: The square root of the arithmetic mean of the squares of the deviation of observed values from their arithmetic mean.

See also: → root; → mean; → square;
→ deviation.

In → graph theory, a → graph that has one of its → vertices, called the → root, distinguished from the others.

See also: → root; → graph.

In → graph theory, a → graph that has one of its → vertices, called the → root, distinguished from the others.

See also: → root; → graph.

In → graph theory, a → tree in which one → vertex is distinguished from the other vertices and is called the root.

See also: → root; → tree.

In → graph theory, a → tree in which one → vertex is distinguished from the other vertices and is called the root.

See also: → root; → tree.

A German X-ray satellite developed through a cooperative program with the United States and the United Kingdom. The satellite,
launched by a Delta rocket (Cape Canaveral) on June 1, 1990, operated until February 12, 1999. ROSAT consisted of two telescopes performing in the → soft X-ray (0.1-2.4 keV) and → extreme ultraviolet (EUV) (006-0.2 keV) ranges. It carried out the first → all-sky surveys with imaging X-ray and EUV telescopes leading to the discovery of 125,000 X-ray and 479 EUV sources. In addition the diffuse Galactic X-ray emission was mapped with unprecedented angular resolution (≤ 1 arcmin). Most of the mission time was devoted to pointed observations at selected targets. ROSAT imaged everything from nearby asteroids and comets to distant quasars during its 8-year mission. The main ROSAT data centers were and are at the Max Planck Institute for Extraterrestrial Physics in Garching (X-rays) and at the University of Leicester (EUV) with mirror sites at the Goddard Space Flight Center and other research institutes.

See also: ROSAT, short for the → ROentgen→ SATellite, in honor of the German physicist.

A German X-ray satellite developed through a cooperative program with the United States and the United Kingdom. The satellite,
launched by a Delta rocket (Cape Canaveral) on June 1, 1990, operated until February 12, 1999. ROSAT consisted of two telescopes performing in the → soft X-ray (0.1-2.4 keV) and → extreme ultraviolet (EUV) (006-0.2 keV) ranges. It carried out the first → all-sky surveys with imaging X-ray and EUV telescopes leading to the discovery of 125,000 X-ray and 479 EUV sources. In addition the diffuse Galactic X-ray emission was mapped with unprecedented angular resolution (≤ 1 arcmin). Most of the mission time was devoted to pointed observations at selected targets. ROSAT imaged everything from nearby asteroids and comets to distant quasars during its 8-year mission. The main ROSAT data centers were and are at the Max Planck Institute for Extraterrestrial Physics in Garching (X-rays) and at the University of Leicester (EUV) with mirror sites at the Goddard Space Flight Center and other research institutes.

See also: ROSAT, short for the → ROentgen→ SATellite, in honor of the German physicist.

A spacecraft launched in March 2004 by the → European Space Agency to be the first man-made object to orbit a → comet’s → nucleus. Rosetta will also be the first spacecraft to fly alongside a comet as it heads toward → perihelion in the inner → solar system. After a ten-year voyage across the solar system, it will reach a → periodic comet known as Comet 67P/ → Churyumov-Gerasimenko. Rosetta will remain in close proximity to the icy nucleus as it plunges toward the warmer inner reaches of the Sun’s realm. Rosetta orbiter’s scientific payload includes 11 different instruments, in addition to a robotic lander and 10 solar panels spanning 32 m tip to tip. In November 2014, Rosetta will launch the 100 kg lander, named Philae, onto the comet. Philae will touch down and then fire a harpoon to anchor itself and prevent it from escaping the comet’s weak gravity. The lander carries 10 instruments, including a drill to take samples of subsurface material. More than a year will pass before the remarkable mission comes to an end in December 2015. By then, both the spacecraft and the comet will have circled the Sun and will be on their way out of the inner solar system. Rosetta’s prime objective is to help understand the origin and evolution of the solar system. The comet’s composition reflects the composition of the pre-solar nebula out of which the Sun and the planets of the solar system formed, more than 4.6 billion years ago. Therefore, an in-depth analysis of comet 67P/Churyumov-Gerasimenko by Rosetta and its lander will provide essential information to understand how the solar system formed. Before arriving at 67P/Churyumov-Gerasimenk, Rosetta flew by the → asteroids 2867 → Steins and 21 → Lutetia in 2008 and 2010, respectively, and gathered data on them.

See also: Named for the Rosetta Stone, a black stele that was inscribed with a royal decree (196 BC) in two languages using three scripts: Egyptian hieroglyphics, Egyptian Demotic, and Greek. The Rosetta Stone was found in a small village in the Nile Delta called Rashid (Rosetta) in 1799. The spacecraft’s robotic lander is called Philae, after a similarly inscribed obelisk found on an island in the Nile River. Both the stone and the obelisk were key to deciphering ancient Egyptian hieroglyphs, carried out by Jean-François Champollion (1790-1832) in 1822. Astronomers hope the Rosetta mission will provide a key to many questions about the origins of the solar system.

A spacecraft launched in March 2004 by the → European Space Agency to be the first man-made object to orbit a → comet’s → nucleus. Rosetta will also be the first spacecraft to fly alongside a comet as it heads toward → perihelion in the inner → solar system. After a ten-year voyage across the solar system, it will reach a → periodic comet known as Comet 67P/ → Churyumov-Gerasimenko. Rosetta will remain in close proximity to the icy nucleus as it plunges toward the warmer inner reaches of the Sun’s realm. Rosetta orbiter’s scientific payload includes 11 different instruments, in addition to a robotic lander and 10 solar panels spanning 32 m tip to tip. In November 2014, Rosetta will launch the 100 kg lander, named Philae, onto the comet. Philae will touch down and then fire a harpoon to anchor itself and prevent it from escaping the comet’s weak gravity. The lander carries 10 instruments, including a drill to take samples of subsurface material. More than a year will pass before the remarkable mission comes to an end in December 2015. By then, both the spacecraft and the comet will have circled the Sun and will be on their way out of the inner solar system. Rosetta’s prime objective is to help understand the origin and evolution of the solar system. The comet’s composition reflects the composition of the pre-solar nebula out of which the Sun and the planets of the solar system formed, more than 4.6 billion years ago. Therefore, an in-depth analysis of comet 67P/Churyumov-Gerasimenko by Rosetta and its lander will provide essential information to understand how the solar system formed. Before arriving at 67P/Churyumov-Gerasimenk, Rosetta flew by the → asteroids 2867 → Steins and 21 → Lutetia in 2008 and 2010, respectively, and gathered data on them.

See also: Named for the Rosetta Stone, a black stele that was inscribed with a royal decree (196 BC) in two languages using three scripts: Egyptian hieroglyphics, Egyptian Demotic, and Greek. The Rosetta Stone was found in a small village in the Nile Delta called Rashid (Rosetta) in 1799. The spacecraft’s robotic lander is called Philae, after a similarly inscribed obelisk found on an island in the Nile River. Both the stone and the obelisk were key to deciphering ancient Egyptian hieroglyphs, carried out by Jean-François Champollion (1790-1832) in 1822. Astronomers hope the Rosetta mission will provide a key to many questions about the origins of the solar system.

A giant H II region of about 1° in diameter, lying about 5000 light-years away in the Milky Way, the constellation → Monoceros. It is ionized by the cluster NGC 2244, a group of hot young stars at the center of the nebula. Also called M16, the brighter portions of the nebula have been assigned different NGC numbers: 2237, 2238, 2239, and 2246.

Etymology (EN): Rosette “a rose-shaped ornament,” from Fr. rosette, from O.Fr. rosette, diminutive of rose “rose;” L. rosa, probably from
Gk. wrodon (Aeolic), then rhodon, a loan from Iranian, as below; → nebula.

Etymology (PE): Miq, → nebula; golsân “resembling rose, flower,” from gol “flower, rose,” variants vard (sohre-vard “red rose”), Semnâni dialect vela “rose;”
Mid.Pers. *vard, gul, loaned in Arm. vard and Ar. ward; Av. varəδa- “rose;” loaned in Gk. wrodon (Aeolic), then rhodon; + -sân “manner, semblance” (variant sun, Mid.Pers. sân “manner, kind,” Sogdian šôné “career”).

A giant H II region of about 1° in diameter, lying about 5000 light-years away in the Milky Way, the constellation → Monoceros. It is ionized by the cluster NGC 2244, a group of hot young stars at the center of the nebula. Also called M16, the brighter portions of the nebula have been assigned different NGC numbers: 2237, 2238, 2239, and 2246.

Etymology (EN): Rosette “a rose-shaped ornament,” from Fr. rosette, from O.Fr. rosette, diminutive of rose “rose;” L. rosa, probably from
Gk. wrodon (Aeolic), then rhodon, a loan from Iranian, as below; → nebula.

Etymology (PE): Miq, → nebula; golsân “resembling rose, flower,” from gol “flower, rose,” variants vard (sohre-vard “red rose”), Semnâni dialect vela “rose;”
Mid.Pers. *vard, gul, loaned in Arm. vard and Ar. ward; Av. varəδa- “rose;” loaned in Gk. wrodon (Aeolic), then rhodon; + -sân “manner, semblance” (variant sun, Mid.Pers. sân “manner, kind,” Sogdian šôné “career”).

A → red dwarf star of → spectral type M4. Other designations: Proxima Virginis, FY Virginis, GJ 447, HIP 57548, and LHS 315. With a distance of just 3.4 → parsecs, it is one of the brightest representatives of this subclass (V = 11.15, J = 6.51, H =5.95, K = 5.65 mag). It is the 13th closest (sub-)stellar system to the Sun, including → brown dwarfs. Ross 128 is moving toward us and will actually become our closest neighbor in just 71,000 years from now. Ross 128 has an → effective temperature,

T_eff = 3192, a mass of 0.168 Msun (→ solar mass), a → luminosity of 0.00362 Lsun (→ solar luminosity), a radius of 0.017 Rsun (→ solar radius), and a → metallicity [Fe/H] of -0.02.

An Earth-sized → exoplanet, → R 128 b, orbits Ross 128 (Bonfils et al., 2017, arXiv:1711.06177).

See also: Star number 128 in the → Ross Catalogue.

A → red dwarf star of → spectral type M4. Other designations: Proxima Virginis, FY Virginis, GJ 447, HIP 57548, and LHS 315. With a distance of just 3.4 → parsecs, it is one of the brightest representatives of this subclass (V = 11.15, J = 6.51, H =5.95, K = 5.65 mag). It is the 13th closest (sub-)stellar system to the Sun, including → brown dwarfs. Ross 128 is moving toward us and will actually become our closest neighbor in just 71,000 years from now. Ross 128 has an → effective temperature,

T_eff = 3192, a mass of 0.168 Msun (→ solar mass), a → luminosity of 0.00362 Lsun (→ solar luminosity), a radius of 0.017 Rsun (→ solar radius), and a → metallicity [Fe/H] of -0.02.

An Earth-sized → exoplanet, → R 128 b, orbits Ross 128 (Bonfils et al., 2017, arXiv:1711.06177).

See also: Star number 128 in the → Ross Catalogue.

An → extrasolar planet around the → red dwarf star → R 128.

The → exoplanet orbits its star every 9.9 days. This Earth-sized world is expected to be temperate, with a surface temperature that may also be close to that of the Earth.

Many red dwarf stars, including → Proxima Centauri, are subject to → flares that occasionally bathe their orbiting planets in deadly → ultraviolet and → X-ray radiation. However, it seems that Ross 128 is a much quieter star, and so its planets may be the closest known comfortable abode for possible life.

Ross 128 b orbits 20 times closer than the Earth orbits the Sun. Despite this proximity, it receives only 1.38 times more irradiation than the Earth. As a result, Ross 128 b’s equilibrium temperature is estimated to lie between -60 and 20°C, thanks to the cool and faint nature of its small red dwarf host star, which has just over half the surface temperature of the Sun.

See also: The letter b, designates the first exoplanet discovered around → R 128.

An → extrasolar planet around the → red dwarf star → R 128.

The → exoplanet orbits its star every 9.9 days. This Earth-sized world is expected to be temperate, with a surface temperature that may also be close to that of the Earth.

Many red dwarf stars, including → Proxima Centauri, are subject to → flares that occasionally bathe their orbiting planets in deadly → ultraviolet and → X-ray radiation. However, it seems that Ross 128 is a much quieter star, and so its planets may be the closest known comfortable abode for possible life.

Ross 128 b orbits 20 times closer than the Earth orbits the Sun. Despite this proximity, it receives only 1.38 times more irradiation than the Earth. As a result, Ross 128 b’s equilibrium temperature is estimated to lie between -60 and 20°C, thanks to the cool and faint nature of its small red dwarf host star, which has just over half the surface temperature of the Sun.

See also: The letter b, designates the first exoplanet discovered around → R 128.

Ross, Frank E., 1926, “New proper-motion stars, (second list)”, Astronomical Journal 36, 856.

See also: Frank Elmore Ross (1874-1960) was the succeeded to E. E. Barnard at Yerkes Observatory. He inheriting Barnard’s collection of photographic plates. Ross decided to repeat the same series of images and compare the results with a → blink comparator. He discovered 379 new variable stars and over 1000 stars of high proper motion.

Ross, Frank E., 1926, “New proper-motion stars, (second list)”, Astronomical Journal 36, 856.

See also: Frank Elmore Ross (1874-1960) was the succeeded to E. E. Barnard at Yerkes Observatory. He inheriting Barnard’s collection of photographic plates. Ross decided to repeat the same series of images and compare the results with a → blink comparator. He discovered 379 new variable stars and over 1000 stars of high proper motion.

A dimensionless number relating the ratio of inertial to Coriolis forces for a given flow of a rotating fluid. It is used in the study of atmospheric motions in planets. In case a small number is involved, cyclones and anticyclones are observed for low and high
pressures. When it is large (Venus) the Coriolis force becomes
negligible and atmospheric motions are barely affected by planetary
rotation.

See also: Named after Carl-Gustav Arvid Rossby (1898-1957), a Swedish-American meteorologist who first explained the large-scale motions of the atmosphere in terms of fluid mechanics; → number.

A dimensionless number relating the ratio of inertial to Coriolis forces for a given flow of a rotating fluid. It is used in the study of atmospheric motions in planets. In case a small number is involved, cyclones and anticyclones are observed for low and high
pressures. When it is large (Venus) the Coriolis force becomes
negligible and atmospheric motions are barely affected by planetary
rotation.

See also: Named after Carl-Gustav Arvid Rossby (1898-1957), a Swedish-American meteorologist who first explained the large-scale motions of the atmosphere in terms of fluid mechanics; → number.

The northward variation of the Coriolis parameter, arising from the sphericity of the Earth.

See also: → Rossby number; → parameter.

The northward variation of the Coriolis parameter, arising from the sphericity of the Earth.

See also: → Rossby number; → parameter.

A wave on a uniform current in a two-dimensional non-divergent fluid system, rotating with varying angular speed about the local vertical.

See also: → Rossby number; → wave.

A wave on a uniform current in a two-dimensional non-divergent fluid system, rotating with varying angular speed about the local vertical.

See also: → Rossby number; → wave.

The → opacity of a gas of given composition, temperature, and density averaged over the various wavelengths of the radiation being absorbed and scattered. The radiation is assumed to be in → thermal equilibrium with the gas, and hence have a → blackbody spectrum. Since → monochromatic opacity in stellar plasma has a complex frequency dependence,
the Rosseland mean opacity facilitates the analysis. Denoted κ_R, it is defined by: 1/κ_R = (π/4σT³) ∫(1/k_ν) (∂B/∂T)_νdν, summed from 0 to ∞, where σ is the → Stefan-Boltzmann constant, T temperature, B(T,ν) the → Planck function, and k_ν monochromatic opacity (See Rogers, F.J., Iglesias, C. A. Radiative atomic Rosseland mean opacity tables, 1992, ApJS 79, 507).

See also: Named after Svein Rosseland (1894-1985), a Norwegian astrophysicist, who obtained the expression in 1924; → mean; → opacity.

The → opacity of a gas of given composition, temperature, and density averaged over the various wavelengths of the radiation being absorbed and scattered. The radiation is assumed to be in → thermal equilibrium with the gas, and hence have a → blackbody spectrum. Since → monochromatic opacity in stellar plasma has a complex frequency dependence,
the Rosseland mean opacity facilitates the analysis. Denoted κ_R, it is defined by: 1/κ_R = (π/4σT³) ∫(1/k_ν) (∂B/∂T)_νdν, summed from 0 to ∞, where σ is the → Stefan-Boltzmann constant, T temperature, B(T,ν) the → Planck function, and k_ν monochromatic opacity (See Rogers, F.J., Iglesias, C. A. Radiative atomic Rosseland mean opacity tables, 1992, ApJS 79, 507).

See also: Named after Svein Rosseland (1894-1985), a Norwegian astrophysicist, who obtained the expression in 1924; → mean; → opacity.

A → spectroscopic phenomenon observed when either an → eclipsing binary’s → secondary star or an → extrasolar planet is seen to
→ transit across the face of the → primary body. Because of the rotation of the star, an asymmetric distortion takes place in the → line profiles of the stellar spectrum, which changes during the transit. The measurement of this effect can be used to derive the → alignment of the → orbit of the transiting exoplanet with respect to the → rotation axis of the star.

See also: Named after Richard Alfred Rossiter (1886-1977) and Dean Benjamin McLaughlin (1901-1965), American astronomers.

A → spectroscopic phenomenon observed when either an → eclipsing binary’s → secondary star or an → extrasolar planet is seen to
→ transit across the face of the → primary body. Because of the rotation of the star, an asymmetric distortion takes place in the → line profiles of the stellar spectrum, which changes during the transit. The measurement of this effect can be used to derive the → alignment of the → orbit of the transiting exoplanet with respect to the → rotation axis of the star.

See also: Named after Richard Alfred Rossiter (1886-1977) and Dean Benjamin McLaughlin (1901-1965), American astronomers.

To turn around an axis. See also → revolve.

Etymology (EN): From L. rotare “to cause to spin, roll, move in a circle,” from L. rota “wheel;” cognate with Pers. râh “way, path” (from Mid.Pers. râh, râs “way, street,” also rah, ras “chariot;” from Proto-Iranian *rāθa-; cf. Av. raθa- “chariot;” Skt. rátha- “car, chariot,” rathyā- “road;” Lith. ratas “wheel;” O.H.G. rad; Ger. Rad; Du. rad;
O.Ir. roth; PIE *roto- “to run, to turn, to roll”).

Etymology (PE): Carxidan “to rotate,” from carx “every thing performing a circulatory motion; a wheel; a cart;” Mid.Pers. chr “wheel,” Parthian cxr “wheel;” Ossetic calx “wheel;” Av. caxra- “wheel;” cognate with Skt. cakra- “wheel, circle; cycle,” carati “he moves, wanders;” Gk. kyklos “circle, wheel,” polos “axis of a sphere,” polein “move around;” L. colere “to dwell in, to cultivate, move around,” colonus “farmer, settler;” O.E. hweol “wheel;” Rus. koleso “wheel.”

To turn around an axis. See also → revolve.

Etymology (EN): From L. rotare “to cause to spin, roll, move in a circle,” from L. rota “wheel;” cognate with Pers. râh “way, path” (from Mid.Pers. râh, râs “way, street,” also rah, ras “chariot;” from Proto-Iranian *rāθa-; cf. Av. raθa- “chariot;” Skt. rátha- “car, chariot,” rathyā- “road;” Lith. ratas “wheel;” O.H.G. rad; Ger. Rad; Du. rad;
O.Ir. roth; PIE *roto- “to run, to turn, to roll”).

Etymology (PE): Carxidan “to rotate,” from carx “every thing performing a circulatory motion; a wheel; a cart;” Mid.Pers. chr “wheel,” Parthian cxr “wheel;” Ossetic calx “wheel;” Av. caxra- “wheel;” cognate with Skt. cakra- “wheel, circle; cycle,” carati “he moves, wanders;” Gk. kyklos “circle, wheel,” polos “axis of a sphere,” polein “move around;” L. colere “to dwell in, to cultivate, move around,” colonus “farmer, settler;” O.E. hweol “wheel;” Rus. koleso “wheel.”

Capable of or having rotation.

See also: From → rotate + → -ing.

Capable of or having rotation.

See also: From → rotate + → -ing.

A black hole that possesses angular momentum, as first postulated by Roy C. Kerr in 1963. Opposite of a stationary black hole. → ergosphere.

See also: → rotating; → black hole.

A black hole that possesses angular momentum, as first postulated by Roy C. Kerr in 1963. Opposite of a stationary black hole. → ergosphere.

See also: → rotating; → black hole.

A star that has a non-zero → angular velocity. In a rotating star, the → centrifugal forces reduce the → effective gravity according to the latitude and also introduce deviations from sphericity. In a rotating star, the equations of stellar structure need to be modified. The usual spherical coordinates must be replaced by new coordinates characterizing the → equipotentials.
See also → von Zeipel theorem.

See also: → rotating; → star.

A star that has a non-zero → angular velocity. In a rotating star, the → centrifugal forces reduce the → effective gravity according to the latitude and also introduce deviations from sphericity. In a rotating star, the equations of stellar structure need to be modified. The usual spherical coordinates must be replaced by new coordinates characterizing the → equipotentials.
See also → von Zeipel theorem.

See also: → rotating; → star.

The motion of a body about its axis.

See also: Verbal noun of → rotate.

The motion of a body about its axis.

See also: Verbal noun of → rotate.

The imaginary line around which an object rotates. Same as → rotational axis and → axis of rotation.

See also: → rotation; → axis.

The imaginary line around which an object rotates. Same as → rotational axis and → axis of rotation.

See also: → rotation; → axis.

A plot of the variation in → orbital velocity of stars and → interstellar matter
with distance from the center of a → galaxy. A “flat” rotation curve indicates that the mass of the galaxy increases linearly with distance from its center. See also: farsi→ Keplerian rotation curve

See also: Rotation; → curve.

A plot of the variation in → orbital velocity of stars and → interstellar matter
with distance from the center of a → galaxy. A “flat” rotation curve indicates that the mass of the galaxy increases linearly with distance from its center. See also: farsi→ Keplerian rotation curve

See also: Rotation; → curve.

The → kinetic energy of rotational motion of an object. It is expressed by E_R = (1/2)Iω², where I is the → moment of inertia and
ω → angular velocity (2π/P).

See also: → rotation; → energy.

The → kinetic energy of rotational motion of an object. It is expressed by E_R = (1/2)Iω², where I is the → moment of inertia and
ω → angular velocity (2π/P).

See also: → rotation; → energy.

1. The number of rotations per unit time of a rotating object.
   

2. The number of → stellar rotations per unit time. The reciprocal of the → rotation period. This parameter usually refers to the equator of the star, because stars do not rotate as solid bodies.

See also: → rotation; → frequency.

1. The number of rotations per unit time of a rotating object.
   

2. The number of → stellar rotations per unit time. The reciprocal of the → rotation period. This parameter usually refers to the equator of the star, because stars do not rotate as solid bodies.

See also: → rotation; → frequency.

The interval of time during which an object turns once about its axis.

See also: → rotation; → period.

The interval of time during which an object turns once about its axis.

See also: → rotation; → period.

A position parameter used in → stellar magnetic field studies. Its zero value represents the moment when, during → stellar rotation, the positive → magnetic pole is nearest to the → line of sight.

See also: → rotation; → phase.

A position parameter used in → stellar magnetic field studies. Its zero value represents the moment when, during → stellar rotation, the positive → magnetic pole is nearest to the → line of sight.

See also: → rotation; → phase.

A type of → turbulence with motions more vigorous in the horizontal than in the vertical direction occurring in internal radiation zone of → rotating stars. Same as → shear turbulence.

See also: → rotation; → induced; → turbulence.

A type of → turbulence with motions more vigorous in the horizontal than in the vertical direction occurring in internal radiation zone of → rotating stars. Same as → shear turbulence.

See also: → rotation; → induced; → turbulence.

A → neutron star that is spinning down as a result of → torques from → magnetic dipole radiation and particle emission. RPPs derive their energy primarily from the → rotation of the neutron star. The energy from their → spin-down appears as broad-band pulsations from → radio to → gamma-ray wavelengths and as a → wind of energetic particles flowing into their surrounding → pulsar wind nebulae. Since the discovery of RPPs through their radio → pulsations in 1967, more than 2000 → radio pulsars are now known with periods ranging from a few milliseconds to several seconds (A. K. Harding, 2013, Front. Phys. 8, 679).

See also: → rotation; → power; → pulsar.

A → neutron star that is spinning down as a result of → torques from → magnetic dipole radiation and particle emission. RPPs derive their energy primarily from the → rotation of the neutron star. The energy from their → spin-down appears as broad-band pulsations from → radio to → gamma-ray wavelengths and as a → wind of energetic particles flowing into their surrounding → pulsar wind nebulae. Since the discovery of RPPs through their radio → pulsations in 1967, more than 2000 → radio pulsars are now known with periods ranging from a few milliseconds to several seconds (A. K. Harding, 2013, Front. Phys. 8, 679).

See also: → rotation; → power; → pulsar.

The spectrum of a molecule resulting from the simultaneous rotation and vibration of its constituent atoms.

See also: → rotation; → vibration;
→ spectrum.

The spectrum of a molecule resulting from the simultaneous rotation and vibration of its constituent atoms.

See also: → rotation; → vibration;
→ spectrum.

Of or pertaining to → rotation.

See also: Rotational, adj., from → rotation + → -al.

Of or pertaining to → rotation.

See also: Rotational, adj., from → rotation + → -al.

The → angular momentum of a body rotating about an axis. The rotational angular momentum of a solid homogeneous sphere of mass M and radius R rotating about an axis passing through its center with a period of T is given by:
L = 4πMR²/5T.

See also: → rotational; → angular; → momentum.

The → angular momentum of a body rotating about an axis. The rotational angular momentum of a solid homogeneous sphere of mass M and radius R rotating about an axis passing through its center with a period of T is given by:
L = 4πMR²/5T.

See also: → rotational; → angular; → momentum.

An imaginary line about which a solid object rotates. Same as → rotation axis and → axis of rotation.

See also: → rotational; → axis.

An imaginary line about which a solid object rotates. Same as → rotation axis and → axis of rotation.

See also: → rotational; → axis.

The spectral line broadening caused by stellar rotation. Light from two rims of the star will be Doppler shifted in opposite directions, resulting in a line broadening effect. The line broadening depends on the inclination of the star’s pole to the line of sight. The derived value is a function of v_e. sini, where v_e is the rotational velocity at the equator and i is the inclination, which is not always known. The fractional width (Δλ/λ) is of the order of 10^-3 for B stars.

See also: → rotational; → broadening.

The spectral line broadening caused by stellar rotation. Light from two rims of the star will be Doppler shifted in opposite directions, resulting in a line broadening effect. The line broadening depends on the inclination of the star’s pole to the line of sight. The derived value is a function of v_e. sini, where v_e is the rotational velocity at the equator and i is the inclination, which is not always known. The fractional width (Δλ/λ) is of the order of 10^-3 for B stars.

See also: → rotational; → broadening.

The → Eddington limit of luminosity for a → rotating star in which both the effects of → radiative acceleration and rotation are important. Such objects mainly include → OB stars, → LBV, → supergiants, and → Wolf-Rayet stars. It turns out that the maximum permitted luminosity of a star is reduced by rotation, with respect to the usual Eddington limit (Maeder & Meynet, 2000, A&A, 361, 159).

See also: → rotational; → Eddington limit.

The → Eddington limit of luminosity for a → rotating star in which both the effects of → radiative acceleration and rotation are important. Such objects mainly include → OB stars, → LBV, → supergiants, and → Wolf-Rayet stars. It turns out that the maximum permitted luminosity of a star is reduced by rotation, with respect to the usual Eddington limit (Maeder & Meynet, 2000, A&A, 361, 159).

See also: → rotational; → Eddington limit.

The → kinetic energy due to the → rotation of and object. Rotational energy is part of the total kinetic energy of the body. It is given by: (1/2)Iω², where I is the
→ moment of inertia and ω is the → angular velocity. Same as → angular kinetic energy.

See also: → rotational; → energy.

The → kinetic energy due to the → rotation of and object. Rotational energy is part of the total kinetic energy of the body. It is given by: (1/2)Iω², where I is the
→ moment of inertia and ω is the → angular velocity. Same as → angular kinetic energy.

See also: → rotational; → energy.

A consequence of → stellar rotation that deforms the star, triggers instabilities (→ shear turbulence and → meridional currents) leading to → transport of chemical species in the star. The efficiency of rotational mixing (measured for instance by the degree of surface → enrichments at a given → evolutionary stage) increases when the initial mass and rotation increase. This efficiency increases also when the initial → metallicity decreases. This is due to the fact that when the metallicity is lower, the stars are more compact. This makes the → gradients of the → angular velocity steeper in the stellar interiors. Steeper gradients produce stronger shear turbulence and thus more mixing. Rotational mixing can bring to the surface heavy elements newly synthesized in the stellar core. Rotation thus produces an increase of the → opacity of the outer layers and activates strong → mass loss through → radiatively driven winds. This effect may be responsible for the loss of large fractions of the initial mass of the star (Meynet et al. 2007, arXiv:0709.2275).

See also: → rotational; → mixing.

A consequence of → stellar rotation that deforms the star, triggers instabilities (→ shear turbulence and → meridional currents) leading to → transport of chemical species in the star. The efficiency of rotational mixing (measured for instance by the degree of surface → enrichments at a given → evolutionary stage) increases when the initial mass and rotation increase. This efficiency increases also when the initial → metallicity decreases. This is due to the fact that when the metallicity is lower, the stars are more compact. This makes the → gradients of the → angular velocity steeper in the stellar interiors. Steeper gradients produce stronger shear turbulence and thus more mixing. Rotational mixing can bring to the surface heavy elements newly synthesized in the stellar core. Rotation thus produces an increase of the → opacity of the outer layers and activates strong → mass loss through → radiatively driven winds. This effect may be responsible for the loss of large fractions of the initial mass of the star (Meynet et al. 2007, arXiv:0709.2275).

See also: → rotational; → mixing.

A very small variation in the surface brightness of a single star due to its rotation. Several types of stars are known to have photospheric spots. Brightness variation occurs as rotation carries star spots or other localized activity across the line of sight.

See also: → rotational; → modulation.

A very small variation in the surface brightness of a single star due to its rotation. Several types of stars are known to have photospheric spots. Brightness variation occurs as rotation carries star spots or other localized activity across the line of sight.

See also: → rotational; → modulation.

Of a → rigid body, a motion in which there are always two points of the body which remain motionless.

See also: → rotational; → motion.

Of a → rigid body, a motion in which there are always two points of the body which remain motionless.

See also: → rotational; → motion.

→ rotation period.

See also: → rotational; → period.

→ rotation period.

See also: → rotational; → period.

A slight change in the energy level of a molecule due to the rotation of its constituent atoms about their center of mass.

See also: → rotational; → transition.

A slight change in the energy level of a molecule due to the rotation of its constituent atoms about their center of mass.

See also: → rotational; → transition.

The velocity of a → rotational motion; same as → angular velocity.

See also: → rotational; → velocity.

The velocity of a → rotational motion; same as → angular velocity.

See also: → rotational; → velocity.

A device that rotates or causes rotation.

See also: Agent noun from → rotate.

A device that rotates or causes rotation.

See also: Agent noun from → rotate.

A rotating part of an electrical apparatus, e.g. the armature of a generator, or of a mechanical device.

A system of several flat blades attached to a hub, which rotates either horizontally to give lift and thrust to a helicopter, or vertically to help control it.
Meteo.: The circulation of flow about a horizontal or nearly horizontal axis.

Etymology (EN): Short for rotator, → rotate, + → -or.

Etymology (PE): Carxâ agent noun of carxidan, → rotate.

A rotating part of an electrical apparatus, e.g. the armature of a generator, or of a mechanical device.

A system of several flat blades attached to a hub, which rotates either horizontally to give lift and thrust to a helicopter, or vertically to help control it.
Meteo.: The circulation of flow about a horizontal or nearly horizontal axis.

Etymology (EN): Short for rotator, → rotate, + → -or.

Etymology (PE): Carxâ agent noun of carxidan, → rotate.

CCD detector: Series of pixels arranged along a line. → column

Etymology (EN): O.E. ræw “a row, line;” cf. Du. rij “row;” O.H.G. rihan “to thread,” riga “line;” Ger. Reihe “row, line, series.”

Etymology (PE): Raj “line, row,” variants raž, rak, râk, rezg (Lori), ris, risé, radé, rasté, râsté, related to râst “right, true; just, upright, straight;” Mid.Pers. râst “true, straight, direct;” Soghdian rəšt “right;” O.Pers. rāsta- “straight, true,” rās- “to be right, straight, true;” Av. rāz- “to direct, put in line, set,” razan- “order;”
cf. Skt. raj- “to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” PIE base *reg- “move in a straight line,” hence, “to direct, rule.”

CCD detector: Series of pixels arranged along a line. → column

Etymology (EN): O.E. ræw “a row, line;” cf. Du. rij “row;” O.H.G. rihan “to thread,” riga “line;” Ger. Reihe “row, line, series.”

Etymology (PE): Raj “line, row,” variants raž, rak, râk, rezg (Lori), ris, risé, radé, rasté, râsté, related to râst “right, true; just, upright, straight;” Mid.Pers. râst “true, straight, direct;” Soghdian rəšt “right;” O.Pers. rāsta- “straight, true,” rās- “to be right, straight, true;” Av. rāz- “to direct, put in line, set,” razan- “order;”
cf. Skt. raj- “to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” PIE base *reg- “move in a straight line,” hence, “to direct, rule.”

An integral constraint used to quantify the uncertainty on the extent of → convective overshooting and its effect on models of stars.

See also: Roxburgh, I. 1989, A&A, 211, 361; → criterion.

An integral constraint used to quantify the uncertainty on the extent of → convective overshooting and its effect on models of stars.

See also: Roxburgh, I. 1989, A&A, 211, 361; → criterion.