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	Encyclopedia topic: RR Lyrae variable (Site not responding. Last check: 2007-10-22)
		RR Lyrae variables are variable star (A star that varies noticeably in brightness) s often used as standard candle (The basic unit of luminous intensity adopted under the Systeme International d'Unites; equal to 1/60 of the luminous intensity per square centimeter of a fl body radiating at the temperature of 2,046 degrees Kelvin) s.
		An RR Lyrae is a pulsating Horizontal_branch (additional info and facts about Horizontal_branch) star, with a mass about half of our Sun (A typical star that is the source of light and heat for the planets in the solar system).
		This type of variable is named after the prototype, the variable RR in the Lyra (A small constellation in the northern hemisphere near Cygnus and Draco; contains the star Vega) constellation.
	www.absoluteastronomy.com /encyclopedia/r/rr/rr_lyrae_variable1.htm (270 words)

	RR Lyrae - Wikipedia, the free encyclopedia
		RR Lyrae is a variable star in the Lyra constellation.
		It is the prototype of the RR Lyrae variables star class.
		As RR Lyrae variables are important standard candles, knowing the precise distance of this star is necessary to determine its luminosity, and hence that of other stars in its class.
	en.wikipedia.org /wiki/RR_Lyrae (173 words)

	RR Lyrae variable - Wikipedia, the free encyclopedia (Site not responding. Last check: 2007-10-22)
		RR Lyrae variables are variable stars often used as standard candles.
		RR Lyrae are pulsating Horizontal branch stars, with a mass of around half the Sun's.
		It should be noted that RR Lyrae stars shed mass prior to becoming RR Lyrae and consequently, RR Lyrae were once stars with similar or slightly greater mass than the sun.
	en.wikipedia.org /wiki/RR_Lyrae_variable (271 words)

	Encyclopedia: RR Lyrae variable (Site not responding. Last check: 2007-10-22)
		A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity.
		Lyra (Latin for Lyre) is one of the 48 constellations listed by Ptolemy, and is one of the 88 modern constellations recognized by the International Astronomical Union.
		RR Lyrae variables RR Lyrae is a variable star in the Lyra constellation.
	www.nationmaster.com /encyclopedia/RR-Lyrae-variable (576 words)

	RR Lyrae star
		A short-period, yellow or white giant pulsating variable; RR Lyrae stars belong to Population II and are often found globular clusters (hence one of their older names of cluster variables) or elsewhere in the galactic halo.
		RR Lyrae variables, however, are older, less massive, and fainter (with luminosities typical around 45 Lsun) than Cepheids.
		The presence of scatter and large amplitude variations in the light curves are often the signature of double-mode pulsation and the Blazhko effect in some of these variables.
	www.daviddarling.info /encyclopedia/R/RR_Lyrae_star.html (272 words)

	Encyclopedia: RR Lyrae (Site not responding. Last check: 2007-10-22)
		RR Lyrae Stars and the Distance to M4 Summary The student finds the distance to the globular cluster, Messier 4 (M4), using observations of an RR Lyrae star and the mean absolute magnitude of RR Lyrae variables.
		RR Lyrae stars (named after the prototype star RR in the constellation Lyra) are evolved population II stars and can be seen in the halos of galaxies, especially in globular clusters.
		Periods of RR Lyrae stars are typically 0.5 to 1 day, making it possible to see one or more periods in a single night of observations.
	www.nationmaster.com /encyclopedia/RR-Lyrae (542 words)

	Variable star - Encyclopedia, History, Geography and Biography
		Variable stars are generally analysed using photometry and photospectrometry.
		For regular variables, the period of variation and its amplitude can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to the next.
		This variation is thought to be caused by episodes of dust formation in the atmosphere of the star.
	www.arikah.com /encyclopedia/Variable_stars (1440 words)

	* RR Lyrae Variable - (Astronomy): Definition
		RR Lyrae variables are old type A stars with periods less than one day, and most of them change brightness by less than one magnitude.
		RR Lyrae Variables: A variable star that has a regularly varying luminosity.
		W Virginis variables are also old stars found in globular clusters, but they are much rarer than RR Lyrae variables.
	en.mimi.hu /astronomy/rr_lyrae_variable.html (273 words)

	RR Lyrae variable stars
		RR Lyrae's have long been recognized to be excellent tracers of old stellar populations, as well as primary distance indicators for Population II systems.
		RR Lyraes and their rôle in establishing the astronomical distance scale have been a major field of study at the OAB since 1984 (e.g.
		At present, a large number of research programs at various levels of completion are carried out, and new projects are proposed to address the RR Lyrae distance scale and the stellar population issues, as well as the impact of the RR Lyraes on stellar evolution and pulsation theories.
	www.bo.astro.it /report00/node19.html (427 words)

	Variable Stars (Site not responding. Last check: 2007-10-22)
		RR Lyrae variables are named after the star RR Lyrae, in the constellation Lyra.
		This cycle repeats as long as the Cepheid (or the RR Lyrae, which pulsates by the same mechanism) is in the instability strip, the region of the H-R diagram where stars are unstable to pulsation.
		RR Lyrae stars can also be used to measure distances, but since they are less luminous, they can only be seen to smaller distances.
	www.stormpages.com /swadhwa/stellarevolution/lecture17.htm (956 words)

	3 Variable stars (Site not responding. Last check: 2007-10-22)
		Variable v7, until now classified as an RR Lyrae star with a period of about 0.515 d, turned out to be of the BL Herculis type with the period approximately twice long.
		Already Hachenberg (1939) noticed that this star is almost a magnitude brighter than RR Lyrae stars in M 92 and thus supposed that it might be in fact a Cepheid.
		Two suspected variables of Nassau (1938): red giant, v15, and an RR Lyrae star, v17, turned out to be constant in light.
	aanda.u-strasbg.fr:2002 /papers/aa/full/2001/15/aah2569/node3.html (1640 words)

	Station Information - RR Lyrae variable
		A RR Lyrae is pulsating red giant, with a mass about half of our Sun.
		The average absolute magnitude of a RR Lyrae is 0.75, only 40 or 50 times our Sun.
		The relationship between a RR Lyrae's variability period and absolute magnitude makes them good standard candles for relatively near objects.
	www.stationinformation.com /encyclopedia/r/rr/rr_lyrae_variable.html (152 words)

	What is an RR Lyrae? (Site not responding. Last check: 2007-10-22)
		An RR Lyrae is a special type of star that changes in brightness with a regular period of a few days.
		RR Lyraes are pulsating, their surface moves radially in and out with a regular rhythm, a little like a balloon that is getting inflated and then deflated at regular intervals.
		RR Lyraes are helpful to astronomers because if we know the period of time it takes for an RR Lyrae to go through its cycle of brightening and dimming, then we can figure the absolute luminosity of that star.
	etacar.umn.edu /~martin/rrlyrae/rrlyrex.htm (594 words)

	AAVSO: Types of Variable Stars
		There are two kinds of variable stars; intrinsic in which variation is due to physical changes in the star or stellar system and extrinsic in which variability is due to the eclipse of one star by another or the effects of stellar rotation.
		The following types of pulsating variables may be distinguished by the pulsation period, the mass and evolutionary status of the star, and the characteristics of their pulsations.
		Cataclysmic variables (also known as Eruptive variables), as the name implies, are stars that have occasional violent outbursts caused by thermonuclear processes either in their surface layers or deep within their interiors.
	www.aavso.org /vstar/types.stm (1008 words)

	BGSU Variable Star Project
		The REU students at Cerro Tololo Inter-American Observatory (Chile) are contributing to this effort by observing RR Lyrae stars in the Southern hemisphere using the Curtis Schmidt telescope.
		RR Lyrae stars are a similar type of pulsating variable.
		RR Lyrae (for simplicity, RRL) are smaller and less luminous than Cepheids.
	physics.bgsu.edu /~layden/VarStarProj/vsp_main.html (2072 words)

	OzDASL: Magnitudes of Variable Star RR Lyrae 1198
		Variable stars are stars for which the intensity of the emitted energy changes over time; for periodic variable stars the change of intensity is periodic over time.
		RR Lyrae stars have periods in the range 0.2 to 0.9 days.
		For example, RR Lyraes and Cepheids lie on a strip called the "instability strip." Different types of variable curves are classified also on the basis of the shapes of their light curves and the relationships of shapes to period, for example.
	www.statsci.org /data/oz/rrl1198.html (749 words)

	RR Lyrae Stars and the Distance to M4
		The other class of variable stars, the ones we are concerned with here, are RR Lyrae variables (named after the prototype star RR in the constellation of Lyra).
		RR Lyrae stars are evolved old, low-mass stars, and can be seen in the uncrowded halos of galaxies, especially in globular clusters.
		Periods of RR Lyrae stars are typically 0.3 to 1 day, making it possible to see one or more periods (cycles) in a single night of observations.
	www.astro.washington.edu /labs/clearinghouse/labs/DistM4/m4.html (1106 words)

	Variable Stars
		As for the physical variable stars, on the contrary, the luminosity variations are due to changes of the characteristics of the star: so the star must find another equilibrium state.
		In other cases these variations occur regularly and periodically; during one period the magnitude, the temperature, the density, the radius and the radial velocity of the stellar gas change, with real oscillations or pulsations.
		The RR Lyrae are pulsating variable stars of a different type and magnitude: these are stars of the horizontal branch, and are going through a phase of instability.
	www.pd.astro.it /E-MOSTRA/NEW/A3041VAR.HTM (701 words)

	Evolution of V79 in M3
		The models of Sweigert and Renzini (1979) and Lee, Demarque, and Zinn (1990) predict that the RR Lyrae variables in the Oosterhoff type I clusters cross the instability strip during their ZAHB phase and are therefore expected to have period decreases, which are followed later by increases, when the stars evolve away from the ZAHB.
		The RR Lyrae variables in type II clusters should switch modes at longer periods, and as a result, their mean periods will be longer than those in the type I clusters.
		In his study of period changes of the RR Lyrae variables in M3, Szeidl (1965) found that the O-C diagram for V79 had a discontinuity between 1926 and 1938, and consequently, he was unable to detect any systematic period change for the star.
	ecf.hq.eso.org /~ralbrech/novdec97apjl/975521.html (2814 words)

	Interstellar Medium and the Milky Way
		RR Lyrae are found in old star clusters called globular clusters and in the stellar halo part of our galaxy.
		All of the RR Lyrae stars in a cluster have the same average apparent magnitude.
		RR Lyrae stars can be used as standard candles to measure distances out to about 760,000 parsecs (about 2.5 million light years).
	www.astronomynotes.com /ismnotes/s5.htm (1165 words)

	RR Lyrae Variable Stars In The CCD/Transit Instrument Survey - Storming Media
		Abstract: RR Lyrae variable stars have long been recognized as important tools in probing the mass, chemical distribution and kinematics of the Galaxy from the inner recesses of the nuclear bulge to the outer environs of the distant Galactic halo.
		This dissertation chronicles an RR Lyrae variable star survey from a thorough description of the initial observations with the CCD/ transit Instrument (CTI), to an examination of RR Lyrae space density and the Galactic mass using the discovered RR Lyrae stars.
		The RR Lyrae space density as a function f Galactocentric distance is shown to be a power-law function (R-3 to -3.5) and consistent with an ellipsoidal distribution in the nuclear bulge and more spherically symmetric distribution in the Galactic halo.
	www.stormingmedia.us /83/8314/A831492.html (197 words)

	AAVSO: Delta Cep, September 2000 Variable Star Of The Month
		The variability of Polaris occurs with a 3.97 day period and fluctuates by only a tenth of a magnitude, but the change is markedly detectable particularly through photometry.
		Pulsating variables of galactic spherical component or old disk population with periods approximately from 0.8d to 35d and amplitudes from 0.3m to 1.2m V. Obey a Period-Luminosity relation different from that for delta Cep variables.
		The light curves of W Vir variables for some intervals of periods differ from the light curves of delta Cep variables for corresponding periods either by amplidtues or by the presence of humps on the descending branch, sometimes turning into broad flat maxima.
	www.aavso.org /vstar/vsotm/0900.stm (3768 words)

	IMSA Astrophysics: Distance Ladder
		RR Lyrae, named for the type star (the first one of its kind recognized) in the constellation Lyra, are a specific variety of variable stars which, though rare, seem to be associated with globular clusters.
		It varies between about 5.74 and 5.84 magnitudes, with a period of 0.799678 days (its period was determined with an accuracy of a few seconds).
		To find the distance to the RR Lyrae star, simply determine its apparent visual magnitude, m, and solve the distance modulus equation.
	staff.imsa.edu /science/astro/astrometry/rung3.html (256 words)

	AAVSO: AAVSO Section
		Variable stars need to be systematically observed over decades in order to: determine the long-time behavior of a star, provide professional astronomers with data needed to analyze variable star behavior, to schedule observations of certain stars, to correlate data from satellite and ground-based observations, and to make computerized theoretical models of variable stars.
		Research on variable stars is important because it provides information about stellar properties, such as mass, radius, luminosity, temperature, internal and external structure, composition, and evolution.
		Cepheid Variables have played a pivotal role in determining distances to far away galaxies and the age of the universe.
	www.aavso.org /vstar (377 words)

	Calvin College - Physics and Astronomy
		RR Lyrae variable stars are known for having a short period of variability of less than a day in length.
		RR Lyrae stars are usually found in globular clusters, which is a cluster of stars from the same source that is gravitationally bound to each other.
		All RR Lyrae stars have a same average absolute magnitude of 0.6.
	www.calvin.edu /academic/phys/observatory/images/Astr212.Fall2002/SWAnd (697 words)

	T.Borkova, V.Marsakov, Subsystemsof RR Lyrae Variable Stars in Our Galaxy (Site not responding. Last check: 2007-10-22)
		However, observations of field RR Lyrae stars and blue hrizontalbranch stars were used in Laydan (1998) to estimate the number ratios of these objects in different directions from the Sun.
		To adequately estimate the chracteristic parameters of the proposed RR Lyrae subsystems, the sample must be representative of the objects in these subsystems.Our choice of stars based solely on their type of variability ensures an absence of kinematic selection effects.
		Thus,we conclude that the variability periods of field RR Lyrae stars cannot be used as an additional criterion to identify objects belonging to the thick-disk subsystem.
	ip.rsu.ru /~marsakov/paper6/article6_1.htm (5709 words)

	Large Magellanic Cloud Microlensing
		However, Connolly (1985) suggests that these faint RR Lyrae may be associated with the LMC and thus may not be RR Lyrae at all.
		RR Lyrae belonging to a dwarf galaxy that is not extended along the line of sight will show as a peak in the luminosity function, while RR Lyrae belonging to the Milky Way halo would be more or less uniformly distributed between the Sun and the LMC.
		The distance distribution of the 20 foreground RR Lyrae stars in the direction of the LMC is shown in Figure 3.
	ecf.hq.eso.org /~ralbrech/novdec97apjl/975552.html (3422 words)

	RR Lyrae variable stars
		A large number of research programs at various levels of completion are presently carried out, and new projects are proposed to address the RR Lyrae distance scale and the stellar population issues, as well as the impact of the RR Lyraes on stellar evolution and pulsation theories (see sections 1.4.1, 1.4.2, 1.4.3, and 1.4.4 below).
		An effort is also being made to organize the observational material on RR Lyrae variables already in hand or coming from future scheduled observations, in order to made it available in electronic form to the astronomical community (see section 1.4.5).
		RR Lyrae in LG galaxies and in globulars
	www.bo.astro.it /report99/node21.html (436 words)

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