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	Cepheid Variable Stars (Site not responding. Last check: 2007-11-03)
		The namesake star in the very important class of stars known as Cepheid variables, this star formed part of the original study in which Henrietta Leavitt first discovered that the periods of luminosity were related to their absolute luminosity.
		Cepheid variable stars have proved to be one of the most valuable methods for distance determination because their period of variability has been shown to be related to their absolute luminosity by a period-luminosity relationship.
		Cepheid variables can be seen and measured out to a distance of about 20 million light years, compared to a maximum distance of about 65 light years for parallax measurements.
	hyperphysics.phy-astr.gsu.edu /hbase/astro/cepheid.html (188 words)

	The Astrophysics Spectator: Cepheid Variable Stars
		Cepheid variable stars are stars that periodically pulsate because of an instability in their internal structure.
		The sources of instability in a Cepheid variable are the regions where helium and hydrogen become ionized, with the region of helium becoming fully ionized the dominant instability region.
		Cepheid variables subdivide into two classes: the classical Cepheid variables, which are population I stars—stars with a high metallicity, and, therefore, of the current generation of stars—and the W Virginis variables, which are population II— stars of a low metallicity stars, and, therefore, early generation stars.
	www.astrophysicsspectator.com /topics/overview/DistanceCepheids.html (648 words)

	Cepheid variable - Wikipedia, the free encyclopedia
		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.
		A Cepheid is usually a population I giant yellow star, pulsing regularly by expanding and contracting, resulting in a regular oscillation of its luminosity.
		The variation in luminosity is caused by a cycle of ionization of helium in the star's atmosphere, followed by expansion and deionization.
	en.wikipedia.org /wiki/Cepheid (650 words)

	Variable Stars
		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.
		These stars have been studied extensively in order to classify their characteristics and are useful distance indicators: they allowed to calculate the distance of nearby galaxies, like M31 (Andromeda), or the Magellan Clouds.
		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)

	A Survey of Cepheids (Site not responding. Last check: 2007-11-03)
		This is primarily due to the recognition of the distinction between Population I and Population II stars and the subsequent exclusion of the RR Lyrae stars in the calibration.
		The reason that Cepheid stars vary in brightness in a regular periodic fashion is that the star itself is actually pulsating, that is, the star expands and contracts in a regular repetitive motion.
		This theory of pulsation is derived from the observations of the regularly varying apparent magnitude and radial velocity of Cepheid variable stars.
	home.comcast.net /~cessna27/a_survey_of_cepheids.htm (2979 words)

	Binary and Variable Stars
		A variable star, on the other hand, is a star that, for one reason of another, changes its luminosity in either a predictable pattern or at random.
		Cepheid variables are only variable for a short time when they are along the Horizontal Giant Branch.
		RR Lyrae stars are a sub-classification of Cepheid variables.
	filer.case.edu /~sjr16/advanced/stars_binvar.html (1239 words)

	cepheids
		Polaris is a cepheid star with a rather small magnitude variation of between 2.5 and 2.6 and a period of under 4 days.
		The brightness variability of cepheids is intrinsic to the stars, and not cause by it being eclipsed by a binary partner.
		In the cepheid variable star the star expands when the force due to the internal pressure is greater than the weight of the star's outer layers.
	www.calstatela.edu /faculty/kaniol/a360/cepheids.htm (622 words)

	Cepheid Variable -- from Eric Weisstein's World of Astronomy
		A Cepheid variable is a young star of several solar masses and roughly 10
		The period of a Cepheid variable is related to its intrinsic luminosity.
		Type I Cepheids are population I stars which have brightnesses which fluctuate with a period between 1 to 100 days.
	scienceworld.wolfram.com /astronomy/CepheidVariable.html (193 words)

	Roller Coaster Stars - Astronomy In Your Hands
		For a variable star this is the time between it reaching a peak of brightness, and reaching its next peak of brightness.
		Cepheids are stars at a particular stage in their life cycles.
		The length of this cycle is your estimate of the period of the star.
	www.astronomyinyourhands.com /activities/rollercoaster.html (1578 words)

	[No title] (Site not responding. Last check: 2007-11-03)
		Cepheid variables constitute a key element in the measurement of distances in the Universe.
		Recall that a Cepheid is special in that the period of its light variations is proportional to the intrinsic luminosity of the star.
		Since the brightness variations are periodic, a better sense of the specific variations is achieved by plotting all the points as though they occurred within one cycle.
	www.lycoszone.com /info/cepheid.html (568 words)

	Polaris (Site not responding. Last check: 2007-11-03)
		For this type of star a complex series of events in its core causes the material of which it is made to alternately expand outward and then contract in a perpetual periodic way.
		For most cepheids, at any given time, all the material making up the star is moving in the same direction; this type of pulsation is called fundamental mode pulsation and is shown in Figure 1a.
		While stars remain as nothing more than simple points of light in most telescopes (just as they looked through Galileo's first telescope 500 years ago) the NPOI and other telescopes like it are finally resolving them into worlds of their own.
	ftp.nofs.navy.mil /projects/npoi/science/cepheids.htm (1129 words)

	AstronomyNook: Variable Stars
		variable star is a star that fluctuates in brightness over a period of time.
		Pulsating variables are stars that show periodic expansions and contractions of their surface layers.
		Exploding stars (also known as Eruptive or Cataclysmic variables), are stars that have occasional violent outbursts caused by thermonuclear processes either in their surface layers or deep within their interiors.
	www.astronomynook.com /variablestars.htm (431 words)

	ASP: A Taste of Real Astronomy — The ESA/ESO Astronomy Exercises
		In this way Cepheid variable stars can be used as one of the 'standard candles' in the Universe that act either as distance indicators themselves or can be used to calibrate (or set the zero point for) other distance indicators.
		Cepheid variables can be distinguished from other variable stars by their characteristic light curves (see Fig.
		The star is located in a star-forming region in one of the galaxy's spiral arms (the star is at the centre of the box).
	www.astrosociety.org /education/publications/tnl/57/realastro3.html (1099 words)

	The Astronomical Distance Scale
		Variable stars have proven to be one of the most reliable types of "standard candles".
		Cepheid Variables are giant stars which have instabilities in their envelopes that cause them to pulsate in size, temperature and luminosity over timescales of a few days.
		With HST Cepheids are being studied in galaxies in the Virgo Cluster.
	casswww.ucsd.edu /public/tutorial/Distances.html (919 words)

	AAVSO: Delta Cep, September 2000 Variable Star Of The Month
		Cepheid variable stars, named for their archetype delta Cephei, have provided astronomers with a wealth of information about our Universe, a feat that no other class can claim.
		As a class, Cepheid variable stars are characterized by a change in brightness of several hundredths to 2 magnitudes, which occurs with extreme regular periodicity of just 1 to 135 days.
		Such stars are commonly of spectral class F at maximum and G to K at minimum, whereby the rule of thumb that applies is the later the spectral class, the longer its period.
	www.aavso.org /vstar/vsotm/0900.stm (3768 words)

	HubbleSite - Release Text about "Hubble Space Telescope Measures Precise Distance to the Most Remote Galaxy Yet"
		HST's detection of Cepheid variable stars in the spiral galaxy M100, a member of the Virgo cluster, establishes the distance to the cluster as 56 million light-years (with an uncertainty of +/- 6 million light-years).
		Once the periods and intrinsic brightness of these stars were established from the careful measurement of their pulsation rates, the researchers calculated a distance of 56 million light-years to the galaxy.
		Cepheid variable stars rhythmically change in brightness over intervals of days (the prototype is the fourth brightest star in the circumpolar constellation Cepheus).
	hubblesite.org /newscenter/newsdesk/archive/releases/1994/49/text (1249 words)

	Measuring cosmic distances with stellar heart beats
		They are named after the star Delta Cephei in the constellation of Cepheus, the first known variable star of this particular type and bright enough to be easily seen with the unaided eye.
		By measuring the period of a Cepheid star, its intrinsic brightness can be deduced and from the observed apparent brightness, the distance may then be calculated.
		With this classical method, the variation of the angular diameter of a Cepheid variable star is inferred from the measured changes in brightness (by means of model atmosphere calculations) as it pulsates.
	www.eurekalert.org /pub_releases/2004-10/eso-mcd102904.php (1487 words)

	ASP: Star Science in the Autumn Sky
		Those stars have emerged as the slayers of modern cosmology, sending theorists back to the flboards in their attempts to understand the origins and future of the universe.
		Variable stars are an ideal way to get high-school students doing real science, with real data, from the real sky.
		Cepheus lies in between the constellation Cassiopeia, the star Deneb in the constellation Cygnus, and the star Polaris (the North Star).
	www.astrosociety.org /education/publications/tnl/32/starscience3.html (1034 words)

	StarsIntro
		A star is a body that at some time in its life generates its light and heat by nuclear reactions, specifically by the fusion of hydrogen into helium under conditions of enormous temperature and density.
		Stars can range up to about 100 times the mass of the Sun (at which point nature stops making them) down to around 8% that of the Sun, at which point the internal temperature is not high enough to run the full range of nuclear reactions (which requires at least 7 million degrees Kelvin).
		Mira variables and other old red giants thus divide into oxygen-rich stars and "carbon stars." Raised up along with the carbon are elements such as zirconium and many others that have been made in a huge variety of nuclear reactions that go on at the same time as helium fusion.
	www.astro.uiuc.edu /~kaler/sow/star_intro.html (5419 words)

	Cepheid Variables - Introduction
		Cepheid stars oscillate between two states: In one of the states, the star is compact and large temperature and pressure gradients build up in the star.
		Cepheid variable stars have masses between five and twenty times the mass of our Sun.
		Because these envelopes are more extended and the density in their envelopes is lower, their variability period, which is proportional to the inverse square root of the density in the layer, is longer.
	imagine.gsfc.nasa.gov /docs/science/mysteries_l1/cepheid.html (693 words)

	Exercise 1.4: Cepheid Period-Luminosity Relation (Site not responding. Last check: 2007-11-03)
		A variable star that has the peculiar problem with achieving the proper balance between the power welling up from the stellar core and the power being radiated from the stellar surface.
		In a futile quest for a steady equilibrium, the atmospheres of these pulsating variable stars alternately expand and contract, causing the star's luminosity to rise and fall.
		It was the Cepheids in the SMC that Henrietta Leavitt studied when she plotted the average apparent magnitude against the logarithm of the period, like the one shown below.
	www.astro.lsa.umich.edu /Course/MMSS/Interactive/Ex1.4 (1102 words)

	Cepheid Variable Stars (Site not responding. Last check: 2007-11-03)
		Certain stars that have used up their main supply of hydrogen fuel are unstable and pulsate.
		Cepheids are important beyond their intrinsic interest as pulsating stars.
		The ``original'' Cepheid variable, Delta Cephei, is close enough that we have a parallax measurement for it.
	zebu.uoregon.edu /~soper/MilkyWay/cepheid.html (267 words)

	Variable Stars (Site not responding. Last check: 2007-11-03)
		Her work at the Harvard College Observatory early in this century was instrumental in the cataloguing of variable stars and in the realization that Cepheid variables could be used to establish a distance scale (Ref).
		The following figure shows the variation in brightness, temperature, spectral class, surface velocity, and radius for the prototype Cepheid variable, delta-Cephei, as a function of the phase of the pulsation period, as does this animation.
		By making careful brightness measurements of Cepheid variable stars detected in this galaxy, it was possible to determine that it lies at a distance of 19.1 parsecs (about 60 million light years) (Ref).
	csep10.phys.utk.edu /astr162/lect/variables/variables.html (174 words)

	Curious About Astronomy: What is a Cepheid variable?
		Cepheid variables are a subgroup of a class of stars called variable stars.
		Cepheid variable stars have masses between five and twenty times the mass of the sun.
		It was the observations of Cepheid variable stars which enabled astronomer Hubble (after whom the Hubble Space Telescope is named) to determine that spiral nebulae like the one in Andromeda were galaxies separate from the Milky Way.
	curious.astro.cornell.edu /question.php?number=33 (263 words)

	The First Cepheid by Ken Croswell
		Cepheids are just as crucial today, for they are key weapons in the ongoing battle over the size and age of the universe.
		While examining variable stars in the Small Magellanic Cloud, she noticed that the brighter variables had longer periods.
		This luminosity, along with the star's mean apparent magnitude and the known absorption of light by interstellar dust, implies that Eta Aquilae is 900 light-years away.
	www.sonic.net /~antares/thefirstcepheid.html (1087 words)

	Cepheid Variables
		The defining characteristics of cepheid variable stars are presented and interpreted in an evolutionary context.
		These are population I yellow or red supergiant stars (type A to F at maximum, G to K at minimum) with periods ranging from 2 to 40 days and varying in brightness by up to one magnitude.
		Several of the stars nearby l Carinae (such as R Carinae) cannot be used as comparison stars since they are themselves variable.
	www.peripatus.gen.nz /Astronomy/CepVar.html (541 words)

	Cepheid Variable Velocity Graph Analysis
		In some Cepheids that appear to have a velocity curve distortion roughly in the center of that slope, that could be an indication of a harmonic pulsing, where a collapsing shell of material momentarily encounters an outward moving shell, causing a brief deceleration of both shells.
		The certain spherical shape of the star means that we would see a Doppler shift that is more representative of a large radiating area of the star centered on the sub-Earth point, and it is generally accepted that the actual radial velocity of the sub-Earth point would always be around 1.4 times what we measure.
		The (Center-of-mass) Relative Radial Velocity column is the value after the constant receding velocity (21.60 km/sec) of the star is eliminated, and is therefore the actual relative radial velocity of the surface, from the reference frame of the center of the star.
	mb-soft.com /public2/cepheid.html (1836 words)

	IMSA Astrophysics: Distance Ladder (Site not responding. Last check: 2007-11-03)
		Although variable stars were previously recognized, the importance of this discovery was monumental.
		Since the stars she studied were too far a way for parallax determination, she had to use their apparent magnitudes in her analysis.
		Let's say you discover an Cepheid variable star and determine its apparent magnitude, mv, to be 9.52, and it is found to have a period of 7.20 days.
	staff.imsa.edu /science/astro/astrometry/rung4.html (981 words)

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