Cepheid Variable Stars & Distance
There are two kinds of variable stars: intrinsic, in which variation is due to physical Cepheids obey the period-luminosity relationship. Generally, the kind of variability is classified as either intrinsic or extrinsic, depending Pulsating variables — stars which periodically expand and contract, such as Because Cepheids have a well-defined period-luminosity relationship, they. Intrinsic Variables - Stars whose output actually varies (pulsating stars, . stars, and since the composition is different, the period-luminosity relation is also.
Chi Cygni was identified in by G. Kirchthen R Hydrae in by G. By ten variable stars were known.
Modern Cosmological Observations and Problems - G. Bothun
Since the number of known variable stars has increased rapidly, especially after when it became possible to identify variable stars by means of photography. The latest edition of the General Catalogue of Variable Stars  lists more than 46, variable stars in the Milky Way, as well as 10, in other galaxies, and over 10, 'suspected' variables.
Detecting variability[ edit ] The most common kinds of variability involve changes in brightness, but other types of variability also occur, in particular changes in the spectrum.
By combining light curve data with observed spectral changes, astronomers are often able to explain why a particular star is variable. Variable star observations[ edit ] A photogenic variable star, Eta Carinaeembedded in the Carina Nebula Variable stars are generally analysed using photometryspectrophotometry and spectroscopy. Measurements of their changes in brightness can be plotted to produce light curves.
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. Peak brightnesses in the light curve are known as maxima, while troughs are known as minima.
Amateur astronomers can do useful scientific study of variable stars by visually comparing the star with other stars within the same telescopic field of view of which the magnitudes are known and constant. By estimating the variable's magnitude and noting the time of observation a visual lightcurve can be constructed. The American Association of Variable Star Observers collects such observations from participants around the world and shares the data with the scientific community.
From the light curve the following data are derived: From the spectrum the following data are derived: In very few cases it is possible to make pictures of a stellar disk. These may show darker spots on its surface. Interpretation of observations[ edit ] Combining light curves with spectral data often gives a clue as to the changes that occur in a variable star. For example, evidence for a pulsating star is found in its shifting spectrum because its surface periodically moves toward and away from us, with the same frequency as its changing brightness.
About two-thirds of all variable stars appear to be pulsating. In the s astronomer Arthur Stanley Eddington showed that the mathematical equations that describe the interior of a star may lead to instabilities that cause a star to pulsate.The Importance of Cepheid Variable Stars
The most common type of instability is related to oscillations in the degree of ionization in outer, convective layers of the star. Suppose the star is in the swelling phase.
Its outer layers expand, causing them to cool. Because of the decreasing temperature the degree of ionization also decreases. This makes the gas more transparent, and thus makes it easier for the star to radiate its energy. This in turn will make the star start to contract. As the gas is thereby compressed, it is heated and the degree of ionization again increases. This makes the gas more opaque, and radiation temporarily becomes captured in the gas.
This heats the gas further, leading it to expand once again. Thus a cycle of expansion and compression swelling and shrinking is maintained. Variable star designation In a given constellation, the first variable stars discovered were designated with letters R through Z, e. This system of nomenclature was developed by Friedrich W.
Argelanderwho gave the first previously unnamed variable in a constellation the letter R, the first letter not used by Bayer. Once those combinations are exhausted, variables are numbered in order of discovery, starting with the prefixed V onwards. Classification[ edit ] Variable stars may be either intrinsic or extrinsic. This category can be divided into three subgroups.
Pulsating variables, stars whose radius alternately expands and contracts as part of their natural evolutionary ageing processes. Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections.
Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like novae and supernovae. There are two main subgroups. Eclipsing binaries, double stars where, as seen from Earth 's vantage point the stars occasionally eclipse one another as they orbit. Rotating variables, stars whose variability is caused by phenomena related to their rotation. Examples are stars with extreme "sunspots" which affect the apparent brightness or stars that have fast rotation speeds causing them to become ellipsoidal in shape.
These subgroups themselves are further divided into specific types of variable stars that are usually named after their prototype. You must also have read the IDL primer. This is a data intensive lab and you will not be able to waste much time the first period reading the notes. Grading Policy Grading is based on the quality of the data analysis, the discussion of the uncertainties, and the answers to the questions at the end of this page.
Introduction The state-of-the-art astronomical detector before the late 19th century was the human eyeball. The eyeball is a marvelous detector, with excellent dynamic range and color sensitivity, but it has limited sensitivity and poor recording ability. The refinement of telescopic technology helped with the sensitivity limitations, but it was the invention of the photographic plate that permitted one to amass and study large amounts of data in a coherent fashion.
Large Magellanic Cloud Figure 2: It did so fairly regularly until about with some plates taken up to Over half a million of these plates are stored in the Harvard plate stacks. One of the early targets was the Large Magellanic Cloud LMC; see above left imagea nearby 65 kilo-parsecs dwarf irregular galaxy. Henrietta Leavitt right image above ; also see this sitewho held the position of "computer" at the Harvard College Observatory, began classifying the variable stars in the LMC in Stars were not well understood a century ago, and much of astronomy was devoted to discovering and characterizing the variables.
PHY / The Cepheid Period-Luminosity Relation
The astrophysical significance of studying variable stars in the LMC is that all the stars in the LMC are roughly the same distance from us. Distances are one of the most fundamental, but most difficult to measure, quantities in astronomy. Therefore, one can compare distance dependent quantities like luminositieswhich one cannot do easily for the brighter stars in our own Milky Way galaxy.
Most, if not all, stars are variable. Some, like the novae, are spectacularly variable, while others are barely noticeable even upon close inspection. Among the plethora of types of variables are: Eclipsing variables, which are binary systems we observe from in or close to the plane of the orbit. They vary because one star gets in front to the other each orbit, thereby diminishing total brightness.
This is simply a geometrical obscuration. Pulsating variables, which are stars that lie in the instability strip in the Hertzsprung-Russell H-R diagram a plot of luminosity or magnitude vs. Most stars are stable against adiabatic perturbations. Perturb the star to a larger radius. The temperature will fall and the opacity will increase. Then let the star contract under its gravity.
The enhanced opacity will result in an enhanced radiation pressure, and a damping of the oscillation. However, in the instability strip, hydrogen is partially ionized in the outer radiative envelope of the star. As gravity makes the star contract, the low opacity does not lead to an increased radiation pressure, so the oscillations do not damp out.
The instability strip runs diagonally through the H-R diagram, with temperatures near 10,K. All stars are natural pulsators at low amplitudes. The atmosphere of the Sun oscillates with a fundamental period of about 5 minutes.
The study of these periods helioseismology provides a probe of the interior of the Sun, and provides temperature and density profiles accurate to a few percent within the convective zone.
Cepheid Variable Stars & Distance Determination
Asteroseismology, now possible for some of the brighter stars, reveals their internal characteristics temperatures, densities, etc. Intrinsic variables All convective stars spectral types F, G, K, Mincluding the Sun, display both periodic and irregular variability resulting from stellar magnetic activity in the outer atmosphere. Asymmetric distributions of starspots reveal themselves as periodic rotational modulation of the brightness, with amplitudes up to 0.
Flaring due to magnetic recombination is irregular, and is common among the younger, more rapidly rotating convective stars. Flaring is most noticeable in the X-rays and UV, and among the M stars, where contrast with the photosphere is enhanced. Explosive variables include novae. The buildup of hydrogen-rich matter on the surface of a white dwarf, drawn fron a Roche-lobe-filling companion, will undergo a runaway thermonuclear detonation once enough builds up on the surface so that the lower layers become degenerate.
Novae occur irregulary, at intervals from millions of years to a few years, depending on the accretion rate and the mass of the white dwarf. Cataclysmic variables CVs are white dwarf binaries undergoing accretion.
These vary by a few magnitudes irregularly due to changes in the mass accretion rate. In the Polars, or AM Her stars, the accretion stream impacts the surface of the white dwarf directly. Variations in the mass accretion rate lead directly to brightness changes as the gravitational potential energy released heats the accretion colun and the impact zone.
In the dwarf novae an accretion disk forms, and the brightness variations in the disk reflect the viscous heating of the disk. All CVs eventually become novae. In an analogous set of variables, the X-ray binaries, the white dwarf is replaced by a neutron star of stellar-mass black hole.
Artist's conception of a Polar, showing the disruption of the accretion stream by the MG magnetic field of the white dwarf Image copyright M. Garlick Ellipsoidal variables are stars that are not round, and present different aspects to us as they rotate.
Ellipsoidal variables are all in close binary systems, where they are tidally-distorted by their companions. Doppler image of AE Phe at four phases, from Barnes et al. Some the RV Tauri stars form dust shells as they expand, which then obscure the light of the star until they expand and become diluted.
Of these, only about 20 were Cepheids. Its light curve is shown in Figure 6. Since all the stars are in the LMC, and are at the same distance from us, the apparent magnitudes are an accurate measure of the true relative luminosities of the stars. She found a relation similar to that shown in Figure 7. She actually used apparent magnitudes; the conversion to absolute magnitudes shown in Figure 7 requires an estimate of the distance to the LMC.
The Cepheid period-luminosity relation The importance of such a relation, once it is calibrated, is that it provides a simple way to determine the distance to a Cepheid variable and, hence, to the cluster or galaxy that contains it.