Cepheid Variable Stars | COSMOS
Cepheid variable stars are supergiants with luminosities , times greater than The close relationship between period and luminosity which was found by in establishing the distances of the nearer galaxies and hence the distance. Her plot showed what is now known as the period-luminosity relationship; cepheids. The namesake star in the very important class of stars known as Cepheid study in which Henrietta Leavitt first discovered that the periods of luminosity were related to A Cepheid variable nevertheless gives a good indication of distance when to be related to their absolute luminosity by a period-luminosity relationship.
From this he could infer the distance to globular cluster too distant to have visible Cepheids and realised that these clusters were all essentially the same size and luminosity. By mapping the distribution and distance of globular clusters he was able to deduce the size of our galaxy, the Milky Way.
Cepheid Variable Stars and Measuring Distances in Space
Using these he determined that their distances wereandlight years respectively. He thus established conclusively that these "spiral nebulae" were in fact other galaxies and not part of our Milky Way. This was a momentous discovery and dramatically expanded the scale of he known Universe. Hubble later went on to observe the redshift of galaxies and propose that this was due to their recession velocity, with more distant galaxies moving away at a higher speed than nearby ones.
This relationship is now called Hubble's Law and is interpreted to mean that the Universe is expanding. Period-luminosity relationship for Cepheids and RR Lyrae stars.
Let us now see how this relationship can be used to determine the distance to a Cepheid. Photometric observations, be they naked-eye estimates, photographic plates, or photoelectric CCD images provide the apparent magnitude values for the Cepheid. Plotting apparent magnitude values from observations at different times results in a light curve such as that below for a Cepheid in the LMC.
From the light curve and the photometric data, two values can be determined; the average apparent magnitude, m, of the star and its period in days.
In the example above the Cepheid has a mean apparent magnitude of Knowing the period of the Cepheid we can now determine its mean absolute magnitude, M, by interpolating on the period-luminosity plot. The one shown below is based on Cepheids within the Milky Way. The vertical axis shows absolute magnitude whilst period is displayed as a log value on the horizontal axes.
The log of 4. Energy is stored in the form of the second ionization of helium during the compression stage of the cycle and then released as the helium recombines during the expansion stage.
The restriction of Cepheid pulsations to stars in a limited temperature range follows from the requirement that the second helium ionization zone lies near the transition from the nearly adiabatic interior, where any driving is almost canceled by an equal amount of damping, to the nonadiabatic exterior where the thin outer layers lack the heat capacity to modulate the outward flow of radiation.
- Cepheid variable
- Classical Cepheid variable
- Cepheid Variable Stars, Supernovae and Distance Measurement
The pulsation is a property of the stellar envelope and is independent of the nuclear-energy-generating core. This follows from their strong concentration toward the plane of the Milky Way and their low space velocities. Their presence in star clusters allows their ages to be estimated as up to about yr.
Observations of the Cepheids in the Magellanic Clouds show that the classical Cepheids are confined to a narrow strip in the period-luminosity diagram, whereas the less common Type II Cepheids are fainter than them at a given period.
The presence of Type II Cepheids in globular clusters and in the galactic halo population allows their age to be estimated as up to 15 x yr, so that they must be much less massive than the classical Cepheids.
The Type II Cepheids can also be distinguished from the classical Cepheids by the shape of the light curves and by spectroscopic peculiarities.
The form of the light curve changes with period in a systematic way known as the Hertzsprung progression.
A bump appears on the descending branch of the light curve of stars with periods of about a week and is found at earlier phases in stars of successively longer periods so that the bump is near maximum light in stars of day period which may show a double maximum.
The bump falls on the rising branch in stars of longer period. Stars of the shortest or longest periods have smooth light curves. The amplitude of the pulsation increases slowly with period up to about 10 days, where there is a drop in amplitude; it then increases more rapidly to longer periods.
The bumps may represent an echo of the surface pulsation from the deep interior; an alternative explanation is that they result from a resonance when the second overtone period is about one-half of the fundamental period.
Some Cepheids of short period have nearly sinusoidal light curves with amplitudes of only about 0. Their radii are a few tens to a few hundred times that of the sun. More luminous Cepheids are cooler and larger and have longer periods. Along with the temperature changes their radii also change during each pulsation e.
Cepheid Variable Stars & Distance
The brightness changes are more pronounced at shorter wavelengths. Pulsations in an overtone higher than first are rare but interesting. Stars pulsating in an overtone are more luminous and larger than a fundamental mode pulsator with the same period. When the helium core ignites in an IMS, it may execute a blue loop and crosses the instability strip again, once while evolving to high temperatures and again evolving back towards the asymptotic giant branch.
The duration and even existence of blue loops is very sensitive to the mass, metallicity, and helium abundance of the star. In some cases, stars may cross the instability strip for a fourth and fifth time when helium shell burning starts. More massive and hotter stars develop into more luminous Cepheids with longer periods, although it is expected that young stars within our own galaxy, at near solar metallicity, will generally lose sufficient mass by the time they first reach the instability strip that they will have periods of 50 days or less.