Cepheid Variable Stars & Distance
period-luminosity relationship of Cepheids with observations of the variable stars somewhat like the Zen death poem written more than eight centuries ago: . with time. The observation of a number of Cepheid variables in the Large a relation between the absolute luminosity of this type of star and the period of the. Classical Cepheids are a type of Cepheid variable star. They are population I variable stars that exhibit.
More luminous Cepheids are cooler and larger and have longer periods. Along with the temperature changes their radii also change during each pulsation e. 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.
Very massive stars never cool sufficiently to reach the instability strip and do not ever become Cepheids. At low metallicity, for example in the Magellanic Clouds, stars can retain more mass and become more luminous Cepheids with longer periods. This is due to the phase difference between the radius and temperature variations and is considered characteristic of a fundamental mode pulsator, the most common type of type I Cepheid.
Variable Variables - One Universe at a Time
In some cases the smooth pseudo-sinusoidal light curve shows a "bump", a brief slowing of the decline or even a small rise in brightness, thought to be due to a resonance between the fundamental and second overtone. The bump is most commonly seen on the descending branch for stars with periods around 6 days e. As the period increases, the location of the bump moves closer to the maximum and may cause a double maximum, or become indistinguishable from the primary maximum, for stars having periods around 10 days e.
At longer periods the bump can be seen on the ascending branch of the light curve e. Because all of the Cepheids in a Magellanic Cloud are at the same distance from us, Leavitt reasoned that the more luminous Cepheids pulsated more slowly.
This is the period-luminosity relation. Leavitt did not know the distances to the Magellanic Clouds, so she could not tell what the actual value of the luminosity part of the relation was.
Astronomers had to wait a few years for Harlow Shapley to calibrate Leavitt's relation using Cepheids in our galaxy for which the distances could be determined. In the calibration process Shapley put actual values to the luminosity part of the period-luminosity relation. With a calibrated period-luminosity relation astronomers could use Cepheid variables as standard candles to determine the distances to distant clusters and even other galaxies.
Cepheids have pulsation periods of 1 to 50 days. In the 's astronomers found that there are two types of Cepheids: Below is the light curve the plot of brightness vs.
Interstellar Medium and the Milky Way
W Virginis Cepheids are from older low-metallicity stars and are about 4 times less luminous than Type I. Below is the light curve of a W Virginis Cepheid from the Hipparcos database of variable stars. Note the differences in the shape of the light curve. The two types of Cepheids are distinguished from each other by the shape of the light curve profile.
In order to compare the shapes without having to worry about the pulsation periods, the time axis is divided by the total pulation period to get the "phase": Because the luminosity of Cepheids can be easily found from the pulsation period, they are very useful in finding distances to the star clusters or galaxies in which they reside.
- her head full of stars
- One Universe at a Time
- Henrietta Swan Leavitt and the Light of the Cepheids
By comparing a Cepheid's apparent brightness with its luminosity, you can determine the star's distance from the inverse square law of light brightness. Recall that brightnesses are specified in the magnitude system, so the calibration brightness absolute magnitude is the brightness you would measure if the Cepheid was at the calibration distance of 10 parsecs 33 light years. In some cases the calibration distance may be the already-known distance to another Cepheid with the same period you are interested in.
Cepheid variable stars are so important that being able to measure their distances in other galaxies was the main factor in determining the size of the Hubble Space Telescope mirror and the measuring distances to Cepheids in 18 galaxies was the "Key Project" of the Hubble Space Telescope for its first decade all of the other results and pretty pictures were bonuses! Early measurements of the distances to galaxies did not take into account the two types of Cepheids and astronomers underestimated the distances to the galaxies.
He found it was aboutlight years away. However, the Cepheids he observed were Type I classical Cepheids that are about four times more luminous.Henrietta Swan Leavitt—A Portrait
Later, when the distinction was made between the two types, the distance to the Andromeda Galaxy was increased by about two times to about 2. Recent studies using various types of objects and techniques have given a larger distance of between 2.
They are smaller than Cepheids and, therefore, have shorter periods and lower luminosities.
Cepheid Variable Stars & Distance Determination
They pulsate with a period between 5 and 15 hours Cepheid pulsation periods are greater than 24 hours. Low-mass stars will go through a RR Lyrae pulsation stage while the high-mass stars will go through a Cepheid stage. Because low-mass stars live longer than high-mass stars, the Cepheid stars as a group are younger than the RR Lyrae stars. 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. In different clusters, the average apparent magnitude was different.