Classical Cepheid variable
[3][4][6][7][8] Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the local spiral arm structure and the Sun's distance from the galactic plane.
Several thousand more are known in the Magellanic Clouds, with more discovered in other galaxies;[9] the Hubble Space Telescope has identified some in NGC 4603, which is 100 million light years distant.
[14] When an intermediate mass star (IMS) first evolves away from the main sequence, it crosses the instability strip very rapidly while the hydrogen shell is still burning.
Stars more massive than about 8–12 M☉ start core helium burning before reaching the red-giant branch and become red supergiants, but may still execute a blue loop through the instability strip.
[citation needed] The rate of change of the period of a Cepheid variable, along with chemical abundances detectable in the spectrum, can be used to deduce which crossing a particular star is making.
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.
However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a month later.
The period-luminosity relation for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds.
[26] Also, in 2008, ESO astronomers estimated with a precision within 1% the distance to the Cepheid RS Puppis, using light echos from a nebula in which it is embedded.
[30] The term s-Cepheid is used for short period small amplitude Cepheids with sinusoidal light curves that are considered to be first overtone pulsators.
[31][32] Small amplitude Cepheids (DCEPS) include Polaris and FF Aquilae, although both may be pulsating in the fundamental mode.
[33][34] Chief among the uncertainties tied to the Cepheid distance scale are: the nature of the period-luminosity relation in various passbands, the impact of metallicity on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on classical Cepheid distances.
[3][4][6][7][8] Resolving this discrepancy is one of the foremost problems in astronomy since the cosmological parameters of the Universe may be constrained by supplying a precise value of the Hubble constant.
The closest class member is the North Star (Polaris) whose distance is debated and whose present variability is approximately 0.05 of a magnitude.
