well well well... noding some APOD content today, and what do I stumble upon but a little astronomy nodeshell...

Certain stars have a variable pulsing intensity. The period of a cycle through this luminosity is directly linked to the star's intrinsic brightness. A ratio between this calculated brightness, and its brightness when observed from earth allows an approximation of its distance.

Some nearby galaxies (like M31), actually have observable Cepheids on their outer edges. The calculated distances really fucked up all of science's perception of the size of the universe. The result was the Cosmological Shapley-Curtis debate, <---- (feel free to node it, otherwise I will some day..) and even extensions on Einstein's theories!

Suddenly a member of The Nodeshell Rescue Team...

Cepheid variables are named after Delta Cephei, the first star of this type to be discovered. It was discovered in 1784 by John Goodricke, a deaf and mute English astronomer. It has a period of 5.4 days, less than any variable star known at the time.

Cepheids, along with RR Lyrae variables, occupy an area on the H-R Diagram known as the instability strip. This lies in the top right, between the main sequence and the long period variables. This means that Cepheids have a surface temperature around that of the Sun but are 1,000-100,000 times more luminous.

The variation in a Cepheid's brightness is caused by the star's oscillating outer envelope. The rate at which this oscillates is determined by the star's metal content and average brightness. As the metal content can be determined through spectroscopy, it is possible to calculate the star's true (not observed) brightness. By comparing this to the observed brightness, the distance to the star can be calculated quite accurately.

For Type I (metal rich) Cepheids, the true luminosity (as a multiple of that of the Sun), is approximately 800 times its period (in days). For Type II (metal poor) Cepheids, the luminosity/period factor is roughly 200.

Known as 'the galactic yardstick', the Cepheid Variable class of stars allow Astrophysicists and Astronomers to work out how far away distant galactic clouds are from our planet with relative ease.

Physical Makeup

There are two types of Cepheid Variable star. The original 'classical' Type I Cepheid Variables are metal-rich yellow supergiant stars in the late F to early K parts of their lives. Their surface temperature is around 6000-8000K, and they are approximately 800 times as luminous as the sun. More recently in 1952, a Type II variant has been discovered by Walter Baade. These stars are less common, far smaller and fainter than the classical variant, and also have a lower metal content enabling them to be distinguished from classical Cepheid using spectroscopy. Their luminosity is only 200 times that of our sun. Both of these types of star inhabit the instability strip of the Hertzsprung-Russell Diagram, alongside another pulating star, the RR Lyrae class, which best covered in seperate node. For a diagram illustrating this strip, look at www.physics.ucla.edu/class/02W/272_Ulrich/notes/NoteSet1.pdf

The cause of the Cepheid Variables' flashing' is actually to do with the radiation pressure within the star itself. To quote Eric Weisstein's World of Physics website:

As radiation streams out, some He+ in the atmosphere of the star is ionised to He2+, making the atmosphere more opaque. The decreased transparency of the stellar material blocks the energy flux and heats the gas, and the increased pressure pushes the envelope out, thus increasing the star's size and luminosity. As the star expands, it cools and He2+ gains an electron, converting back to He+. The enhanced transparency causes the atmosphere to shrink again.
This fluctuation in radiation pressure causes the star to actually grow larger and smaller in radius over the course of time. As it does, they get brighter and fainter usually with a period in the range of 1 to 50 days

How are they used?

The first Cepheid Variable was Delta Cephei which was discovered in 1784 by the English astronomer John Goodricke, but their usefulness was not discovered until 1912 when Henrietta S. Leavitt and Harlow Shapley noticed that their fluctuation in brightness or apparent luminosity is related to its actual power output or absolute luminosity. This becomes useful when you bear in mind that it is possible to calculate how far away a star is by applying the distance-luminosity formula:

d = sqrt(L/4πb)


  • d is the distance to the star
  • L is the absolute luminosity
  • b is the apparent luminosity, or brightness

All Cepheid variables with the same period have almost the same absolute luminosity within their type, but their apparent luminosity’s differ because they are at different distances from us. Due to their pulsing, Cepheid Variables are quite easy to pick out in space, due to their flashing, and once their average apparent luminosity is known, it can be compared to the known absolute luminosity, and the remainder is trivial.

One of the main flaws with this method is the impossibility of calculating the amount of interstellar dust between us and the Cepheid we are observing can throw the result off a little by reducing the amount of light reaching us, and therefore the apparent luminosity.

Disclaimer : I am not an Astrophysicist, nor an Astronomer, so if you spot any gaping flaws in this node /msg me.

Sources include:

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