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)
Where
- 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:
http://physics.bgsu.edu/~layden/VarStarProj/vsp_main.html
http://nedwww.ipac.caltech.edu/level5/ESSAYS/Evans/evans.html
http://scienceworld.wolfram.com/astronomy/CepheidVariable.html
http://antwrp.gsfc.nasa.gov/apod/ap960110.html