Reasons to Believe

Cepheid Variable Stars

Not until Albert Einstein produced his theory of general relativity in 1916 did scientists even consider the possibility of continual cosmic expansion.

Yet six different Bible authors wrote extensively and explicitly about this continual expansion, “the stretching out of the heavens” under fixed physical laws.1 For over two thousand years the Bible stood alone as the only text making such claims about the universe. Consequently, any additional evidence in favor of continual cosmic expansion can be taken as further evidence for the reliability and predictive power of the Bible and as support of the Christian worldview.

Moreover, cosmic expansion—specifically, the different expansion rates throughout the universe’s history—provides researchers with the most compelling scientific evidences for supernatural, super-intelligent design. Thus, any advance in astronomers’ capabilities to measure the cosmic expansion rates provides additional opportunities to put the biblical cosmic creation model to the test.

The distance ladder method has been the limiting factor in measuring the cosmic expansion rates. It uses direct distance measurements on nearby objects to calibrate indirect methods for more remote objects. Direct distance measurements rely upon plane geometry theorems. Traditionally, the diameter of Earth’s orbit has served as the base of the triangle for such theorems. But because that base is only 186 million miles across, direct distance measurements deliver accurate distances only for stars located out to a few thousand light-years. Within those few thousand light-years, however, are a number of Cepheid variable stars.

Cepheid variable stars are pulsating stars whose light output varies in response to their pulsation. They have the property that their period of variability is tightly correlated with their intrinsic luminosity (or brightness). The longer the period of variability the brighter the light output from the star.

Because of the correlation between the brightness of the star and its period of variability, a Cepheid variable star can be used as a standard candle (a recognizable object that can be used to estimate distances). As seen through telescopes, two stars manifesting identical periods of variation will differ in light output in proportion to their different distances from Earth. For example, if one of the stars is twice as far away it will appear exactly one quarter as bright.

Direct distance measurements on the nearest Cepheid stars can be used to calibrate the period-luminosity relationship. Consequently, an astronomer only needs to measure the brightness of a remote Cepheid variable star to determine its distance. Since these stars are intrinsically very bright, they can be used to calculate distances to galaxies as far away as 100 million light-years.

Currently, Cepheid variable stars provide the most accurate determinations of the cosmic expansion rate. In three papers published in the April 20 issue of the Astrophysical Journal, two different teams of American astronomers demonstrate how this measuring technique can be dramatically improved.

The first team noted that astronomers had not yet exploited ultra-long period Cepheids.2 So far, scientists have used only Cepheids with periods of variability between 1 and 50 days. While rare, fundamental mode Cepheid variable stars with periods between 80 and 210 days do exist. Because these ultra-long period stars are so much brighter than normal Cepheids, they extend the effectiveness of this distance measuring method out to much greater distances. Instead of being limited to measuring galaxy distances out to just 100 million light-years, astronomers can now exploit ultra-long period Cepheids to measure galaxy distances out to 350 million light-years.3

Meanwhile, the second team published their results on using the Cepheid variable star distance method at infrared wavelengths.4 They discovered that the period-luminosity relationship had much less scatter at infrared wavelengths than at the visual wavelengths used in previous endeavors. Less scatter means that the Cepheid method can deliver much more accurate results than before. This research group is in the process of re-measuring the Cepheid distances to a number of nearby galaxies.

Greater forthcoming accuracy and trustworthiness in the measured value for the cosmic expansion rates throughout cosmic history resulting from improvements in the Cepheid variable star method will soon give cosmologists a more detailed history of the universe and a more exact picture of the cosmic creation event. This superior understanding will allow deeper and more rigorous tests of the big bang creation model. It could provide even more evidence for the Bible’s description of cosmic creation and history. Reasons To Believe scholars predict that the results will strengthen the validity of the Bible’s eloquent creation accounts.

Subjects: Big Bang, Origin of the Universe, Universe Design

Dr. Hugh Ross

Reasons to Believe emerged from my passion to research, develop, and proclaim the most powerful new reasons to believe in Christ as Creator, Lord, and Savior and to use those new reasons to reach people for Christ. Read more about Dr. Hugh Ross.


  1. Hugh Ross, The Creator and the Cosmos, 3rd ed. (NavPress, Colorado Springs, 2001), 23–29.
  2. Jonathan C. Bird, K. Z. Stanek, and Jose I. Prieto, “Using Ultra-Long Period Cepheids to Extend the Cosmic Distance Ladder to 100 Mpc and Beyond,” Astrophysical Journal 695 (April 20, 2009): 874–82.
  3. Jonathan C. Bird, K. Z. Stanek, and Jose I. Prieto: 874.
  4. Barry F. Madore et al., “The Cepheid Period-Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths. II. Second-Epoch LMC Data,” Astrophysical Journal 695 (April 20, 2009): 988–95; Wendy L. Freedman et al., “The Cepheid Period-Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths. IV. Cepheids in IC 1613,” Astrophysical Journal 695 (April 20, 2009): 996–98.