Cepheid variables are massive, pulsating stars, valued by astronomers for the precise link between their brightness and steady pulsation. Let’s look at the history of Cepheid variables and how recent discoveries about these stars shatter established theories of stellar evolution.
Cepheid variable stars have been known since the 18th century. The closest to Earth and best-known Cepheid is Polaris, the North Star about 430 light years (132 parsecs) away. Regularly brightening to a peak over days, weeks or months before fading these variable stars were remarkable curiosities to astronomers in past centuries. The term Cepheid is taken from the star Delta Cephei in the constellation of Cepheus, which was identified as variable in 1784 by the teenaged prodigy John Goodricke (1764-96). The talented Goodricke was the first to explain the light variations of another type of variable star, like Algol, when he showed these to be examples of binary star systems where the dimmer orbiting companion passes in front of the brighter primary, temporarily obscuring it. Based on the laws of gravitation, measuring the companion star’s orbital period enables us to calculate the two stars’ relative masses. Goodricke’s discovery of these eclipsing binary stars won him a gold medal from the Royal Society. (Since I wrote this paragraph I have read Michael Hoskin’s 2011 Discoverers of the Universe which tells of how Goodricke was allowed to take credit for this discovery as an act of generousity by his friend Edward Piggott, According to Hoskin, Piggott alone came up with the explanation.)
Cepheids are not such eclipsing binaries, being intrinsically variable, that is their fluctuating brightness comes from some process inside them. Cepheids literally shrink as they dim and swell as they brighten. In 1908, Henrietta Swan Leavitt (1864-1921) discovered that Cepheids pulse at a rate governed by their brightness. This discovery, published in 1912, was based on painstaking measurements of 25 stars’ characteristics recorded at Harvard College Observatory when Leavitt was employed as a ‘calculator’, a lowly paid female astronomer who performed mathematical calculations for the Observatory’s research staff. Sadly she received little credit for her work on Cepheids during her lifetime.
It is an odd coincidence but Goodricke and Leavitt, both astronomers remembered for research on variable stars were each rendered deaf by illness.
The relationship between pulsation period and luminosity for Cepheids made them a perfect cosmic yardstick to measure the Universe. Astronomers now could easily tell how far away a Cepheid is just by observing how long it takes to cycle from minimum to maximum brightness. Until the 1920s many astronomers believed that our galaxy was essentially the entire Universe. Other galaxies were visible but were then thought to be ‘spiral nebulae’, relatively small objects inside the Milky Way. Then Edwin Hubble, a flamboyant character of an astronomer, recognized Cepheids in the Andromeda Nebula (M31), the largest of these spiral nebulae. Hubble did the calculations and discovered that the Andromeda Nebula was very distant indeed (2.2 million light years away is the accepted figure today); so far in fact that it could only be a galaxy in its own right. Extra-galactic Cepheids have helped astronomers to make highly accurate distance estimates to other galaxies.
Yet, embarrassingly, astronomers are ignorant of the detailed workings of Cepheids. The processes deep in their hearts which make these stars pulse so predictably are not fully understood. For more than four decades, astronomers have been aware that calculations of the masses for Cepheids give results significantly lower than expected from our theories of stellar evolution. What was needed was an accurate measurement of a Cepheid variable star’s mass.
Astronomers at the European Southern Observatory(ESO) have recently done just that. They searched for an eclipsing binary star system containing a Cepheid where, like Algol system, the less bright star’s orbit is edge-on with respect to Earth. Such star systems are very rare, none are known in the Milky Way. The team at ESO instead looked to the Large Magellanic Cloud, a small neighbouring galaxy, where they found OGLE-LMC-CEP0227. This system consists of a Cepheid orbited every 310 days by a larger, less luminous companion star. The astronomers watched the two stars orbiting and passing in front of one another, recording their brightness variations, and measured by spectroscopy both the speeds of the Cepheid’s surface movements as it pulsated and the orbital velocity of the companion star.
As a result we now know the orbital parameters, sizes and masses of the two stars with unparalleled precision. The Cepheid star’s actual mass is now known to an accuracy of about 1%. The value matches exactly the predictions of the theory of stellar pulsation, showing that the larger mass predicted by stellar evolution theory to be incorrect. Once the reason for this failure of theory is uncovered it will again be time to rewrite the textbooks.
(Article by Colin Johnston, Science Communicator)