The star RS Puppis, one of the brightest Cepheid variables in the Milky Way, imaged by Hubble. Credits: NASA, ESA, and the Hubble Heritage Team (Stsci/Aura)-Hubble/Europe Collaboration Acknowledgment: H. Bond (Stsci and Penn State University)
The name cepheids indicates three different categories of variable stars (stars whose brightness varies periodically with time) which have the characteristic of being excellent sample candles, whose individual distances can be estimated with an accuracy of a few percent. Classical cepheids pulsate with periods ranging from days to a few hundred days and are tracers of young stellar populations (less than 200-300 million years old) typically associated with star-forming regions, while type II cepheids pulsate with periods ranging from one day to a hundred days, they are tracers of old stellar populations (older than 10 billion years) and are mainly identified in the core and halo of our galaxy. Between these two main categories are the less numerous anomalous cepheids that pulsate with periods ranging from a few dozen hours to a few days, associated with stellar populations of intermediate age (a few billion years).
The paths of modern astrophysics and those of Cepheids have intertwined several times.
The first galactic Cepheid was discovered in October 1784 by a British amateur astronomer, John Goodricke, who, making regular observations with his telescope, realized that the delta star in the constellation Cepheus was a variable star. In reality the first Cepheid had been discovered the month before Edward Pigott and was the star Eta of the constellation Aquila. Edward was also an amateur astronomer and was not only young John’s neighbor but also his friend and mentor. He decided to take a step back and give young John (seventeen year old, deaf and dumb) the chance to announce his discovery of him first. If Edward had not made this great gesture of nobility of spirit today we would call them ‘aquileids’ and not ‘cepheids’.
On the left, Cepheid stars in the spiral galaxy NGC 5584 (credits: NASA, ESA, and L. Frattare/Stsci). On the right, original title page of Edwin Hubble’s article “Cepheids in Spiral Nebulae” (credits: Huntington Digital Library)
Next year marks the centenary of the publication of Edwin Hubble’s paper (1925) on the discovery of classical cepheids in the Andromeda Galaxy. This article marks the birth of observational cosmology and marks the end of the long-standing dispute between Curtis and Shapley regarding the nature of the so-called “nebulae”. Curtis was right: they were external galaxies similar to ours and not nebulae belonging to our galaxy.
It was thanks to Walter Baade’s (1956) discovery of stellar populations that it was realized that classical cepheids and type II cepheids obeyed two different period-luminosity (PL) relationships. This was a revolution of Copernican proportions, both in the estimate of the age of the universe (which for the first time was greater than the age of the Earth based on radioactive decays) and in its size. The PL relationship was discovered by Henrietta Leavitt in her study of the cepheids of the Magellanic Clouds (1912) and allows, once calibrated, to provide very accurate distances.
To these must be added many other relevant discoveries which have used Cepheids as lighthouses to investigate the rotation curve of our galaxy or as laboratories to provide very accurate estimates of the physical properties (evolutionary, pulsational) of stars in advanced evolutionary stages with masses that they range from just under a solar mass to about ten solar masses.
The revolutions and discoveries we have discussed are found in many texts on the history of astrophysics. It would be reasonable to think that Cepheids could be considered from an astrophysical point of view as old women on sunset boulevard. But this is a baseless inference. Cepheids continue to be at the crossroads of important astrophysical and cosmological problems.
The recent identifications of classical cepheids in eclipsing binary systems in the Magellanic Clouds have made it possible to measure their mass with an accuracy of one percent and to provide very stringent limits on the phenomena of mixing that happen within them. These same Cepheids were used to measure, for the first time with a geometric method, the distance of the Large and Small Magellanic Clouds with an accuracy of one and two percent.
Using two of the most powerful space telescopes in the world – NASA’s Hubble and ESA’s Gaia – astronomers have made the most precise measurements to date of the rate of expansion of the universe. This is calculated by measuring distances between nearby galaxies using special types of stars called Cepheid variables as cosmic meters. By comparing their intrinsic brightness as measured by Hubble, with their apparent brightness as seen from Earth, scientists can calculate their distances. Gaia further refines this metric by geometrically measuring distances to Cepheid variables within our Milky Way galaxy. This allowed astronomers to more precisely calibrate the distances to the Cepheids observed in external galaxies. Credits: NASA, ESA and A. Feild (STScI)
The Magellanic Clouds play a fundamental role in the cosmic distance ladder, because they are used as the first rung in determining the Hubble constant. The most recent results on the estimate of this constant suggest a tension at the eight-sigma level between the direct estimate based on a local sample of 42 type Ia supernovae calibrated with classical cepheids (H0=73.04±1.04 m/s/Mpc) and the estimate of the same constant based on the cosmic background radiation of the Planck satellite (H0=67.4±0.5 km/s/Mpc). This tension seems to suggest that, currently, our universe is expanding at a speed that is about 8 percent faster than predicted by the most accredited cosmological model (LambdaCdm). This difference, if confirmed, would imply not only an overcoming of the standard model but also an interesting opportunity for new physics.
The astrophysics community is moving along three different paths: a) verify the universality, and in particular the dependence on metallicity, of the PL relations used to estimate the distances of Cepheids; b) use sample candles tracers of ancient stellar populations such as type II Cepheids and the typ of the giant branch; c) search for possible systematic errors. This titanic effort is still underway, but there are good reasons to think that the new results of the Gaia satellite and the new survey spectroscopic tests from the ground will allow us to unravel the problem.
Measurement of Cepheid abundances are important not only for the cosmic distance scale but also for investigating the chemical enrichment history of our galaxy. Classical Cepheids are used to measure the chemical composition gradients of the thin disk in which our Solar System is located and help us understand how many chemical elements are formed, including those that underlie life – the so-called Chnops (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulphur).
It seems appropriate to underline that next year also marks the centenary of the publication of the doctoral thesis of Cecilia Payne, the first woman to obtain a doctorate in astrophysics. She uses the Saha equation for the first time to determine stellar abundances and demonstrates that the most widespread element in the universe is hydrogen. The approach she takes paves the way for quantitative spectroscopy and nucleosynthesis.
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