Date: September 12th, 2012
Title: Astronomy Word of the Week : Cepheid
Podcaster: Dr. Christopher Crockett
Organization: United States Naval Observatory
Link: http://christophercrockett.com
http://earthsky.org/team/christophercrockett
Description: Astronomers have many tricks up their sleeves when it comes to measuring distances to stars and galaxies. Sometimes, variable stars come to the rescue. The astronomy word of the week is “Cepheid”.
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Today’s Sponsor: This episode of 365 days of Astronomy is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.
Transcript:
Astronomers have devised a number of ways to measure distances across the Universe. One such mechanism employs Cepheid variable stars. These stars exhibit a relationship between their average brightness and period of variability. Leveraging this property allows researchers to use them as “standard candles” which serve as distance beacons across intergalactic space.
Cepheids were discovered in 1784 and are named after one of the first of their type to be catalogued: Delta Cephei in the constellation Cepheus. In the centuries that followed, astronomers identified hundreds more Cepheids in our Galaxy alone. The star Polaris isn’t just famous for being the North Star, it is also the closest Cepheid variable to Earth.
In 1908, American astronomer Henrietta Swan Leavitt (info?), discovered something remarkable while studying variable stars in the Magellanic Clouds—satellite galaxies of the Milky Way. Leavitt noticed that the brighter Cepheids varied more slowly than dimmer ones. Her observations uncovered a tight relationship between a Cepheid’s luminosity and its period of variability. This revelation would go on to forever change how we view the Universe.
Measuring distances to the stars is a phenomenally tricky business. Our inability to stretch a tape measure across the cosmos makes nailing down the Universe’s third dimension a perennial thorn in the astronomer’s side. Our only direct way of measuring stellar distances—monitoring the subtle apparent shift (or parallax) of stars as the Earth moves around the Sun—only works out to about 10,000 light-years or so.
The Cepheid period-luminosity relationship lets us smash through this limit. If an astronomer measures how quickly a Cepheid is varying, she can turn that around to calculate the star’s intrinsic brightness. Comparing the calculated brightness to how bright the star appears from Earth then reveals how far away the star is.
To make this work, though, astronomers need to independently measure the intrinsic brightness of several Cepheids for calibration. We can do that by directly measuring the distance of some nearby Cepheids, like Delta Cephei, with some other method (like the aforementioned parallax). These close Cepheids provide absolute brightnesses which can then be used to measure far more distant ones. This trick of using one distance technique to calibrate another (and then another, and another…) is called the “cosmological distance ladder”. Each new technique builds on older ones and lets astronomers measure distances from the Moon to the edge of the Universe!
Leavitt’s discovery of the period-luminosity relationship vastly extended how deep into space we could determine distances. in 1924, Edwin Hubble used Cepheids to measure the distance to the Andromeda Galaxy. Up to that point, astronomers disagreed on the nature of the strange spiral nebula in Andromeda and others like it across the sky. Some argued that these were relatively nearby gas clouds; others thought they were whole other star systems lying beyond the edge of the Milky Way. Hubble’s Cepheid measurements put that debate to rest. At a reported distance of 900,ooo light-years, Andromeda was far beyond the outer reaches of our galaxy. Andromeda was, in fact, a separate galaxy. Cepheids, it could be argued, fundamentally altered our view of how the Universe is structured.
In fact, Cepheids helped reveal two astonishing tidbits about the Universe. One was that our Milky Way was just one galaxy among many. The other revealed itself when Hubble plotted the distances he was getting versus measurements of each galaxy’s velocity relative to us. The result was game-changing: not only was nearly every galaxy receding from the Milky Way, but their velocities were dependent on their distances! The most distant galaxies were receding the fastest, while the closest ones were doing so much more slowly. Cepheids provided Hubble with a crucial piece in revealing the uniformly expanding Universe: the first observational evidence of the Big Bang!
Of course, nothing is as simple as it first seems. Later generations of astronomers would discover that Cepheids can be classified into a few different groups, each with its own period-luminosity relationship. Not knowing this led to large errors in many of the early measurements: Hubble originally placed Andromeda two or three times closer to the Milky Way than its actual distance of over two million light-years. But science is an iterative process. Over the decades, our understanding of the nuances of Cepheids has matured, and with it our techniques for using them as cosmic rulers has been refined.
There are many types of variable stars and they all have something unique to tell us about stellar structure and evolution. But sometimes they reveal something much greater than themselves. Cepheids are a perfect example. They go beyond the physics of variable stars to help us understand the structure of the Galaxy and even the origin of the Universe itself.
End of podcast:
365 Days of Astronomy
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