How can a cepheid variable indicate distance

As we saw, the difference between the two allows them to calculate the distance. Leavitt discovered hundreds of variable stars in the Large Magellanic Cloud and Small Magellanic Cloud , two great star systems that are actually neighboring galaxies although they were not known to be galaxies then.

A small fraction of these variables were cepheids Figure 3. Figure 2: Henrietta Swan Leavitt — Leavitt worked as an astronomer at the Harvard College Observatory. While studying photographs of the Magellanic Clouds, she found over variable stars, including 20 cepheids. Since all the cepheids in these systems were at roughly the same distance, she was able to compare their luminosities and periods of variation. She thus discovered a fundamental relationship between these characteristics that led to a new and much better way of estimating cosmic distances.

These systems presented a wonderful opportunity to study the behavior of variable stars independent of their distance. For all practical purposes, the Magellanic Clouds are so far away that astronomers can assume that all the stars in them are at roughly the same distance from us.

Of course, if you are in Los Angeles, you will notice annoying distances between the suburbs, but compared to how far away New York City is, the differences seem small. If all the variable stars in the Magellanic Clouds are at roughly the same distance, then any difference in their apparent brightnesses must be caused by differences in their intrinsic luminosities.

Figure 3: Large Magellanic Cloud. It was in this galaxy that Henrietta Leavitt discovered the cepheid period-luminosity relation. Leavitt found that the brighter-appearing cepheids always have the longer periods of light variation. Thus, she reasoned, the period must be related to the luminosity of the stars. When Leavitt did this work, the distance to the Magellanic Clouds was not known, so she was only able to show that luminosity was related to period.

She could not determine exactly what the relationship is. To define the period-luminosity relation with actual numbers to calibrate it , astronomers first had to measure the actual distances to a few nearby cepheids in another way. This was accomplished by finding cepheids associated in clusters with other stars whose distances could be estimated from their spectra, as discussed in the next section of this chapter.

But once the relation was thus defined, it could give us the distance to any cepheid, wherever it might be located Figure 4. Here at last was the technique astronomers had been searching for to break the confines of distance that parallax imposed on them. Cepheids can be observed and monitored, it turns out, in many parts of our own Galaxy and in other nearby galaxies as well. In the s, Edwin Hubble made one of the most significant astronomical discoveries of all time using cepheids, when he observed them in nearby galaxies and discovered the expansion of the universe.

As we will see, this work still continues, as the Hubble Space Telescope and other modern instruments try to identify and measure individual cepheids in galaxies farther and farther away. The most distant known variable stars are all cepheids, with some about 60 million light-years away. The brief life of John Goodricke is a testament to the human spirit under adversity.

Born deaf and unable to speak, Goodricke nevertheless made a number of pioneering discoveries in astronomy through patient and careful observations of the heavens. Figure 5: John Goodricke — This portrait of Goodricke by artist J.

Scouler hangs in the Royal Astronomical Society in London. There is some controversy about whether this is actually what Goodricke looked like or whether the painting was much retouched to please his family. Born in Holland, where his father was on a diplomatic mission, Goodricke was sent back to England at age eight to study at a special school for the deaf.

He did sufficiently well to enter Warrington Academy, a secondary school that offered no special assistance for students with handicaps. His mathematics teacher there inspired an interest in astronomy, and in , at age 17, Goodricke began observing the sky at his family home in York, England. Most stars, including Cepheids, are in hydrostatic equilibrium where there is a balance between the inward force of gravity and the outward pressure of the energy the star radiates.

As stars evolve, that equilibrium can be perturbed and in some circumstances leads to stable oscillations, as in the case of Cepheids. For mechanical systems including those regulated by gravity , the natural period of oscillation is largely controlled by the average density, which is mass divided by volume or equivalently the cube of the radius. The bottom line is that low-density stars have longer periods. And variables like Cepheids also tend to have larger radii.

Larger radii translate into larger surface areas, which for a fixed surface brightness means higher luminosity. Longer-period Cepheids will then have higher luminosities. Periods predict luminosities. Snapshot : Live fast, die young. Target a changing crown jewel. Hubble spots heavy, ancient, metal-filled stars pulsing away. Galaxy quakes could improve hunt for dark matter. New infrared view of the Trifid Nebula reveals new variable stars far beyond.

Cosmos: Origin and Fate of the Universe. Astronomy's Moon Globe. Galaxies by David Eicher. Astronomy Puzzles. Jon Lomberg Milky Way Posters. Astronomy for Kids. Want to leave a comment?

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