How Hot Is

Stars are too distant to stick a thermometer under their tongue. We can't even do that with our own star, the sun. But you can get a pretty good feel for a star's temperature simply by looking at its color.

As we discussed in Chapter 7, the temperature of a distant object is generally measured by evaluating its apparent brightness at several frequencies in terms of a black-body curve. The wavelength of the peak intensity of the radiation emitted by the object can be used to measure the object's temperature. For example, a hot star (with a surface temperature of about 20,000 K) will peak near the ultraviolet end of the spectrum and will produce a blue visible light. At about 7,000 K, a star will look yellowish-white. A star with a surface temperature of about 6,000 K—such as our sun—appears yellow. At temperatures as low as 4,000 K, orange predominates, and at 3,000 K, red.

So simply looking at a star's color can tell you about its relative temperature. A star that looks blue or white has a much higher surface temperature than a star that looks red or yellow.

Close Encounter

Career opportunities for women were limited at the turn of the century. However, women with specialized training in astronomy were able to find employment at the nation's observatories. The Harvard College Observatory first hired women in 1875 to undertake the daunting task of the classification of stellar types. Under the direction of Edward Pickering, the observatory employed 45 women over the next 42 years. Photography and the telescopes of the HCO were beginning to generate vast amounts of astronomical data, in particular photographic plates filled with individual stellar spectra. It was more economical to pay college-educated women to pore over these data sets than to hire the same number of male astronomers.

The observatory employed a series of women whose experience, intelligence, and familiarity with the data allowed them to make valuable contributions to astronomy. Williamina Fleming (appointed Curator of Astronomical Photographs in 1898) made the spectral classifications of the bulk of the stars in the first Henry Draper catalog. (Henry Draper was a wealthy physician whose widow gave money to the observatory in his memory.) Annie Jump Cannon rearranged the spectral classification of Fleming and, in the end, classified over 500,000 stars. She was so expert at classifying stellar spectra from photographic plates that she could classify them at a rate of three per minute, calling out the star name and its spectral type to an assistant. These catalogs of stellar spectra were invaluable to later astronomers, and the HD prefix is still used for stars originally classified by these women.

Finally, Henrietta Swan Leavitt made one of the most fundamental astronomical discoveries of the early part of the century. She was involved in the effort to catalog variable stars, and identified a number of so-called "Cepheid" variable stars, with variation in brightness that occurs over a period of 2 to 40 days. Much like massive bells that ring with a lower tone than a tiny bell, massive (intrinsically brighter) stars have a longer period than low-mass (intrinsically fainter) stars. This result meant that astronomers could make a leap from the timing of the period of a star's fluctuations in brightness directly to its intrinsic brightness. And knowledge of the intrinsic brightness of a star (and its observed brightness) let us know how far away the star might be. This discovery earned Leavitt a posthumous nomination for the Nobel Prize in 1925.

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