The Temperatures of Stars Image credit: NOAO
Understanding Stars Starlight contains a lot of information. This information doesn t decay or expire as the light travels through space. By examining the light of distant objects (stars, galaxies, or anything else), we can learn a great deal about the objects without touching them. Today we understand the information that light carries, but that wasn t the case 100 years ago. It all began with a group of women at Harvard College Observatory about 1880.
Pickering and his Computers The Observatory director Edward Pickering hired women to work as computers : assistants to make measurements and do computing tasks.
The Harvard Computers At the same time, the Observatory received funding from a wealthy donor to complete a large survey of stellar spectral types: the Henry Draper Survey. The work of collecting measurements, cataloging the results, and analyzing them fell to the female computers. Since they could be paid half what men earned, Pickering could hire twice as many. Some of the women computers had studied astronomy at nearby colleges in New England. Others had no formal education at all. None of them could teach at the colleges as professors.
The Harvard Computers Advances in telescopes and photography allowed the Harvard computers to record the spectra of nearly 1 million stars and classify each. A spectrum is the range of colors emitted by an object that contains information on the object s temperature, chemical composition, or motion. The Draper Catalog provided a wealth of data that would reveal much about the nature of stars.
A Stellar Spectrum A stellar spectrum is produced by taking starlight and spreading it out into its constituent colors, such as: Notice that some colors are less intense (fainter) than other colors. These dark lines are known as spectral lines. The Harvard astronomers were trying to explain why stars had certain patterns of lines.
Understanding Stellar Colors First hired by Pickering to be his housekeeper, Williamina Fleming was later made an assistant and eventually supervisor for the other women. Once the Draper survey began, Fleming was given the huge task of photographing, cataloging, and analyzing about 10,000 stars over 9 years. She developed her own labeling system for the spectra using the letters A-Q. At the time, no one understood the differences between spectra.
Understanding Stellar Colors Annie Jump Cannon graduated from Wellesley and Radcliffe Colleges, studying astronomy & physics. In 1896, Cannon joined the staff at Harvard and began improving on the system that Fleming had developed while also recording more stellar spectra and classifying them by eye. Cannon discovered a new sequence that simplified the lettering to O B A F G K M. This is the system in use today. Cannon analyzed 400,000 stars in her career.
Stellar Spectra Here are other stellar spectra: Credit: NOAO/AURA/NSF Two things to note: (1) blue-to-red brightness shift, (2) change in the dark spectral lines
Stellar Spectra Annie Jump Cannon also divided each letter into 10 sub-types: 0 through 9 A G5 star is hotter than a G6 star. A G9 star is hotter than a K0 star. Under this scheme, the Sun is a G2 star. Spectral classification has become so precise that decimal subclasses are assigned occasionally; such as, an F3.5 star or an M5.5 star.
Stellar Spectral Types O and B stars are very hot ( > 10,000 Kelvin) and have a bluish-white color. A and F stars are cooler (6000 10,000 Kelvin) and have a white color. G and K stars are cooler still (3500 6000 Kelvin) and have a yellowish or orange color. M stars are coolest (< 3500 Kelvin) and appear a pale red color.
Stellar Spectral Types Simply by recording the color of a star you can make an estimate of the star s temperature. More precise measure of temperature requires looking at the dark lines present in the spectrum.