Notes Compiled by Paul Woodward Department of Astronomy University

Stars Notes compiled by Paul Woodward Department of Astronomy University of Minnesota This week, we will be discussing the way astronomers and astrophysicists have been able to develop a detailed understanding of the structure and evolution of stars. A key point to appreciate is the inability of astronomers to observe this evolution as it proceeds, since the time for a star to evolve from birth to death can be billions of years. We can only see a snapshot of the heavens during our lifetimes. We must therefore assume that much of the variety that we observe comes from catching the stars in different stages of their evolution. How we have sorted out which differences are due to evolution and which are due to fundamental differences in the objects involved is a fascinating story. The process of understanding the stars began very simply by trying to classify them in a number of different ways. The first classification was by apparent brightness. However, the brightness of a star is largely an accident of its distance from us. If our brightness categories are to have any close relationship with properties of the stars themselves, and not just of where they happen to be located, then we need to remove the distance factor. We need to classify the stars by luminosity, the amount of energy they radiate per unit time in all directions, not by apparent brightness. The total luminosity is the energy radiated per unit time in all wavelengths of the electromagnetic spectrum. The apparent brightness of a star decreases as the inverse square of the distance to the star. The luminosity of the star is independent of the distance to the star. It is an intrinsic property of the star itself, not of its position. Since we can only measure the apparent brightness directly, we must somehow also measure the distance to the star in order to calculate the star’s luminosity. If a star is near us, that is, within about 30 light years, then the most reliable method of measuring its distance from us is to observe its parallax. Recently, the Hipparcos satellite measured the positions and parallaxes of 120,000 stars with an accuracy of 0.002 arcsec. The apparent brightness of a star can be measured in Watts per square meter, or in units of the Sun’s luminosity, L . Some astronomers still use the ancient magnitude system, devised by Hipparchus (c. 190 - 120 B.C.). In this system, stars of first magnitude were brightest, stars of second magnitude somewhat less bright, and so on. The magnitude system has been modernized, and now the brightest star, Sirius, has a magnitude of -1 (two magnitudes brighter than 1). Absolute magnitudes are like luminosities. They are the apparent magnitudes that stars would have if they were placed at a distance of 10 parsecs from the Sun. A parsec is a distance at which a star has a parallax of one arc second. Thus a parsec is 3.26 light years. Once we get the distances to stars from their parallaxes, we can compute their intrinsic luminosities from their apparent brightness, and this allows us to begin to really learn something about the stars. In particular, we can learn how many stars there are in various luminosity bins. This is shown on the previous slide. We will see that when we put this together with other information, we can learn how many stars have what masses and how many stars are likely to be in which phases of their evolution from formation to “death.” We do not live long enough to watch stars evolve. Therefore we must observe a great many stars to put the story of what stars are and what they do during their “lifetimes” into a clear picture. The work of classifying the stars in order to get the essential information to do this was extremely tedious and was done largely by women, it turns out. But there is no other way to discover what the stars are. By engaging intelligent and dilligent women to do this work, male astronomers of the day gave up the opportunity to make many very important discoveries and ceded this fundamental role to women astronomers. They may not have seen it that way at the time, but we do so now. These women astronomers showed themselves perfectly able to make their own discoveries from their work, and they brought about great advances in our understanding of the stars. In addition to classifying stars by their luminosities, we can also classify them by their colors. Because of the properties of the black body spectrum discussed near the beginning of the course, the color of a star is fairly directly related to its surface temperature. The Color of Stars Figure 18-4 Universe – Kaufmann Blackbody Curves Figure 17-4 Universe - Kaufmann Wien’s Displacement Law – temperature vs. maximum intensity wavelength Colors of stars can be measured quantitatively and repeatably by measuring their luminosities in separate regions of the spectrum that are selected by colored filters. Still more information can be obtained by examining the spectrum of the star. Lines in the spectrum from highly ionized elements indicate high surface temperatures, while lines of molecules (which dissociate at high temperatures) indicate low surface temperatures. Astronomers have thus devised a classification system of spectral types, according to what lines of what strength are observed in the spectrum of a star. These spectral types are related directly to stellar surface temperature. In order of decreasing temperature, they are O, B, A, F, G, K, M. Measuring temperatures of Stars from spectra - composition of M stars Like many classification schemes in astronomy, this one for stellar spectra turned out to be weird because it was originally devised in a fairly complete absence of understanding of what the spectra meant. Only much later was it understood, and rationalized, but, characteristically, the classification scheme retained its original nomenclature. This now seems to us obscure. In the 1880s, Edward Pickering, of Harvard College Observatory, suggested that the stellar spectra be classified according to the strength of their hydrogen lines. Type A had the strongest hydrogen lines, then came type B, and so on to type O, with the weakest hydrogen lines. Pickering hired several women, among them Williamina Fleming (1857-1911) to classify the stellar spectra in this way, and Ms. Fleming classified 10,000 stars by 1890. In 1896, Annie Jump Cannon (1863-1941) joined Pickering’s team at Harvard. Building on the work of Fleming and of Antonia Maury, Cannon realized that the spectral classes fell into a natural order when more than just the hydrogen lines were considered, but this order was not the A,B,C order originally proposed. Some of the classes overlapped and some could therefore be eliminated. Cannon determined that the natural order of spectral classes was OBAFGKM. Cannon also added the numerical subclasses that we use today. During her lifetime she personally classified over 400,000 stars. She was the first woman ever awarded an honorary degree by Oxford University. It fell to Cecilia Payne-Gaposchkin to figure out why Cannon’s spectral sequence was the natural one. She was educated at Cambridge, and worked with Rutherford, but chose to come to the U.S. because Harlow Shapley at Harvard gave women the opportunity to play important roles in the science of astronomy (or at least more important roles than seemed available to women in science in England at the time). Payne-Gaposchkin showed that the differences between the spectra of the different classifications did not reflect different chemical compositions but instead reflected different ionization levels of the emitting atoms. O stars, for example, have weak hydrogen emission lines because at their high surface temperatures nearly all the hydrogen is ionized. Payne-Gaposchkin published her dissertation in 1925. Williamina Paton Stevens Fleming (1857-1911) Mina Stevens Fleming, the first to discover stars called "white dwarfs", was born May 15, 1857 in Dundee, Scotland. She attended public schools in Dundee and then taught in Dundee from age fourteen until her marriage to James Fleming in 1877. The couple emigrated to Boston when she was twenty-one. A year later she was abandoned by her husband while pregnant with their child. To support herself and the baby, Mina Fleming obtained work as a maid in the home of Prof. Edward Pickering, the director of the Harvard Observatory. Pickering was unhappy with the work performed by his male employees and declared that his maid could do a better job than they did. In fact, he hired her in 1881 to do clerical work and some mathematical calculations at the Observatory. Fleming soon proved that she was also capable of doing science. She devised a system of classifying stars according to their spectra, a distinctive pattern produced by each star when its light is passed through a prism. She used this system, which was later named after her, to catalog successfully over 10,000 stars within the next nine years. This work was published in 1890 in a book titled Draper Catalogue of Stellar Spectra. Her duties were expanded and she was put in charge of dozens of young women hired to do mathematical computations, the work nowadays done by computers. She also edited all publications issued by the observatory. The quality of her work was so superior that in 1898 Harvard Corporation appointed her curator of astronomical photographs. This was the first such appointment given to a woman. In 1906 she was the first American woman elected to the Royal Astronomical Society. In 1907 she published a study of 222 variable stars she had discovered.

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