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On February 22, 1901, after racing through space for 1,500 years, the light from a nova burst into view in the constellation Perseus. Nova Persei reached a maximum brightness equal to that of Capella, the sixth-brightest star in the sky. Within a year, astronomers observed an expanding shell of gas, called the Firework Nebula, around the nova. This was the first nova to be studied using the new tool of spectroscopy. Yet, it would be decades before theories were developed to explain the meaning of this observational data.

In 1901, astronomers did not know how stars evolved or how long they "lived." Astronomy was mostly a mathematical science involved with measuring the positions, distances, movement, and brightness of stars. This emphasis began to change as astronomers examined spectra gathered in sky surveys of the 1890s. By comparing the spectra to those of hot gases in laboratories on Earth, astronomers discovered that stars were made of familiar elements. (Helium was first discovered on the Sun and then found in Earth's atmosphere in 1895.) Spectra offered another way (besides brightness, movement, and color) to distinguish one star from another. Some stars had many absorption lines, showing a range of elements in their atmospheres, while others had only a few. The brightness and spacing of the lines varied as well. Practitioners of what was called the new astronomy focused on the physical properties of stars that might explain these differences. The new astronomy became known as astrophysics.

Spectra were captured on glass photographic plates. To compare the spectra of one star with another and with itself over time, astronomers measured the location, width, intensity, and spacing between the often very faint lines. Much of this important work was done by Edward Pickering's "computers" at the Harvard College Observatory. These computers were not electronic but were actually a group of talented women headed by Williamina Fleming.

Fleming was originally hired as Edward Pickering's maid. When Pickering became frustrated with his male assistants' work classifying spectra at the observatory, he famously declared that his maid could do a better job. So in 1881, he hired Williamina Fleming. She devised and helped implement a system to classify stars by the amount of hydrogen in their spectra. The stars with the most were given the letter A, and the next-most a B, and so on. Later, Annie Jump Cannon (1863-1941) rearranged the spectra by temperature but kept the letter designations. In nine years at Harvard, Fleming catalogued the glass plates of more than 10,000 stars. In 1907, she published a list of 222 variable stars that she had discovered through changes in their spectra.

One of the most famous "new astronomers" was George Hale of the University of Chicago, who was honored with a Bruce Medal in 1916 (see page 249). He founded the Astrophysical Journal in 1895 and played a key role in forming the American Astronomical Society in 1899. Many new theories, techniques, observations, and methods to analyze them were published in the pages of the Journal or debated at society meetings. Hale was also pivotal in providing the community with world-class observing facilities to provide more accurate data. He founded the Yerkes Observatory in Wisconsin in 1897 and equipped it with the world's largest telescope. As the observatory's first director, he invented the spec-troheliograph, a device to take photos of the light of a single element in the Sun. (The sunspot cycle had been shown to be 11.1 years in 1851, but the cause of sunspots was not yet known.) The famous astronomer William Herschel had speculated much earlier that they were holes in the Sun and that the surface underneath was lush with vegetation. In 1905, Hale took the first photographs of a sunspot's spectrum. His observations proved that sunspots were cooler areas on the Sun and not the "holes" that Herschel had suggested.

To spread the spectrum out and observe finer details, Hale needed a larger telescope. The Yerkes 40-inch (1-m) telescope remains the biggest refractor ever built. Hale realized that a lens any larger would sag from its own weight or be so thick that it would block the light. What he needed was a reflecting telescope (see figure on page 13) that used a mirror instead of a lens.

Hale's father paid $25,000 to have a 60-inch (1.5-m) mirror made for his son. Yerkes Observatory could not afford to build the mounting and dome for this telescope, so the mirror was kept in storage for 12 years. In pursuit of better seeing conditions, Hale moved to sunny Pasadena, California, where he founded the Mount Wilson Solar Observatory in 1904. He moved the 40-inch (1-m) refractor there and obtained a $300,000 grant from the Carnegie Foundation to finish and install the 60-inch (1.5-m) reflector on Mount Wilson. When it went into operation in 1908, it was the largest telescope in the world. With it, Hale helped settle the Mars debate and provided the data for a coming revolution in astrophysics.

One of Hale's first discoveries in 1908 was that spectral lines from sunspots showed the Zeeman effect. This effect is named for a Dutch physicist, Pieter Zeeman (1865-1943), who discovered in 1896 that spectral lines split in the presence of a strong magnetic field. Hale found solar magnetic fields hundreds of miles across that persist long after the sunspot associated with them disappeared.

Binary and variable stars were also of special interest to astrophysicists. In the early 1800s, John William Herschel discovered that stars did


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© Infobase Publishing not just appear double because of their direction relative to Earth but because some orbited each other. In 1827, Mizar, the middle star in the handle of the Big Dipper and the first star identified as a double in 1650, was found to be two stars: Mizar A and Mizar B that orbit each other every 60 years. In 1889, Edward Pickering discovered that the absorption lines in the spectra of the atmospheres of binary stars double as one star moves away from Earth and the other moves toward Earth. Edward Pickering observed that Mizar A's spectrum had double lines, and thus was two stars, the first discovered spectroscopic binary. In 1908, E. B. Frost (1866-1935) found that Mizar B is also a spectroscopic binary. So Mizar is actually four stars revolving around each other.

Some spectroscopic binaries were found to eclipse one another and thus brighten and fade periodically. But other variable stars changed in brightness and had no companion to block their light. Delta Cephei, first identified as a variable star in 1784, did not have a companion. Yet, studies of its spectra showed that it went from bright to dim and back to the same brightness level every five days. Stars whose brightness

Refracting telescopes use lenses and reflecting telescopes use mirrors to gather and focus light.

Refractors are limited in size because of the weight of the lens.

varies in a periodic way like Delta Cephei are called Cepheid variables. The North Star, Polaris, is a Cepheid variable with a period of just under four days.

In 1908, Henrietta Leavitt, another of Edward Pickering's "computers," published "1777 variables in the Magellanic Clouds" in the Annals of Harvard College Observatory. The list included 16 Cepheid variables with periods between 1.25 to 127 days. She reported on page 107, "it is worthy of notice that . . . the brighter variables have the longer periods." Although her report of this relationship between brightness and period went unnoticed for many years, Henrietta Leavitt had made a discovery that would provide astronomers with a new way to measure distances in space.

The amount of accumulating data on stars led astrophysicists to see relationships between properties of stars that would lead to theories of stellar evolution. In 1905, Danish astronomer Ejnar Hertzsprung (18731967) observed that the brightness and spectral type (the letters A, B, and so forth that Williamina Fleming assigned) were correlated. All "O" stars were brighter than "A" stars, for example. At the same time, American Henry Norris Russell (1877-1957) used orbital data from binary stars to determine their densities. Being able to correlate density with brightness and spectral lines led directly to breakthroughs in understanding of stellar evolution in the next decade.

The collection of spectra of binary systems also led to a serendipitous discovery by German Johannes Hartmann (1865-1936) in 1904. In addition to the usual two sets of spectral lines that shifted back and forth as two stars orbited each other, he found a narrow line of ionized calcium that did not move. The line did not shift because it was not associated with the stars. Hartmann had discovered that the space between stars was not an empty void but contained a thin veil of gas.

Vesto Slipher at Lowell Observatory confirmed Hartmann's discovery in 1908 when he also found lines of calcium in the spectra of double stars. Slipher provided evidence for interstellar gas in three widely spaced regions of the Milky Way. In 1910, Slipher showed that the Pleiades in Taurus are bright because light from nearby stars reflects off a cloud of interstellar gas.

In 1906, Joel Stebbins (1878-1966), director of the University of Illinois Observatory, added to the tools that astrophysicists could use to study stars. He began to measure the brightness of astronomical objects using a photometer. In photometry, the light collected by the telescope is focused on a metal surface or container of gas called a photoelectric cell. This cell produces an electric current when struck by particles of light. The current can be amplified and measured to determine the brightness of the source. Before this, the only way to measure brightness was to compare an image of a new star with a known one.

Joel Stebbins used a selenium photometer to observe a lunar eclipse on July 24, 1907. In 1908, he measured the change in brightness of the eclipsing binary star Algol. In 1910, he studied Halley's comet. From 1909

to 1925, Stebbins and Mount Wilson astronomer Albert Whitford (19052002) continued to improve the photoelectric cell. Both men received Bruce Medals for their work (see page 249). As the sensitivity of the photometer increased, they were able to use it to measure the light of the solar corona during a total eclipse. Photometry would eventually become a more accurate method of measuring brightness than photography.

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