The Lake County Astronomical Society

Astronomy Bio...Alfred H. Joy

Jay Bitterman

Alfred Harrison Joy was an American astronomer most famous for his work on stellar distances, the radial motions of stars and variable stars. He was born on September 23, 1882 in Greenville, Illinois, the son of a merchant of New England ancestry. He was educated locally, obtaining his Ph.D. from Greenville College in 1903. For the next 10 years (1904 to 1914) he was a teacher at the American University of Beirut, in Lebanon, and then a Professor of Astronomy and Director of the Observatory. In 1914 he returned to the United States to work at the Yerkes Observatory as an instructor for a year. While at Yerkes Observatory he was also involved in a program of measuring stellar distances, using the 40-inch (1m) Yerkes refractor, by direct photography and parallax measurements, which was an extremely tedious and out-of-date method

In 1916 he joined the staff of Mount Wilson Observatory where he began to make spectroscopic observations and he was subsequently invited to take part in their research program to obtain stellar distances. A new method of finding the distance to stars was to compare the apparent magnitude of the star with its absolute magnitude. For more than 20 years Joy continued with this program, and in collaboration with his colleagues Walter Adams (1876-1956) and Milton Humason (1891-1972), their results enabled them to ascertain the spectral type, absolute magnitude, and stellar distance of more than 5,000 stars.

Joy and his colleagues also studied the Doppler displacement of the spectral lines of some stars to determine their radial velocities. By noting the variations in radial velocity, he and his team were able to show that many stars are spectroscopic binary stars and that their period and orbit can be interpreted. From their observations of eclipsing binary stars they deduced the absolute dimensions, masses and orbital elements of some specific stars within eclipsing binary systems. Using radial velocity data of 130 Cepheid variable ,stars, they determined the distance and direction of the center of the Galaxy and calculated the rotation period for bodies moving in circular orbits at the distance of the Sun, with a view to calculating the rotation period of the Galaxy.

He spent many years of his life observing variable stars and classifying them according to their characteristic spectra. While studying the long-period variable star, Mira Ceti, he observed the spectrum of a small hot companion object, that was later named Mira-B and determined to be a white dwarf, it can be seen visually today. Joy later became interested in the parts of the Galaxy where dark, absorbing clouds of gas and dust exist, and by carefully observing these areas he found examples of a particular kind of variable star, called a T-Tauri star, which is strongly associated with these areas. T-Tauri stars have a wide range of spectral types combined with characteristically low magnitudes. He showed that these characteristics indicated that they were very young stars in an early stage of their evolutionary history.

Joy remained at Mount Wilson Observatory until 1952. In 1946 he became Vice-President of the American Astronomical Society and then, in 1949, it's President. He died in 1973.