All the stars that can be seen with the unaided eye from the earth belong to our galaxy. They are members of a flat spiral system that rotates around a massive center some 10,000 parsecs from the sun. (One parsec is 3.26 light-years.) It is difficult to study the internal motions of the galaxy directly because we are located in its central plane, which is clogged with interstellar dust and gas that at visible wavelengths obscure the galactic center and most of the more distant stars. Thus although we can study some of the motions of the galaxy from the observation of relatively nearby stars and from the radio waves emitted by distant clouds of hydrogen, in order to learn more about the dynamical behavior of galaxies we must turn to other systems. The nearest galaxy closely resembling our own is the Great Nebula in Andromeda, and its internal motions have recently been studied in considerable detail. Some of the results of these studies are quite unexpected.
On a clear night away from city lights in the Northern Hemisphere the Andromeda nebula is just barely visible to the unaided eye as a faint elongated. patch of light. It was described by the Persian astronomer Umar al-Sufi Abd-al-Rahman in the 10th century; it appeared on Dutch star charts in 1500. It was first observed with a telescope in 1612 by Simon Marius of Germany, who described it as resembling the light of a candle flame seen through translucent horn. In 1781 Charles Messier of France listed it as No. 31 in his catalogue of nebulous objects, and to this day the Andromeda nebula is also commonly known as Messier 31, or M3l.
William Parsons of England, better known as the Earl of Rosse, began observing M31 in 1848 with his 72-inch speculum-metal reflecting telescope. His journal of observations was published some 40 years later, in 1885. That same year a brilliant new star-a supernova appeared near the center of M31. Ultimately this star served as a link in a chain of reasoning that established that spiral nebulas were not nearby clusters of stars or clouds of gas but were stellar systems outside our own. Another link was provided by Sir William Huggins, who obtained the first spectrogram of M31 in 1890, and by Julius Scheiner, who first discussed the spectrum of M31 in 1899. Scheiner recognized that the spectrum arose from the light of many stars rather than from a glowing cloud of gas.
Modern observations of M31 date from 1914, when V. M. Slipher, using the 24-inch refracting telescope at the Lowell Observatory in Flagstaff, Ariz., determined that the solar system and the center of M31 are approaching each other at a speed of 300 kilometers per second. It is now known that most of this observed velocity reflects the motion of the sun around the center of our galaxy. The sun is rotating around the galactic center at a velocity of about 250 kilometers per second in the direction of M3l. If we could make observations from the galactic center, they would show that our galaxy and M31 are actually approaching each other at a rather modest speed: some 50 kilometers per second.