Adrian van Maanen and the Rotation
of the Spiral Nebulae
In the early 1900’s, astronomers were probing deeper and deeper into the observable universe. Techniques to image the sky were becoming well developed and professional observatories were routinely taking photographic plates every night. These plates were being used routinely for stellar parallaxes and spectroscopic studies. The first stellar parallax was made in 1844 by Fredrick Bessel and many more followed. But the parallax technique was limited to nearby stars, and most nearby stars had larger proper motions, and proper motions could only be determined after a baseline of many years.
Spectroscopy, which was applied to astronomical studies in the nineteenth century, held the key to many procedures for estimating distances. Working with a sample of stars with known distances determined from the parallax technique, in 1905, Henry Norris Russell and Ejnar Hertzsprung, working independently on different continents, discovered a direct relationship between a stars color and absolute brightness. In honor of both astronomers, this relationship has been called the H-R diagram and is the basis of nearly all distances in the Universe.
Distances to open clusters were extrapolated outward to determine distances to globular clusters using the period-luminosity relation (discovered by Henrietta Leavitt) found by studying Cepheid variable stars. Other astronomers were trying to determine the structure of our galaxy (which was thought to be the known Universe) and locate the relative position of the Sun in the vast array of stars.
Early Models of the Galaxy
In the time period 1900-1920, two astronomers made news about the size of the observable Universe. J.C. Kapteyn used statistics and proper motions to determine the distances to stars between successive magnitudes. He found a model for the Galaxy to be a flattened disk 10 kpc in diameter and 2 kpc in thickness with the Sun slightly off center. Kapteyn also knew that his results were based on the assumption that the light coming from stars was not absorbed. (As an example, when light is absorbed in the Earth’s atmosphere, some of it is absorbed and thus not all the light reaches the ground. The air that absorbs the light gets heated, and the rest makes it to the ground, heating the ground. As the light is absorbed, it loses energy and turns redder than it was before it entered the atmosphere, which is why sunrises and sunsets are orange red.)
Now if the stars light were suffering from some absorption, the effect on the distance would be greatest on the most distant stars thus making the Galaxy larger and have a much lower density. Kapteyn knew that even moderate amounts of absorption would negate many of the conclusions of his analysis. Kapteyn began studies of absorption and his results were inconclusive, thus by 1918 he determined that absoption was a negligible effect on his Galactic system theory. The “Kapteyn Universe” – as his model was often called, was at least a rough approximation of the structure of our Galaxy.
Harlow Shapely in 1914 began studying globular clusters and noted they were widely distributed above and below the galactic equator. Yet he found they were largely concentrated around one hemisphere in galactic longitude. Shapely noticed that this skewing in galactic longitude was unique to globular clusters and no other type of object – nebulae, open clusters, double stars, etc. Shapey determined the distances to the globular clusters using a the newly found technique, the period-luminosity relation. In 1916, he found the distance to M13 as 30kpc, which placed well outside the size of Kapteyn’s Universe. Shapely also discovered by deductive reasoning that since the globular clusters were of probably similar size, and if they are outside of our galaxy but associated with it, then their distribution might suggest that we - not they – are in a skewed position !! Perhaps the Sun is located toward the edge of an enormous system, 5 times larger than previously thought, defined by the globular clusters.
During this time period, photographic patrol plates routinely showed “spiral nebulae” of which the nature was unknown. Some astronomers postulated that they were island universes just like our own Galaxy (but they had no proof) and most astronomers suspected they were part of our own Galaxy since just like other nebulae and clusters. But recent studies had shown them to have very high radial velocities. If only astronomers could determine the distance to these spiral nebulae, their nature could be tied into the structure of the Galaxy.
By accident in 1917, while George Ritchey at Mt. Wilson was taking long exposures of spiral nebulae to measure their proper motions and rotation, he found a nova of apparent magnitude +15. Soon other observatories were searching their plate collections and in 2 months, 11 more nova were discovered, many by Heber Curtis, who was also photographing spiral nebulae in a long term program to determine their proper motions, and thus get a handle on their direct distances. But Curtis had made an even more remarkable result – he found that there was an average difference of 10 magnitudes between galactic novae and those novae found in the spiral nebulae. If this was the case, then if spiral novae were 10 magnitudes fainter, then they were 100 times further distant than galactic novae. Curtis even suggested that if there were absorption in the spiral nebulae, then they would be actually further away. This novae research supported the island universe theory that the spiral nebulae were stellar systems just like our own and thus the observable Universe was at least 10 – 15 times larger than previously thought by anyone.
At the same time Curtis was carrying out his novae studies, a Dutch astronomer with a high degree of credibility, Adrian van Maanen, a student of Kapteyn, was making measurements that were to be used as strong evidence against the island universe view of the spirals. van Maanen had measured internal motions of spirals in later years and his results indicated that the spiral nebulae were rotating. His results for internal motions in M33 are shown below in Figure 1.
Figure 1. M33 internal motions found by Adrian van Maanen in 1923. The arrows indicate the direction of motion of individual stars.
Van Maanen measurements of several other spiral nebulae were fairly consistent – the spirals seemed to rotating and his measured parallaxes of the spirals placed them locally inside our galaxy. For example, van Maanen’s parallax of M31, the Andromeda Galaxy was 0.004 ± 0.005" which placed it at a distance of 250 pc. But notice the error of 0.005" which is larger than the actual value of the parallax. Here lies one of van Maanen’s problems: many of his measurements were at the limit of detectable precision, which means they could be spurious. Van Maanen’s average finding of the rotational motion of spirals seemed to hover around 0.020 radians/yr, (1 radian = 57.3°) indicating a complete rotation in less than 1,000 years.
Van Maanen’s efforts of measuring internal motions of the spiral nebulae was in held high regard by the astronomical community, and rightfully so. Earlier in 1912 he was appointed to the staff of Mt. Wilson Observatory where he began measuring proper motions and parallaxes of stars. He was ideally suited for this work not only because of his dissertation (The Proper Motions of 1418 Stars in and Near the Clusters h and χ Persei), but also because of his work at Yerkes during 1911-1912 had been involved in making such measurements. All of van Maanen’s spiral nebulae measurements were done with a blink stereo comparator, (similar to one used by Clyde Tombaugh in his discovery of Pluto).
Enter Joel Stebbins who in 1924 studied the spectroscopic data on the spirals and concluded that they were indeed rotating – but in the opposite direction that van Maanen’s proper motions indicated ! Stebbins spectroscopic results showed the spirals winding up, while van Maanen’s measures showed them to be unwinding. Another observer studying internal motions of M51 was W. Schouten. He found internal motions in the spirals by careful measurements – but again in the opposite direction of van Maanen ! It seems that every observer who attempted to measure the internal motions of the spiral nebulae came up with different results.
By 1921, the only two astronomers supporting the island universe theory (the galaxies were at huge distances and not local objects) theory was Curtis and Knut Lundmark, a German astronomer. Lundmark made his own studies of the internal motions of the spirals and his results of internal motions came up negative – just random motions as one would expect for a distant object. See Figure 2 below of Lundmark's measurements of M33.
Figure 2. M33 internal motions as measured by Lundmark. Notice the random arrows. Compare this to van Maanen’s results above.
Lundmark rejected van Maanen’s results based upon his own studies of nebular motions and he concluded that van Maanen’s measurements of internal motions arose out of personal error.
Van Maanen attacked Lundmark’s conclusions by questioning several of his assumptions. He also reaffirmed his results by discussing possible sources of error in the telescope, the plates, and the measuring instruments. Van Maanen was convinced that the internal motions he found in the spirals was real and not the result of some sort of personal systematic error. In the 1920’s, Van Maanen’s work was widely accepted for a number of reasons. He was a staff member of one of the world’s finest observatories, had use of the finest astronomical equipment at his disposal, and he was renowned as a meticulous observer, moreover he was supported by prominent scientists.
The true check of van Maanen’s results came later that decade when Edwin Hubble announced the discovery of Cepheid variables in M31. Hubble found Cephieds in other galaxies also and was able to show a direct relation between the galaxies distance and the redshift. The spiral nebulae were confirmed to be “island universes” (galaxies just like our own) and at huge distances, so there was no possible way van Maanen could have seen rotational motion. What did van Maanen see? He was measuring quantities on the verge of delectability of his techniques and instrumentation. More than likely a personal systematic error plagued his results.
Further information can be found from:
Berendzen, R., Hart, R., Seeley, D., 1976, Man Discovers the Galaxies, Science History Publications, a division of Neale Watson Academic Publications, Inc., 156 Fifth Avenue, New York, 10010, ISBN 0-88202-023-4