M81 Internal Motions, van MaanenInstalled 19 July 2003. Latest update 28 Aug 2013.
New material or changes are in bold.
The photo on the right was made with Mt. Wilson's 100 inch reflector. (Source NASA). This photo was digitally quantized into six brightness levels to form the background for the van Maanen information on the left.
Both photographs above are misleading to a degree because over-exposure in the central region of M81 makes it look as though that region is comprised of thousands or millions of stars (which may or may not
be true). To see a clearer picture of the central region, go to the Space.com article
For further evidence of a small (star like?) core for M81 see NRAO's page Radio Images of Galaxies. [Added 05 Sep 2003.]
Recommended sites for other photographs of M81.
[Page no longer available.]
M81 - Spiral Galaxy (Type Sb) - Multi-wavelength photos.
Note of Caution
"Hubble's preference for a recessional or expanding model is further revealed in a 1936 paper communicated to the National Academy of Sciences. In this paper he reviewed previous counts of nebulae and reported the results of two new surveys. The data, enlarged from that of the previous year, still pointed to the same conclusion: the observations more nearly fit the non-expanding model." [Emphasis added.]
It is fitting to quote van Maanen's tribute to James Jeans in the final two paragraphs of his 1921 paper (MA21) on Messier 81.
The close agreement of the displacements in direction with the spiral arms suggests that we have here a realization of the motions described by Jeans in Problems of Cosmonogy and Stellar Dynamics, from which I quote freely.
"If so, we must then suppose that before the formation of the spiral arms the nebular masses were rotating and had reached a lenticular shape. The formation of Laplace's ring requires perfect symmetry of mass about the axis of rotation. The distances of adjacent masses in space are in general so great that their gravitational influence will be extremely small, but even the slightest external gravitational field will be sufficient to preclude the formation of a ring; instead of this, the matter will will be thrown off at two antipodal points. The masses first thrown off at the antipodal points form in themselves a tide-generating system which concentrates the region of ejection more and more at two points. The result is an extension farther and farther into the equatorial plane as the evolution of the nebula proceeds. The determination of the shape of the arms seems at present to be beyond the reach of mathematical analysis, but the long streams of gaseous matter must become longitudinaly unstable and will tend to break up into condensations or nuclei." [Adams Prize Essay, University of Cambridge, 1917.]
Jeans' other publications on spiral nebulae (JJ17), (JJ23), and (JJ25) should be of interest to students of the History of Astronomy.
Readers are encouraged to see the Hubble Site 07 Feb 2002 release,
'Backwards' Spiral Galaxy". Future findings may invalidate the picture
but, at present, astronomers suggest that "the galaxy called NGC 4622
appears to be rotating in the opposite direction to what they expected."
The apparent direction of rotation is similar to that for van
Maanen's measured internal motions for M81 shown above.
See Jay Bitterman's Astronomy Bio... "Adriaan van Maanen" at http://www.bpccs.com/lcas/Articles/maanen.htm . [Added 27 August 2003. Article no longer online, 28 Aug 2013.]
Adriaan van Maanen (left), Bertil Lindblad (center), Unidentified man (right),
on the platform for observation at the Newton focus of the 1 meter reflector of
the Stockholm Observatory in Saltsjöbaden Sweden. August 1938.*
(Lindblad was Director of the Stockholm Observatory from 1927 to 1965.)
Photograph by Dorothy Davis Locanthi,
* Thanks to Per Olof Lindblad for the location and date of the photograph.
ReferencesHN71 - Hetherington, N., Astronomical Society of the PacificLeaflet 509, 473-480 (1971) - NADS
HB92 - Hetherington, N., Brashear, R., Journal for the History of Astronomy 23, 53-56 (1992).
JJ17 - Jeans, J., Internal Motion in Spiral Nebulae, The Observatory 40 60-61(1917) - NADS [No article or abstract]
JJ23 - Jeans, J., Internal Motions in Spiral Nebulae, MNRAS 84, 60-76 (1923) - NADS
JJ25 - Jeans J., Note on the Distances and Structure of the Spiral Nebulae, MNRAS 85, 531-534 (1925) - NADS
MA21 - van Maanen, Adriaan, Investigations on Proper Motion - Fifth Paper: The Internal Motion in the Spiral Nebula Messier 81, Astrophysical Journal 54, 347-356 (1921) - NADS