Images being compared: Ritchey 1910(1) - Pilato 2003(2)
van Maanen's comparison stars(3) are annoted in red.
Velocity vectors for van Maanen's objects are shown in red and their lengths
correspond to the distance the objects are calculated to travel in 2500 years.
Selected serendipity objects are annotated in green.
M33 Blink Comparators by Quadrant
In the 93 years between 1910 and 2003, the measured proper motion of van
Maanen's object No. 18, which appears to be a gaseous region, should have
moved it 1.3 arcsec West and 2.2 arcsecs North. The calculated 2003 location
of object 18 is marked on the 1910 image by a short line
perpendicular to the object's velocity vector. Object 18
seems, however, to have moved in the opposite direction, i.e. to the South
and East. This may turn out to be a piece of coroborating evidence relating
to reports (Refs pending.) from the 1920s that internal motions in spiral
nebulae were indeed astrometrically measurable but that their directions
were, in general, opposite to those that van Maanen reported.
There doesn't seem to be any obvious motions for van Maanen's objects 21 and 22.
aa - At present, the author doesn't know whether object aa is a field star,
or is associated with M33, or even if the respective images actually correspond
to a single object. If the connection is real, then during the 93 years between
photos, the object has moved 2 arcsec in RA and -2 arcsec in DEC (+/- 0.5 arcsec).
That equates to a proper motion of 21 +/- 5 mas/yr in both RA and DEC.
ab - Proper motion: 0 mas/yr RA, -4 mas/yr DEC. Field star?
ac - Semi-stationary opaque hydrogen cloud?
(1) Hubble, Edwin, "A Spiral Nebula as a Stellar System," Contributions from the
Mt. Wilson Observatory," No. 310, 1 (1926).
Reprinted from the Astrophysical Journal,
63, pp. 236-274, (1926).
(2) 2003 astrograph of M33 courtesy of
Dark Star Images by Tony Pilato.
(3) van Maanen, A., "Investigations on Proper Motion - Tenth Paper: Internal Motion
in the Spiral Nebula Messier 33, N.G.C. 598" - ApJ 57, 264-278 (1923) -
- (van Maanen's Plate XV is an annotated version of Milton Humason's 1922 photograph of M33.)