| Astrophysics (Index) | About |
The Kapteyn universe is an early (late 19th century/early 20th century) model of the universe developed by Jacobus Kapteyn based upon observed stars, which consisted of what is now known as the Milky Way galaxy. It was a model developed before it was established that the spiral nebulae seen throughout the sky were, in fact, extremely distant structures similar to the Milky Way itself. When this was realized, such nebulae were eventually termed galaxies, borrowing a term that was originally a synonym for what was then termed the Milky Way: a large portion of the night sky that was not as black as the rest. The Great Debate took place before this realization was well-established.
Kapteyn's size-estimate of his universe was smaller than the currently-known size of the disk of the Milky Way galaxy: he estimated a diameter of about 17 kpc rather than the current estimate of 40 kpc. Kapteyn discounted any significant effect of reddening, his own analysis having concluded reddening to be a minor factor. This would lengthen his distance-estimates based magnitudes, but his estimated total size was small because much of the Milky Way is obscured (extinction), reddening being a major contributor.