Our Milky Way Galaxy has three major divisions:

(1) The disk, which contains gas and newly formed population I stars, dust, and stars of all ages. Most of the star formation and the bulk of the interstellar gas and dust is tied up in giant molecular clouds which are interspersed with large hot "bubbles". The molecular clouds are the seats of star formation and trace out a spiral arm structure. The 21cm line shows us that the atomic hydrogen is confined to a thin (100pc) disk extending some 25kpc from the center. Most of the dust is found inside the Sun's orbit which lies 8.5 kpc from the center, orbital speed of 250 km/s and has a period of about 200 Myr.

(2) A nuclear bulge composed of a dense packing of old metal deficient population II stars, and hot ionized gas moving hundreds of km/s around... a black hole?

(3) A spherical halo consisting of very old population II stars, globular clusters and RR Lyrae stars. Gas at temperatures of a million Kelvin called the galactic corona probably ejected from the disk via supernova explosions is found in the inner galactic halo.

The mass of the Galaxy inferred from the galactic rotation curve is a trillion (10¹²) solar masses, only ten percent of which an be "seen" by any form of electromagnetic radiation. The other 90% is called dark matter and is of unknown composition unless that X-files episode is correct. Astronomers have proposed brown dwarfs (not likely in view of a recent MACHO search relying on gravitational lensing), white dwarfs (not likely in view of a recent Hubble Space Telescope result), massive black holes, magnetic monopoles (do they exist?), neutrinos (it seems they do have mass but not enough), or something really exotic.

The earliest and best known catalogue of 103 nebulous sky objects was prepared in 1781 by Charles Messier as an aid for comet seekers. Today we know the Andromeda galaxy as M31 and the great nebula in Orion as M42. M1 is the Crab Nebula in Taurus, a supernova remnant and home of the first discovered pulsar. Around 1800 William Herschel prepared three catalogues containing 2500 nebulae and in 1864 John Herschel published the General Catalogue of Nebulae. This GC was revised and enlarged to cover 7840 nebulae and clusters by J. Dryer in 1888 and today most bright nebulae are known by their numbers in Dryer's New General Catalogue. So the Andromeda galaxy is known as "the great galaxy in Andromeda", M31, and NGC224. Two supplements to the NGC known as the first and second Index Catalogue's were produced in 1895 and 1908 covering some 15,000 objects. Some were correctly identified as star clusters, some as gaseous nebulae, but most were unexplained.

Immanual Kant knew about the unexplained patches of light, in 1755 he wrote The analogy with the system of stars we find ourselves -- is in perfect concept that these objects are just (island universes) -- in other words, Milky Ways. But astronomers didn't like this idea and their true nature remained a subject of hot debate until 1924.

By 1920 evidence was very strong but still hotly debated over the nature of certain faint patches of light or nebulosities showing elliptical or wheel shaped symmetry that avoided the Milky Way as opposed to other irregular amorphous nebulae such as Orion's sword (M42) that abound in the Milky Way. Were they within or outside like Kant's Island Universes. H. D. Curtis and Harlow Shapley had a "great debate" before the national Academy of Sciences, Curtis supported the island universe theory and Shapley opposed it.

The controversy ended when Cepheid variables, known at that time to be brighter than absolute magnitude -1, were found in M31/NGC224, M33/NGC598 and NGC6822 at apparent magnitude +18 by Hubble who used the new 100 inch telescope at mount Wilson in 1923 and 1924. Now how far away are these island universes or galaxies?