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In 1923, following the Great Debate, it became clear that many "nebulae" were in fact galaxies far from the Milky Way. Slipher and Edwin Hubble continued to collect the spectra from many different nebulae, finding 29 that showed emission spectra and 33 that had the continuous spectra of star light. [18]
Box Nebula NGC 6445: 1786 4.5 11.2 Sagittarius: Eye of Sauron Nebula M 1-42: 10 14 Sagittarius
The theory was developed by Immanuel Kant and published in his Universal Natural History and Theory of the Heavens (1755) and then modified in 1796 by Pierre Laplace. Originally applied to the Solar System , the process of planetary system formation is now thought to be at work throughout the universe .
The following articles contain lists of nebulae: List of dark nebulae; List of diffuse nebulae; List of planetary nebulae; List of protoplanetary nebulae;
In this map of the Observable Universe, objects appear enlarged to show their shape. From left to right celestial bodies are arranged according to their proximity to the Earth. This horizontal (distance to Earth) scale is logarithmic.
Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances. Starting from the 1990s, Hubble Space Telescope images revealed that many planetary nebulae have extremely complex and varied morphologies. About one-fifth are roughly spherical, but the majority are not spherically symmetric.
1923 – Edwin Hubble measures distances to a few nearby spiral nebulae (galaxies), the Andromeda Galaxy (M31), Triangulum Galaxy (M33), and NGC 6822. The distances place them far outside the Milky Way, and implies that fainter galaxies are much more distant, and the universe is composed of many thousands of galaxies.
In 1920 a debate took place between Harlow Shapley and Heber Curtis, the Great Debate, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as ...