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The Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble, and others. Edwin Hubble discovered that most nebulae are associated with stars and illuminated by starlight.
Planetary nebulae are generally faint objects, and none are visible to the naked eye. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula, observed by Charles Messier in 1764 and listed as M27 in his catalogue of nebulous objects.
1912 — Vesto Slipher's spectrographic studies of spiral nebulae find high Doppler shifts indicating recessional velocity. 1917 — Heber Curtis finds novae in Andromeda Nebula M31 were ten magnitudes fainter than normal, giving a distance estimate of 150,000 parsecs supporting the "island universes" or independent galaxies hypothesis for spiral nebulae.
To estimate the age of the Solar System, scientists use meteorites, which were formed during the early condensation of the solar nebula. Almost all meteorites (see the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, suggesting that the Solar System must be at least this old. [141]
Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
Safronov's ideas were further developed in the works of George Wetherill, who discovered runaway accretion. [1] While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe; as of 24 July 2024 astronomers have discovered 7,026 extrasolar planets in our galaxy. [14]
Instead of large gas clouds collapsing to form a galaxy in which the gas breaks up into smaller clouds, it is proposed that matter started out in these “smaller” clumps (mass on the order of globular clusters), and then many of these clumps merged to form galaxies, [3] which then were drawn by gravitation to form galaxy clusters. This still ...
1802 – Due their star-like apparience, William Herschel suggested Ceres and Pallas, and similar objects if found, be placed into a separate category, named asteroids, although they were still counted among the planets for some decades. [129] 1804 – Karl Ludwig Harding discovers the asteroid Juno. [130]