While other stars receive their light from the Sun (but have their glow covered by solar rays), the Milky Way is the glow of stars that are not directly visible due to Earth's shadow. [66] Aristotle himself thought that the Milky Way was a product of stars burning that did not evaporate due to its outermost position in the atmosphere (composing its enormous circle). He also thought that the Milky Way was a component of the Earth's upper atmosphere, together with the stars. He added that the Earth's atmosphere is responsible for the Milky Way Galaxy'sowing to refraction. [67] [68] [69] This theory was challenged by the Neoplatonist philosopher the Younger (c. 495–570 AD), who asserted that if the Milky Way were sublunary, it should appear differently at various times and locations on Earth.
In 1917, Curtis observed the nova S Andromeda within the Great Andromeda Nebula (Messier object 31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were independent galaxies. [86] [87] In 1920, the Great Debate took place between low Shape and Curtis 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 well as the significant Doppler shift. [88]
The Milky Way is one of the two largest galaxies in the Local Group (the other being the Andromeda Galaxy), although its galactic disc size and how much it defines the diameter are unknown. [11] According to an extrapolation of the exponential disk with the scale length of the inner disc, the significant bulk of stars in the galaxy lie within the 26 (80,000 light-years) diameter, and the number of stars beyond the outermost disc dramatically decreases to a very low number. [113] [11]
In astronomy, various approaches are used to define the size of a galaxy, and each of them might produce different answers when compared to one another. The most often used
The Milky Way galaxy has a total mass of 890 billion to 1.54 trillion times that of the Sun (8.91011 to 1.541012 solar masses), [7] [8] [9], with stars and planets accounting for only a minor portion of this. The mass of the Milky Way is estimated differently depending on the approach and data used. The bottom end of the estimation range is 5.81011 solar masses (M), which is somewhat less than the Andromeda Galaxy. [126][127][128] In 2009, measurements with the Very Long Baseline Array revealed velocities as high as 254 km/s (570,000 mph) for stars at the Milky Way's periphery. [129] Because the orbital velocity is determined by the overall mass within the orbital radius, this implies that the Milky Way is more massive, roughly equaling the mass of the Sun.
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