The Andromeda Galaxy was later independently noted by Simon Marius in 1612. In 1734, philosopher Emanuel Swedenborg in his Principia speculated that there may be galaxies outside our own that are formed into galactic clusters that are minuscule parts of the universe that extends far beyond what we can see.
In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.Available in English translation by Ian Johnston at: Vancouver Island University, British Columbia, Canada The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky.
In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.Available in English translation by Ian Johnston at: Vancouver Island University, British Columbia, Canada The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky.
In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae.
He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture. In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition.
He found that the majority of these nebulae are moving away from us. In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier object M31, was then known).
Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center.
He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent 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 well as the significant Doppler shift. In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.
In 1936 Hubble produced a classification of galactic morphology that is used to this day. === Modern research === In 1944, Hendrik van de Hulst predicted that microwave radiation with [line|wavelength of 21 cm] would be detectable from interstellar atomic hydrogen gas; and in 1951 it was observed.
In 1936 Hubble produced a classification of galactic morphology that is used to this day. === Modern research === In 1944, Hendrik van de Hulst predicted that microwave radiation with [line|wavelength of 21 cm] would be detectable from interstellar atomic hydrogen gas; and in 1951 it was observed.
In 1936 Hubble produced a classification of galactic morphology that is used to this day. === Modern research === In 1944, Hendrik van de Hulst predicted that microwave radiation with [line|wavelength of 21 cm] would be detectable from interstellar atomic hydrogen gas; and in 1951 it was observed.
With improved radio telescopes, hydrogen gas could also be traced in other galaxies. In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas.
Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter. Beginning in the 1990s, the Hubble Space Telescope yielded improved observations.
As of March 2016, GN-z11 is the oldest and most distant galaxy observed.
This followed a 2016 estimate that there were two trillion () or more galaxies in the observable universe, overall, as many as an estimated stars (more stars than all the grains of sand on planet Earth).
Particularly, galaxy surveys in the Zone of Avoidance (the region of the sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies. A 2016 study published in The Astrophysical Journal and led by Christopher Conselice of the University of Nottingham used 20 years of Hubble images to estimate that the observable universe contains at least two trillion () galaxies.
It has a comoving distance of 32 billion light-years from Earth, and is seen as it existed just 400 million years after the Big Bang. In 2021, data from NASA's New Horizons space probe was used to revise the previous estimate of 2 trillion galaxies down to roughly 200 billion galaxies ().
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