Edwin Hubble proved galaxies exist but refused to call them galaxies
Hubble used Cepheid stars to show that spiral nebulae lie far beyond the Milky Way, settling one of astronomy's great arguments. Yet he kept calling them nebulae, and the word galaxy only took over after his death in 1953. There may be between 200 billion and 2 trillion of them in the observable universe.
A galaxy is a gravitationally bound swarm of stars, gas, dust and dark matter, with dark matter making up most of the mass and visible stars only a few percent. The name comes from the Greek for milky, after the pale band our own Galaxy paints across the night sky. Sizes vary enormously, from dwarfs holding fewer than a thousand stars to supergiants with a hundred trillion, and supermassive black holes commonly sit at the centre.
Thinkers argued over that milky band for millennia. Democritus guessed it was distant stars; Aristotle thought it a fiery glow high in the atmosphere. Ibn al-Haytham tried to measure its parallax, found none, and concluded it was far beyond the air. Galileo's telescope confirmed in 1610 that it was packed with faint stars. In 1750 Thomas Wright proposed a rotating disc seen from inside and suggested some nebulae might be other Milky Ways. The Persian astronomer al-Sufi had already recorded Andromeda as a small cloud in the 10th century.
Measuring our own Galaxy was tricky. William Herschel counted stars in 1785 and placed the Sun near the centre, as did Jacobus Kapteyn in 1920 with a small ellipsoid; Harlow Shapley, mapping globular clusters, found a vast flat disc with the Sun far off-centre. Both missed the dimming effect of interstellar dust, which Robert Trumpler measured in 1930. Today the Milky Way is known to span at least 87,400 light-years, with Andromeda about 2.4 million light-years away.
The status of spiral nebulae was fought out in the 1920 Great Debate between Shapley and Heber Curtis, who had used faint novae in Andromeda to argue it was an island universe. Vesto Slipher had already found that most spirals are rushing away from us. Ernst Öpik's 1922 distance estimate supported Curtis, and Hubble, with the 100-inch Mount Wilson telescope, resolved individual stars and in 1926 devised the shape classification still used. Galaxies gather into groups, clusters and filaments around immense voids.
Source: Galaxy
