Discovering galaxies
Only a century ago, astronomers debated whether the Milky Way was the entire universe. Edwin Hubble showed it is one galaxy among billions.
The Great Debate
In 1920, Harlow Shapley and Heber Curtis debated whether spiral nebulae were clouds within the Milky Way or separate 'island universes' far beyond it. The question could only be settled by measuring their distances.
Hubble's discovery
In 1923-1924, Edwin Hubble, using the 100-inch telescope at Mount Wilson, identified Cepheid variable stars in the Andromeda nebula. Using Henrietta Leavitt's period-luminosity relation, he showed it was far beyond the Milky Way, now known to be about 2.5 million light-years away. Andromeda was a galaxy, and the universe was vastly larger than thought.
The Hubble sequence
Hubble classified galaxies by shape in a 'tuning fork' diagram. Elliptical galaxies (E0-E7) are smooth, featureless ellipsoids. Spiral galaxies (Sa-Sc) have discs and arms; barred spirals (SBa-SBc) have a central bar. Lenticular (S0) galaxies have discs but no arms. Irregular galaxies have no regular shape.
Spiral galaxies
Spiral galaxies have rotating discs with gas, dust, and ongoing star formation, giving them blue arms with young stars, plus central bulges of older stars. About two-thirds of spirals, including the Milky Way, have bars. Their rotation curves reveal dark matter halos.
Elliptical galaxies
Elliptical galaxies contain mostly old, red stars, little cool gas, and little new star formation. Their stars move in random orbits rather than a rotating disc. They range from dwarf ellipticals to giant ellipticals with trillions of stars, often found at the centres of galaxy clusters, likely built through mergers.
Irregular and dwarf galaxies
Irregular galaxies, such as the Magellanic Clouds, lack a regular structure, often due to gravitational interactions. Dwarf galaxies, small and faint, are the most common type in the universe, though bright galaxies dominate what we see.
How many galaxies
Deep images such as the Hubble Ultra Deep Field, a tiny patch of sky containing about 10,000 galaxies, suggest the observable universe contains hundreds of billions of galaxies, possibly up to about two trillion according to some estimates. JWST deep fields are revealing galaxies from when the universe was only a few hundred million years old.
Worked example
Hubble measured a Cepheid in Andromeda with a period of about 31 days. From Leavitt's relation, that period corresponds to a known high luminosity. Comparing that luminosity with its faint apparent brightness, the inverse-square law gave a distance far greater than the Milky Way's size, ending the Great Debate.
Key takeaways
The 1920 Great Debate asked whether spiral nebulae were separate galaxies. Hubble's Cepheid measurements in 1923-1924 proved Andromeda is a distant galaxy. The Hubble sequence classifies ellipticals, spirals, barred spirals, lenticulars, and irregulars. Spirals have discs, gas, and young stars; ellipticals have old stars and little gas; irregulars lack regular shape; dwarfs are most common. The observable universe holds hundreds of billions of galaxies or more.
Flashcard
1920 Shapley-Curtis debate
Check your understanding
How did Edwin Hubble prove that the Andromeda nebula was a separate galaxy?