What was the Great Debate of 1920?

A debate between Einstein and Newton over whether gravity caused the expansion of the universe A public debate between Harlow Shapley and Heber Curtis over whether spiral nebulae were inside the Milky Way or were separate distant galaxies A controversy between Hubble and Slipher over the correct value of the Hubble constant A dispute between American and European astronomers about whether the Milky Way was the largest galaxy

How did Edwin Hubble prove that the Andromeda Nebula was a separate galaxy?

He photographed a supernova in Andromeda and compared it to supernovae in the Milky Way He identified Cepheid variable stars in Andromeda and used their known luminosities to calculate a distance far too great to be inside the Milky Way He measured the Doppler redshift of Andromeda and calculated its distance from the recession velocity He measured the parallax of stars in Andromeda directly using a baseline of Earth's orbit

What discovery by Henrietta Swan Leavitt made Hubble's work possible?

She discovered that the Milky Way is a spiral galaxy by mapping the positions of all visible stars She invented the spectroscope technique that allowed Doppler shift measurements of distant nebulae She discovered in 1912 that Cepheid variable stars have a precise relationship between their pulsation period and their intrinsic luminosity She discovered the first extragalactic nebula by noticing it moved differently from background stars

What is a Cepheid variable star?

A binary star system where two stars eclipse each other as they orbit A pulsating star that alternately expands and contracts, brightening and dimming with a period that reveals its true luminosity A star that periodically flares up when it accretes material from a companion A star on the boundary between a red giant and a white dwarf, caught in the act of transitioning

What contribution did Vesto Slipher make to the discovery of the expanding universe?

He built the 100-inch Hooker Telescope that Hubble used to photograph Andromeda He proved mathematically that the universe must be expanding using general relativity He measured the recession velocities of spiral nebulae between 1912 and 1917, finding that most were moving away from us at high speeds He discovered the Cepheid period-luminosity relationship independently of Henrietta Leavitt

What was Hubble's original estimate of Andromeda's distance, and how does it compare to the modern value?

Hubble estimated 2.5 million light-years, which turned out to be exactly correct Hubble estimated about 900,000 light-years; the modern value is about 2.5 million light-years — off by nearly a factor of 3 Hubble estimated 100 billion light-years — he badly overestimated the size of the universe Hubble estimated 10,000 light-years, suggesting Andromeda was just barely outside the Milky Way

After Hubble proved Andromeda was a separate galaxy, what did this imply about the scale of the universe?

It confirmed that the Milky Way was the only true galaxy — all others were just small dwarf companions The universe was vastly larger than previously imagined, filled with billions of galaxies each containing billions of stars It suggested the universe was smaller than thought, because Andromeda was just barely outside the Milky Way It proved Einstein's cosmological constant was correct and the universe is static

What role did the Mount Wilson Observatory play in Hubble's discovery?

It provided a radio telescope that could detect the recession velocities of distant nebulae Its 100-inch Hooker Telescope was the most powerful in the world at the time and could resolve individual stars in Andromeda It housed the computer that processed Hubble's photographic plates and identified the Cepheid stars automatically It proved that the Andromeda Nebula was a spiral galaxy by photographing its spiral structure for the first time

Why is Henrietta Swan Leavitt's contribution to astronomy historically significant beyond just the Cepheid discovery?

She was the first woman to work at a major observatory, opening astronomy to women permanently Her period-luminosity relationship became the first rung of the cosmic distance ladder used by all subsequent astronomers to measure distances across the universe She invented the spectroscope, making it possible to measure Doppler shifts of distant objects She discovered that the Andromeda galaxy was approaching us, not receding

The term island universe was used before Hubble's discoveries. What did it mean?

The theory that the universe was finite and enclosed, like a sphere — an island of matter in an infinite void The idea that stars were surrounded by habitable planets — islands of life scattered through space The idea that spiral nebulae were separate, distant galaxies comparable to the Milky Way — each an island of stars in a vast sea of empty space The belief that the Milky Way was isolated in the universe with no other large galaxies nearby