26.1 The Discovery of Galaxies

Edwin Hubble's groundbreaking observations of distant nebulae revolutionized our understanding of the universe. Using the powerful Hooker telescope, he identified Cepheid variable stars in the Andromeda nebula, proving it was a separate galaxy far beyond our Milky Way.

This discovery resolved the confusion between nebulae and galaxies, revealing that many fuzzy objects in the night sky were actually distant island universes. Hubble's work expanded our cosmic perspective, showing the universe was vastly larger than previously thought and filled with countless galaxies.

The Discovery of Galaxies

Confirmation of external galaxies

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• Edwin Hubble used the 100-inch Hooker telescope at Mount Wilson Observatory to observe distant nebulae
• He identified Cepheid variable stars within the Andromeda nebula (M31)
  • Cepheid variables are pulsating stars with a known relationship between their luminosity and pulsation period enabling astronomers to determine their distance (standard candles)
  • This relationship, discovered by Henrietta Swan Leavitt, allows astronomers to determine the distance to Cepheid variables by measuring their apparent brightness and pulsation period
• By measuring the period of the Cepheid variables in M31, Hubble calculated its distance
  • He found that M31 was much farther away than any known object within the Milky Way (2.5 million light-years)
  • This proved that M31 was a separate galaxy, not a nebula within our own galaxy
• Hubble's discovery provided conclusive evidence for the existence of galaxies beyond the Milky Way confirming the "island universe" hypothesis

Nebulae vs galaxies confusion

• Before Hubble's discovery, astronomers observed distant, fuzzy objects called nebulae
  • Nebulae appeared similar to gas clouds within the Milky Way (Orion Nebula, Crab Nebula)
  • Many astronomers believed that these nebulae were part of our galaxy and represented early stages of star formation
• The true nature of nebulae was unclear due to the limitations of early telescopes
  • Smaller telescopes could not resolve individual stars within distant galaxies making them appear as diffuse, cloud-like objects
  • This led to the misconception that nebulae were gaseous objects within the Milky Way and not separate galaxies
• Hubble's observations with the powerful Hooker telescope allowed him to resolve individual stars in M31
  • This revealed that some nebulae, like M31, were actually separate galaxies far beyond the Milky Way composed of billions of stars
• Hubble's findings resolved the confusion between nebulae and galaxies
  • It became clear that many of the observed nebulae were, in fact, distinct galaxies outside our own while others were truly gas clouds within the Milky Way
• The study of galactic morphology helped astronomers classify and understand the different types of galaxies observed

Significance of Andromeda's distance

• Measuring the distance to the Andromeda galaxy (M31) was a crucial step in comprehending the vast scale of the universe
• Before Hubble's measurement, the size of the universe was thought to be limited to the Milky Way
  • Astronomers believed that all observed objects were contained within our galaxy spanning about 100,000 light-years
• Hubble's distance measurement to M31 revealed that it was far beyond the boundaries of the Milky Way at 2.5 million light-years away
  • This discovery expanded our understanding of the universe's size dramatically by showing it was at least 25 times larger than previously thought
• The distance to M31 provided a new benchmark for cosmic distances
  • It demonstrated that the universe was much larger than previously imagined containing vast expanses of apparently empty space between galaxies
  • This realization led to a significant shift in our perception of the cosmos from a relatively small, single-galaxy universe to an immense realm of countless island universes
• Hubble's work laid the foundation for the concept of a universe filled with countless galaxies
  • It opened the door to further exploration and discovery of the large-scale structure of the universe (galaxy clusters, superclusters, filaments, and voids)
• The measurement of M31's distance was a crucial step in the development of modern cosmology
  • It paved the way for future studies of galaxy clusters, superclusters, and the overall structure of the universe leading to our current understanding of a vast, expanding cosmos originated in the Big Bang

Advancements in Extragalactic Astronomy

• Spectroscopy became a crucial tool in studying distant galaxies, allowing astronomers to analyze their composition and motion
• The discovery of redshift in galaxy spectra provided evidence for the expansion of the universe and became a key method for measuring cosmic distances
• These advancements led to the establishment of extragalactic astronomy as a distinct field, focusing on the study of objects and phenomena beyond our own galaxy