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AP Psychology

28.1 Observations of Distant Galaxies

3 min readLast Updated on June 12, 2024

Gazing at distant galaxies is like peering into the universe's past. As light travels billions of years to reach us, we see galaxies as they were long ago. This cosmic time machine reveals how galaxies have changed over billions of years.

Early galaxies were smaller, more irregular, and formed stars at a frantic pace. They had fewer heavy elements and looked quite different from the majestic spirals and ellipticals we see nearby. Studying these ancient galaxies helps us understand how the universe evolved.

Observations of Distant Galaxies

Information from distant galaxies

Top images from around the web for Information from distant galaxies
Top images from around the web for Information from distant galaxies
  • Light travel time enables observing the universe as it was in the past since light from distant galaxies takes billions of years to reach Earth (Andromeda galaxy)
  • Redshift occurs when light from distant galaxies is shifted towards longer, redder wavelengths due to the expansion of the universe, with greater redshift indicating more distant galaxies and observations further back in time (quasars)
  • Hubble's law expresses the relationship between a galaxy's distance and its redshift as v=H0×dv = H_0 \times d, where vv is the galaxy's recessional velocity, H0H_0 is the Hubble constant, and dd is the galaxy's distance (Hubble Space Telescope)
  • Lookback time represents the time in the past when the light observed from a distant galaxy was emitted, calculated using the galaxy's redshift and the expansion rate of the universe (cosmic microwave background)
  • Spectroscopy is used to analyze the light from distant galaxies, providing information about their composition, temperature, and motion

Evidence of early star formation

  • Lyman-alpha emission, an ultraviolet emission line produced by hydrogen atoms in star-forming regions, indicates the presence of young, massive stars in early galaxies (starburst galaxies)
  • Infrared emission suggests ongoing star formation in distant galaxies as dust in star-forming regions absorbs ultraviolet light and re-emits it in the infrared (Spitzer Space Telescope)
  • Hubble Ultra Deep Field, a long-exposure image revealing thousands of galaxies in the early universe, shows many galaxies with blue colors, indicating young stellar populations and active star formation (gravitational lensing)
  • Spectroscopic observations revealing the presence of heavy elements, produced by stars, in distant galaxies confirm that stars had already formed and enriched the interstellar medium (Very Large Telescope)
  • The James Webb Space Telescope provides unprecedented views of early galaxies, allowing for more detailed studies of their formation and evolution

Early vs modern galaxy characteristics

  • Size and morphology differ as early galaxies tend to be smaller and more irregular in shape compared to modern galaxies, with many classified as "proto-galaxies" or "galaxy fragments" (Hubble sequence)
  • Stellar populations in early galaxies are younger and more metal-poor compared to nearby galaxies, with the most distant galaxies potentially containing Population III stars, the first generation of stars formed from pristine hydrogen and helium (globular clusters)
  • Star formation rates are higher in early galaxies compared to nearby galaxies, with the cosmic star formation rate peaking around 10 billion years ago and declining since then (Milky Way)
  • Chemical composition shows lower abundances of heavy elements in early galaxies compared to nearby galaxies, as the chemical enrichment of galaxies over cosmic time is driven by successive generations of stars (metallicity gradient)

Large-scale structure and galaxy environment

  • Galaxy clusters are the largest gravitationally bound structures in the universe, containing hundreds to thousands of galaxies
  • The cosmic web describes the large-scale structure of the universe, with galaxies arranged in filaments and sheets separated by vast voids
  • Active galactic nuclei are extremely luminous centers of galaxies, powered by supermassive black holes, and are more common in the early universe
  • Dark energy is believed to be responsible for the accelerating expansion of the universe, affecting the distribution and evolution of galaxies over cosmic time

Term 1 of 33

Active Galactic Nuclei
See definition

Active galactic nuclei (AGN) refer to the extremely luminous and energetic centers of some galaxies, powered by supermassive black holes that are actively accreting matter. These galactic nuclei emit radiation across the entire electromagnetic spectrum, from radio waves to high-energy gamma rays, making them some of the most energetic phenomena in the universe.

Key Terms to Review (33)

Term 1 of 33

Active Galactic Nuclei
See definition

Active galactic nuclei (AGN) refer to the extremely luminous and energetic centers of some galaxies, powered by supermassive black holes that are actively accreting matter. These galactic nuclei emit radiation across the entire electromagnetic spectrum, from radio waves to high-energy gamma rays, making them some of the most energetic phenomena in the universe.

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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

Term 1 of 33

Active Galactic Nuclei
See definition

Active galactic nuclei (AGN) refer to the extremely luminous and energetic centers of some galaxies, powered by supermassive black holes that are actively accreting matter. These galactic nuclei emit radiation across the entire electromagnetic spectrum, from radio waves to high-energy gamma rays, making them some of the most energetic phenomena in the universe.



© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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