Extragalactic astronomy
Extragalactic astronomy studies objects and phenomena beyond the Milky Way, from individual galaxies to the universe’s large-scale structure.
Galaxy: A gravitationally bound system of stars, gas, dust, stellar remnants, and dark matter. Galaxies are the principal objects whose populations and properties extragalactic astronomy investigates.
Galaxy formation and evolution: The processes by which galaxies originate, grow, change, and sometimes cease forming stars. Comparing galaxies across cosmic time tests models of their life histories.
Astronomical spectroscopy: The analysis of astronomical light by wavelength to infer an object’s composition, motion, and physical conditions. Spectra provide redshifts and clues to the contents of distant galaxies.
Edwin Hubble: An American astronomer whose observations helped establish that galaxies exist beyond the Milky Way and that the universe expands. His work made external galaxies central to modern astronomy.
Dark matter: Matter inferred from its gravitational effects that does not emit, absorb, or reflect detectable light. Its distribution helps explain galaxy rotation and the growth of cosmic structure.
Cosmological distance ladder: A sequence of calibrated methods for measuring distances to astronomical objects at progressively greater ranges. Distance estimates turn observed galaxy brightness and redshift into physical scales.
Galaxy cluster: A gravitationally bound structure containing hundreds to thousands of galaxies, hot gas, and dark matter. Clusters expose how galaxies behave in dense environments and map massive structures.
Photometry: The measurement of an astronomical object’s brightness through specified wavelength bands. Multiband brightness measurements estimate galaxy properties and identify distant sources.
Henrietta Swan Leavitt: An American astronomer who discovered a relation between Cepheid variable stars’ periods and intrinsic luminosities. Her relation enabled distance measurements to nearby galaxies.
Dark energy: The name given to whatever drives the observed accelerated expansion of the universe. Distant galaxies and supernovae help constrain the expansion history it affects.