Galaxies

How many galaxies are in the universe?

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Science  18 Nov 2016:
Vol. 354, Issue 6314, pp. 844
DOI: 10.1126/science.354.6314.844-a

Hubble eXtreme Deep Field view of distant galaxies

PHOTO: NASA, ESA, G. ILLINGWORTH, D. MAGEE, AND P. OESCH, R. BOUWENS, HUDF09 TEAM

Counting the number of galaxies is a complicated problem because astronomical surveys are biased and incomplete: It is easier to detect a bright nearby galaxy than a faint distant one. Small galaxies are the most numerous, but a boundary must be drawn between them and large star clusters. Distant galaxies are seen as they were earlier in their lifetime, and galaxy numbers can fall through merging. Taking into account these effects and more, Conselice et al. combined and extrapolated results from numerous surveys to determine that there are 2.0 ± 0.6 trillion galaxies in the observable universe. The vast majority still await discovery.

Astrophys. J. 830, 83 (2016).

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