How Does Space Become Transparent?

Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all

The Orion Nebula demonstrates the answer.

“So numerous are the objects which meet our view in the heavens, that we cannot imagine a point of space where some light would not strike the eye; — innumerable stars, thousands of double and multiple systems, clusters in one blaze with their tens of thousands of stars, and the nebulae amazing us by the strangeness of their forms and the incomprehensibility of their nature, till at last, from the limit of our senses, even these thin and airy phantoms vanish in the distance.” –Mary Fairfax Greig Somerville

Named “nebulae” because of their structure on the sky, these bright objects are actually nothing more than neutral gas.

*Image credit: Wikimedia Commons user Rawastrodata, under c.c.a.-s.a.-3.0.*

The reason they appear illuminated at all are because they either reflect/disperse starlight or emit light themselves, as Orion’s pink patches show.

*Image credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team.*

This emission occurs when electrons recombine with nuclei post-ionization, where ionization is caused by the intense ultraviolet light from hot, blue stars.

The hottest, most massive stars ionize the gas fastest, with the least gas-dense regions becoming completely ionized first.

When ionization is complete, not only are even the faintest stars revealed, but light from all the stars and galaxies beyond streams through.

*Image credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit.*

The finals stages of this evaporation are visible through the tadpole-like tendrils known as evaporating gaseous globules (EGGs): the last gasps of the neutral gas.

Principal Investigator(s): John Bally and Adam Ginsberg, University of Colorado and the GeMS/GSAOI commissioning team; Data processing/reduction (using the Gemini IRAF package v1.12beta): Rodrigo Carrasco, Gemini Observatory; Color image composite: Travis Rector, University of Alaska Anchorage. Image credit: Gemini Observatory / AURA.

Until the gas is completely ionized, visible light can be reflected or absorbed, depending on the orientation of the stars and gas with respect to us.

The only way to see through neutral gas is by looking in the infrared, which is sensitive to other features.

*Image credit: ASA/JPL-Caltech/D. Barrado y Navascués (LAEFF-INTA), from the Spitzer Space Telescope.*

Once the gas is 100% ionized, it’s 100% transparent, and the entire Universe is revealed.

Image credit: Infrared (R): NASA; K.L. Luhman (Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.); and G. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson (Steward Observatory, University of Arizona, Tucson, Ariz.); visible-light picture (L): NASA, C.R. O’Dell and S.K. Wong (Rice University).

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words.

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Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all