This image shows a laser guide star being created towards the galactic center of the Milky Way. While the Milky Way’s plane shines prominently overhead in this photo, optical observatories find very few spiral and elliptical galaxies close to the Milky Way’s plane compared to the large numbers they find elsewhere. This led to the Milky Way’s plane being known as the “Zone of Avoidance,” a mystery that wasn’t solved until the 1960s. (Credit: G. Hüdepohl / atacamaphoto.com / ESO)
Astronomers see spiral and elliptical nebulae nearly everywhere, except by the Milky Way’s plane. We didn’t know why until the 20th century.
From their earliest discovery, grand cosmic spirals have posed a tremendous puzzle.
The featured image shows galaxy NGC 7331 along with other members of its galactic group, including the prominent galaxies NGC 7335, 7336, 7337, and 7340. We now know that a large fraction of galaxies beyond the Milky Way are spiral-shaped in nature, and that all of the spiral nebulae we were considering in ~1920 are indeed galaxies beyond our own. Like nearly all galaxies visible to even the aided human eye, these galaxies are located far away from the Milky Way’s central galactic plane. (Credit: Vicent Peris/c.c.-by-2.0)
Most nebulae — dark nebulae, star clusters, planetary nebulae — are found everywhere: omnidirectionally.
Near Orion’s Belt, the reflection nebula known as the Flame Nebula (left) as well as the star-forming emission nebula known as IC 434 (in red) are joined by a series of dark molecular clouds in the foreground that create spectacular silhouettes known as dark nebulae. The Horsehead Nebula (at center) is arguably the most famous dark nebula of them all, with nebulous features found wherever new stars are forming or will form in the future within the Milky Way. (Credit: Stephanh/Wikimedia Commons)
Some prefer the Milky Way’s plane: where stars, gas, and dust are most concentrated.
This complex of some 20 independent star-forming regions is within a 3-degree span and inside the Milky Way’s galactic plane. Taken with the far-infrared Herschel spacecraft, this region, Westerhout 43, is located at the intersection of our central galactic bar and one of our spiral arms. Where multiple infalling streams of cold matter intersect, new star-formation events usually arise. (Credit: ESA/Herschel/PACS, SPIRE/Hi-GAL Project. Acknowledgement: UNIMAP / L. Piazzo, La Sapienza — Università di Roma; E. Schisano / G. Li Causi, IAPS/INAF, Italy)
But not spiral nebulae; they’re found everywhere except in or near the galactic plane.
