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Starts With A Bang

This Is How Astronomers Solved The ‘Zone Of Avoidance’ Mystery

Galaxies are found uniformly everywhere in the Universe, except in the Milky Way’s plane. Here’s why.

From the time of their very first discovery, the Universe’s grand spirals have puzzled astronomers.

This ultraviolet composite image of the Andromeda galaxy, taken by the GALEX spacecraft, showcases the youngest, bluest stars of all, which trace out the spiral arms and the galactic bulge. Andromeda was the first spiral nebula to be identified as a galaxy beyond our own. Note the extended nature of the arms, which indicates that new waves of star formation may be triggered by mild tidal disruptions.(NASA / JPL-CALTECH / GALEX)

While stars, star clusters and other nebulae were concentrated in the plane of our Milky Way, there were no spiral nebulae present.

The Milky way’s central region in visible light, with the location of the galactic center marked by E. Siegel. Billions of stars can be found there, and Pan-STARRS has collected data on more of them than ever before. Near the plane of the galaxy, however, there are no spiral nebulae to be found. At least, not in visible light. (JAIME FERNÁNDEZ)

For some reason, they eschewed our galaxy’s plane, which became known as the Zone of Avoidance.

A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, the Large and Small Magellanic Clouds (our two largest satellite galaxies), and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. There are a great many galaxies to be discovered, but within about 10 degrees above and below the galactic plane, visible light cannot reveal them. (ESA/GAIA)

Upon discovering that spiral nebulae were galaxies beyond our own, the problem made more sense.

A small selection of the galaxy as seen by Pan-STARRS, where dust is very dense, but the grains themselves are little different than anywhere else. This survey provides the most comprehensive 3D data ever taken. (DANNY FARROW, PAN-STARRS1 SCIENCE CONSORTIUM AND MAX PLANCK INSTITUTE FOR EXTRATERRESTIAL PHYSICS)

Dust, gas, and concentrated matter blocks the light from more distant objects, obscuring them.

Visible (left) and infrared (right) views of the dust-rich Bok globule, Barnard 68. The infrared light is not blocked nearly as much, as the smaller-sized dust grains are too little to interact with the long-wavelength light. At longer wavelengths, more of the Universe beyond the light-blocking dust can be revealed. (ESO)

The dust itself is composed of matter grains of specific sizes, preferentially blocking shorter-wavelength photons.

The dark regions show very dense dust clouds. The red stars tend to be reddened by dust, while the blue stars are in front of the dust clouds. These images are part of a survey of the southern galactic plane. (LEGACY SURVEY/NOAO, AURA, NSF)

Even modern 3D dust maps show this; dust grain size is independent of its location in the galaxy.

As a result, infrared telescopes can see through the dust, revealing the material behind it.

The view of the galactic center in four different wavelength bands. Atop, from the ATLASGAL survey at 870 microns; below that, from Spitzer in the mid-IR; below that, from ESO’s VISTA in the near-IR, and at the bottom in visible light, where the dust obscures everything of interest. (ESO/ATLASGAL CONSORTIUM/NASA/GLIMPSE CONSORTIUM/VVV SURVEY/ESA/PLANCK/D. MINNITI/S. GUISARD ACKNOWLEDGEMENT: IGNACIO TOLEDO, MARTIN KORNMESSER)

Not only can we reveal the structure of our own galaxy from within, but we at last found galaxies behind it.

Italian astronomer Paolo Maffei’s promising work on infrared astronomy culminated in the discovery of galaxies — like Maffei 1 and 2, shown here — in the plane of the Milky Way itself. Maffei 1, the giant elliptical galaxy at the lower left, is the closest giant elliptical to the Milky Way, yet went undiscovered until 1967. (WISE MISSION; NASA/JPL-CALTECH/UCLA)

The first galaxies found in the Zone of Avoidance are named Maffei 1 and 2, after Paolo Maffei, who pioneered infrared astronomy.

What we call “the Zone of Avoidance” isn’t, as we commonly present it, a nearby region with very few galaxies. Although we’ve seen very few galaxies, in reality it’s most probably a region with just as many galaxies as the rest of the Universe, that just happens to be hard to see from our vantage point! (COSMIC FLOWS PROJECT/UNIVERSITY OF HAWAII)

Galaxies are just as rich in the Zone of Avoidance as anyplace else.

Many galaxies, particularly young and dusty ones, emit most of their energy in the infrared portion of the spectrum. If we want to find the brightest galaxies of all, we’ll need a next-generation infrared space telescope. The Fireworks galaxy, from NASA’s Spitzer space telescope, is a local example of a predominantly infrared galaxy, and galaxies such as this can be revealed in the infrared thanks to infrared observatories such as Spitzer and WISE. (NASA / JPL-CALTECH / SSC / R. KENNICUTT ET AL.)

Thanks to viewing the Universe with infrared eyes, the mystery is now solved.

Although the vast majority of infrared emission comes from the plane of the Milky Way itself, where stars and gas and dust are primarily located, many galaxies can be viewed beyond it. When you look in the right wavelengths of light, the distribution of galaxies appears random; the Zone of Avoidance is an artifact of looking in visible wavelengths. where light-blocking is very efficient. (NASA/JPL-CALTECH/UCLA, FOR THE WISE MISSION)

Mostly Mute Monday tells the scientific story of an astronomical phenomenon, object, or problem in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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