I Really Wasn’t Trying To Avoid You

The band of the summer Milky Way dominates the southern sky in a photo taken in late June. The chunky clouds are rich concentrations of stars that are so far away they blend into a blur. The dark lane cutting the band in two are enormous dust clouds in the plane of our galaxy. The Milky Way blocks the view of the rest of the universe behind it, hence its other name - Zone of Avoidance. Photo: Bob King

Fact of life. There are some people and places we choose to avoid either for fear of getting roped into a conversation we don’t care to have or simply because we know what’s better for us. We give these individuals and situations a wide berth.

Something like this happens in the sky as well, and astronomers call it the Zone of Avoidance (ZOA). For all its wonders, the Milky Way galaxy is so full of gas, dust and stars that it obscures some 20% of the extragalactic sky – everything behind it – in visible light. Early astronomers observers noticed that most of the fuzzy things they saw through their telescopes, which we now know to be galaxies, seemed to ‘avoid’ the Milky Way. And for good reason. Only a few are bright or lucky enough to shine through the galactic flotsam.

A fisheye panorama of the evening sky in late April shows that the Milky Way hovers near the horizon, leaving the rest of the sky wide open to the extragalactic universe. Created with Stellarium

During summer, winter and much of fall, the Milky Way band stretches across the sky from horizon to horizon. Its presence is formidable, especially from dark, rural skies, where it’ll take your breath away. Either side of the band, the number of stars drops off fairly quickly, allowing views into the extragalactic realm. Amateur astronomers have their choice of a rich assortment of star clusters and nebulas should they decide to point their telescopes at the Milky Way. Or we can look off to the sides to peer beyond the galactic neighborhood at galaxies external to our own.

It’s different in early spring. The winter portion of the Milky Way band is now slung around the western horizon, while the summer portion has yet to rise in the east. That leaves the sky almost completely free of the Milky Way band. If you’re into galaxies, spring’s your season. With a small to medium telescope, you can revel in hundreds of them without any trouble from the home galaxy. They range from individuals scattered across the Big Dipper and Leo to huge clusters of several thousands in Virgo and Coma Berenices.

The Coma Galaxy Cluster in Coma Berenices contains thousands of galaxies. We get a great view because it gets no interference from the Milky Way. The cluster is located about 300 million light years from Earth. Credit: NASA/ESA

Stay up late enough, say around midnight, and the rotation of the Earth will finally carry up the constellations of the Northern Cross and Aquila the Eagle from the eastern horizon. Both are gorged with bright stars, clusters and dust clouds of the summertime Milky Way.

Galaxies that are otherwise veiled by the Milky Way galaxy are revealed in dust-penetrating infrared light in this montage from the 2MASS survey. Credit: 2-Micron All-Sky Survey

Like many of us, astronomers won’t be denied something they really want. Nowadays, surveys in other wavelengths of light like infrared and radio, ‘see’ through the dust to reveal long-hidden galaxies beyond our own. Two very large nearby galaxies, Maffei 1 and Maffei 2, were discovered in the Zone of Avoidance by Paolo Maffei by their infrared or heat emission in 1968. Were it not for obscuring dust, Maffei 1 would be one of the brightest galaxies in the sky.

More recent studies, like the Two Micron All-Sky Survey (2MASS), have given us an even better picture of the ZOA. Despite great strides in this galactic game of hide-and-seek, the plane of the galaxy, where the stars are densest, remains a tough nut to crack. Although a few external galaxies have been found there, there are undoubtedly many more lurking in the deeps.

4 Responses

  1. Cherokawa

    Hi Bob,
    I really enjoy your posts – they are full of information and totally engrossing. The photographs you post – and not just of celestial objects – are excellent. Please keep the articles coming and know that you have a faithful and grateful following.

    Clear skies,

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