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A Galactic Traffic Jam
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Name: NGC 3887
Description: Spiral Galaxy
Position (J2000): RA 11h 47m 4.56s Dec -16° 51' 16.62"
Constellation: Crater
Distance: 70 million light years
Visual magnitude: 10.6
Angular size: 4.95 by 3.3 arcmin
Field of view: 0.64 x 0.67 arcminutes
Orientation: North is 74.6° right of vertical
Image Credit: ESA/Hubble & NASA, P. Erwin et al.
Release date: March 2, 2020

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The central region of the barred spiral galaxy NGC 3887, seen here as viewed by the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope, lies over 60 million light-years away from us in the southern constellation of Crater (The Cup). It was discovered on December 31, 1785 by the German/British astronomer William Herschel.

Its orientation to us, while not exactly face-on, allows us to see NGC 3887's spiral arms and central bulge in detail, making it an ideal target for studying a spiral galaxy's winding arms and the stars within them.

The very existence of spiral arms was for a long time a problem for astronomers. The arms emanate from a spinning core and should therefore become wound up ever more tightly, causing them to eventually disappear after a (cosmologically) short amount of time. It was only in the 1960s that astronomers came up with the solution to this winding problem; rather than behaving like rigid structures, spiral arms are in fact areas of greater density in a galaxy's disc, with dynamics similar to those of a traffic jam. The density of cars moving through a traffic jam increases at the center of the jam, where they move more slowly. Spiral arms function in a similar way; as gas and dust move through the density waves they become compressed and linger, before moving out of them again.