Translucent Giant

Credit: NASA, ESA and K. Cook (Lawrence Liv­er­more National Lab­o­ra­tory, USA)

A sky full of stars and galax­ies is dom­i­nated by the translu­cent giant spi­ral galaxy called NGC 4921 in this image from NASA’s Hub­ble Space Tele­scope. It has taken light from NGC 4921 about 320 mil­lion years to reach our eyes on Earth. Galax­ies in the rich back­ground are even more remote stretch­ing back to the early Universe.

Explore the image and the stun­ning back­ground. What do you notice?

NGC 4921 bears some resem­blance to other spi­ral galax­ies includ­ing our Milky Way Galaxy. But instead of dis­tinct, sweep­ing spi­ral arms with bright areas of star for­ma­tion, NGC 4921 has a smooth swirl of dust almost devoid of gas. It glows like a giant, translu­cent jel­ly­fish. Look close, and you may notice a ring of bright blue between the galaxy’s core and edge. Hubble’s sharp vision shows us the new, blue stars clearly. This del­i­cate swirl of color is the only tell-tale sign of new star for­ma­tion. Astronomers refer to this as an “ane­mic” galaxy where few stars are formed.

NGC 4921 is part of the Coma Galaxy Clus­ter in the con­stel­la­tion Coma Berenices, the hair of Queen Berenice. The rich clus­ter, also known as Abell 1656, is one of the clos­est col­lec­tions of galax­ies with more than 1,000 mem­bers; most of them ellip­ti­cal galax­ies. Galax­ies in crowded clus­ters often undergo many col­li­sions and merg­ers. These inter­ac­tions strip spi­ral galax­ies of their plen­ti­ful gas and dust. Gas and dust are essen­tial for star for­ma­tion. Even­tu­ally after bump­ing and col­lid­ing, astronomers believe spi­ral galax­ies evolve into larger ellip­ti­cal galax­ies with less new star formation.

Far beyond NGC 4921, we see thou­sands of remote galax­ies of all sizes, shapes and col­ors. While some of the far-off galax­ies have spi­ral shapes, oth­ers are uneven and ragged. These galax­ies formed in an ear­lier uni­verse before the grace­ful spi­rals and giant ellip­ti­cal galax­ies formed.