Credit: NASA,ESA, and theHubble SM4 ERO Team& ST-ECF
Explanation: What is that strange arc?While imaging the cluster of galaxies Abell 370, astronomers had noted an unusual arc to the right of many cluster galaxies. Although curious, oneinitial response was to avoid commenting on the arc because nothing like it had ever been noted before. In the mid-1980s, however, better images allowed astronomers toidentify the arc as a prototype of a new kind of astrophysical phenomenon —the gravitational lenseffect of entire cluster of galaxieson background galaxies.Today, we know that this arc actually consists oftwo distorted images of a fairly normal galaxy thathappened to lie far behind the huge cluster. Abell 370’s gravity caused the background galaxies’ light — and others — tospread out and come to the observer alongmultiple paths, not unlike a distant light appears through the stem of awine glass.In mid-July, astronomers used thejust-upgraded Hubble Space Telescopeto image Abell 370 and its gravitational lens images in unprecedented detail. Almost all of the yellow imagespictured above are galaxies in the Abell 370 cluster.An astute eye can pick up manystrange arcs anddistorted arclets, however,that are actually images of more distant galaxies.Studying Abell 370and its images gives astronomers a unique window into the distribution of normal anddark matter ingalaxy clusters and the universe.
