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Discovering true distances in space

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Discovering true distances in space

NGC 1356, LEDA 467699, LEDA 95415 and IC 1947

Hubble Space Telescope image showing the spiral galaxies NGC 1356, LEDA 467699, LEDA 95415 and IC 1947. Credit: ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/NOIRLab/NSF/AURA, confirmation: L Shatz

This Hubble Picture of the Week shows a wealth of spiral galaxies: the large, visible spiral galaxy on the right side of the image is NGC 1356; the two apparently smaller spiral galaxies flanking it are LEDA 467699 (above) and LEDA 95415 (very close to the left), respectively; and finally IC 1947 is on the left side of the photo.

This image is a really interesting example of how difficult it can be to tell whether two galaxies are actually close to each other, or if it just appears that way from our perspective here on Earth. A quick glance at this image will likely lead you to the conclusion that NGC 1356, LEDA 467699, and LEDA 95415 were close companions, while IC 1947 was more distant. However, we must remember that two-dimensional images such as this only give an indication of angular separation, or how objects are distributed within the sphere of the night sky. What they cannot represent is the objects’ distance from Earth.

For example, although NGC 1356 and LEDA 95415 appear to be so close to each other that they must certainly be interacting, the former is about 550 million light-years from Earth and the latter is about 840 million light-years away, so there is an almost whopping 300 million light-year separation between them. This also means that LEDA 95415 is probably not as much smaller than NGC 1356 as it appears.

On the other hand, while NGC 1356 and IC 1947 appear to be separated by a relative gulf in this image, IC 1947 is only about 500 million light-years from Earth. The angular distance visible between them in this image is only less than four hundred thousand light-years, so they are actually much closer neighbors in three-dimensional space than NGC 1356 and LEDA 95415!

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