Discovering real distances in space

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

This Hubble Image of the Week is rich in spiral galaxies: the large spiral galaxy prominent on the right side of the image is NGC 1356; The two smaller surrounding spiral galaxies are LEDA 467699 (above) and LEDA 95415 (very close to the left), respectively; Finally, IC 1947 is located on the left side of the image.

This image is a really interesting example of how difficult it is to know whether two galaxies are actually close together, or appear to be from our perspective here on Earth. A quick look at this image would probably lead you to believe that NGC 1356, LEDA 467699, and LEDA 95415 were all close companions, while IC 1947 was farther away. However, we have to remember that 2D images like this one only give an indication of angular separation: how objects are spread across the night sky. What they cannot represent is the distance objects are from Earth.

For example, while NGC 1356 and LEDA 95415 appear so close that they must be interacting, the former is about 550 million light-years away from Earth and the latter is about 840 million light-years away, so there's almost a colossal difference. 300 million Light year Separate them. This also means that LEDA 95415 is likely 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 gap in this image, IC 1947 is located only about 500 million light-years from Earth. The angular distance between them in this image is only about four hundred thousand light-years, so they are actually much closer neighbors in 3D space than NGC 1356 and LEDA 95415!